Retrospective Theses and Dissertations Iowa State University Capstones, Theses andDissertations

1-1-1977

Effect of irrigation on the potatoInnocent AgwatuIowa State University

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Recommended CitationAgwatu, Innocent, "Effect of irrigation on the potato" (1977). Retrospective Theses and Dissertations. 17468.https://lib.dr.iastate.edu/rtd/17468

Approved:

Effect of irrigation on the potato

by

Innocent 0. I. Agwatu

A Thesis Submitted to the

Graduate Faculty in Partial Fulfillment of

the Requirements for the Degree of

MASTER OF SCIENCE

Major: Horticulture

Signatures have been redacted for privacy

Iowa State University

Ames, Iowa

1977

ii

TABLE OF CONTENTS

DEDICATION

INTRODUCTION

REVIEW OF THE LITERATURE

MATERIALS AND METHODS

RESULTS AND DISCUSSION

SUMMARY AND CONCLUSION

LITERATURE CITED

ACKNOWLEDGEMENTS

Page

iii

1

3

19

27

51

54

63

iii

DEDICATION

To Chukwuma D. Agwatu,

the pioneer and bulwark

of higher education in

the Agwatu family.

1

INTRODUCTION

Iowa's water source is scarce and expensive. Rainfall

is frequently erratic. The growing seasons of 1975 and 1976

were dry with temperatures above normal in July-August.

Consequently, interest in irrigation has accelerated to elu-·

cidate means of judiciously managing this limited resource

to offset the year to year yield and quality variations of

such crops as the potato, a cool season and drought sensitive

crop.

To perform at its best the root zone of the potato crop

requires near field capacity moisture conditions. Mounting

evidence indicates that of the methods of irrigation prac-

ticed, the trickle system comes close to maintaining near

field capacity conditions, and so has aroused considerable

interest. With its capacity of small volume, frequent

irrigation application through small orifices, the trickle

irrigation system has enhanced yield production with higher

water efficiency. The sprinkler irrigation system, on the

other hand, wets the cropland to field capacity only once a

week or so. Excess water is expended because some areas

without roots or root extensions are also wetted to field

capacity. Coupled with this, there is the obvious loss of

water in the form of run-off and high pumping energy.

Irrigation enhances root growth and development. More

root growth increases demand on soil nutrients and this calls

2

for higher fertilization. Nitrogen fertilizer is a landmark

in this regard because it is the most universally deficient

of the soil derived nutrients for plants. In recent years,

there has been a steady increase in the use of nitrogen

fertilizer to achieve maximum potato yields. Higher

fertilizer costs have created a need for additional research

dealing with fertilizer usage in potato production,

especially nitrogen.

Although nutrients may be present in the soil in

sufficient quantities, they may become unavailable for crop

use because the surface soil, the location for most

nutrients, may be dry. In such a situation, crops may start

to show nutrient deficiency symptoms even before they start

to exhibit moisture stress.

The primary objective of this study was to determine if

there is any advantage of trickle versus sprinkler irrigation

to satisfy water requirements for the production of fresh

market potatoes in Iowa. A second objective was to

investigate how management of fertilizer nitrogen changes

with the trickle irrigation system.

3

REVIEW OF THE LITERATURE

Irrigation

Irrigation is the practice of supplementing natural

precipitation by applying water to the soil with a view to

preventing and/or correcting plant water stress (23).

Irrigation offers adequate moisture for the rapid growth

and development of the potato crop and results in increased

tuber yield (18, 26, 27, 28, 44, 81). Galien (28) and Lucke

(48) have established that irrigation helps to keep down

insect damage. Working on the effect of supplementary

sprinkler irrigation on cut worm damage of potatoes, Lucke

(48) found that not only did the irrigated plots out-yield

the non-irrigated controls (23t/ha vs. 13.4t/ha); but also

there was more reduction in tuber damage by cut worm in

irrigated than in non-irrigated plots (15.1% vs. 44.6%).

The rate of consumptive use of water by the potato in

any particular locality is probably affected more by

temperature than by any other factor. Abnormally low

temperatures retard crop growth and unusually high tempera-

tures produce dormancy (41) . Irrigation cools through water

evaporation from the cropland and the plant canopy. Since

the potato requires a cool micro-climate for optimum

performance (13, 44, 70), irrigation of some sort is

decidedly necessary. Peterson and Weigle (68) have

4

established that mist irrigation, in cooling the plant

canopy, causes the stomates to stay open longer, and other

factors being equal, photosynthetic intensity is enhanced and

more carbohydrates are consequently translocated to the tubers.

Further, they showed reduced transpiration rate to be

associated with lower temperatures.

The amount of water utilized by the potato crop increases

with rise in temperature (94). If rainfall is uncertain

during this period of increased moisture need, irrigation

serves as a good substitute. Phene and Sanders (70) pointed

out that as far as possible, potatoes should be grown at

temperatures averaging 70°F, so as to produce tubers with

high specific gravity and high starch content. Higher

temperatures delayed tuber formation and lowered tuber

quality.

Although irrigation prolongs maturation of the potato,

thereby allowing more time for tuber bulking, the dry matter

and the total nitrogen content of the tubers are lowered (12,

79). Also, excessive irrigation enhances the incidence of

secondary growth and hollow heart. Quite often, immature

tubers, containing more reducing sugars and less starch, are

produced when compared with tubers grown under natural

rainfall conditions.

Peterson and Weigle (68} pointed out that under mist

irrigation conditions, the specific gravity is increased

5

rather than decreased. Challaiah (13), sharing the above

viewpoint, stated that more frequent irrigation increased the

specific gravity of tubers. However, decreasing soil water

potential increases the number of misshapen tubers (23, 39,

45, 70, 76). Such tuber shape defects have been attributed

to moisture tension. Maintaining a non-fluctuating soil

materic potential in the root zone would help to minimize

growth defects.

Different types of irrigation systems, some new, others

old, have generally been employed to the advantage of some

crop plants. While economics is the guiding factor as to

which system to employ, other factors such as the crop to be

irrigated, the topography of the land and the operating

environmental conditions need to be considered. Surface,

furrow, mist, sprinkler, and trickle irrigation are some of

the systems that are generally employed.

Sprinkler irrigation

Sprinkler irrigation system has been employed to a

greater advantage than some other systems. In rotating

sprinkler irrigation, water is broken up into fine drops,

sprayed into the air and distributed over the ground. Water

then proceeds into the root zone by infiltration and capillary

action. The amount of water that actually infiltrates

depends on soil type. By capillary action water tends to

creep from one soil particle to another because of its

6

attraction to the surfaces of the soil particles. This

attraction may be multi-directional--upwards, lateral, and

downwards.

The sprinkler irrigation system has been credited with

the following benefits--yield increases, keeping air 0 temperature within the plant canopy below 80 F throughout the

growing season (44, 62), and decreasing the percentage of

tubers with Streptomyces scabies. However, this irrigation

system has certain obvious short-comings which may seem to

overwhelm some of its benefits. Such short-comings include

excessive wetting of the cropland which may result in

anaerobic situations and lots of run-off water. Krogman and

Torfason (44) have observed that with low volume sprinkler, a

deposit of salt may accumulate on the leaves. Crop growth is

impaired because the salt-deposits have a tendency to absorb

water and therefore pull water out of the leaves. Salt depos-

its on leaves also result in high amounts of certain micro-

elements, which otherwise the crop would require only in very

small amounts. Thus, saline water should not be used with

the sprinkler irrigation practice.

Trickle irrigation

The trickle irrigation system is, at least for now, the

newest system in irrigation technology. It has been given

different names in different countries, hence one oftentimes

encounters such synonymous names as ~~trickle" which

7

originated in England; "drip" in Israel; "daily flow" in

Australia, and dibble irrigation (41). Although trickle

irrigation has been employed in the greenhouse for several

years, its use in the field on vegetable and orchard crops

has evolved only recently. While other systems may aim at

correcting water stress, trickle irrigation tries to tackle

the stress problem before it ever arrives, tries to prevent

it.

Proponents of trickle irrigation claim that it could pro-

duce higher yields, improve quality of production, decrease

water usage, decrease pumping energy, increase fertilizer

efficiency, and reduce labor costs. There is accumulated

data on higher yields resulting from trickle than from

sprinkler or any other irrigation systems (11, 65, 70, 93).

Bucks et al. (11) have claimed that through application of

small quantities of water at frequent intervals by trickle

irrigation, in contrast to larger, infrequent water applica-

tion by conventional irrigation methods, a possible savings

of 50-90% in water requirements may result. Kenworthy (42)

pointed out that frequent light irrigations can be applied

to crop plants to keep moisture available in a limited root

zone, 26 percent, without wetting the soil beyond the extent

of the root system. Thus, only small portions of the soil

volume are irrigated, surface evaporation is decreased, water

movement below the root zone is reduced, all culminating in

8

higher irrigation efficiency.

Phene and Beale (71) working in the sub-humid coastal

plains have reported that controlled high frequency trickle

irrigation can be used to accurately regulate soil matric

potentials in the root zone without inhibiting the oxygen

diffusion rate in soil and to minimize plant water stress of

sweet corn (Zea mays). Other workers (2, 5, 42, 80) feel

that widely spaced tree crops and orchard crops utilize the

trickle irrigation system to more advantage than row crops,

and have concentrated their work on such high value crops.

The trickle system is also reputed for its ability to utilize

saline water for irrigation over and above the other systems

(42). As the crop is watered, moisture tends to soak the

soil unilaterally so that the salt is precipitated at the

edge of the wetted zone. As soil moisture in each field

varies from year to year, highest profits are seldom realized

by irrigations timed by calendar alone. Available soil mois-

ture has to be predicted. Trickle irrigation has proved

efficient. Sprinkler irrigation can over-wet the plots there-

by giving rise to standing water or temporary flooding,

puddling soil and consequent compaction. When soils stay

wet in this manner for a long period of time, plant growth

might be delayed. Such soils may run the risk of having

their nitrogen leached out. Although irrigation has a

tendency to check the incidence of disease (28, 48) allowing

9

standing water may enhance the severity of disease. Excess

moisture also promotes anaerobic conditions which promote the

proportion of raised lenticels on tubers, a depressing

external quality, an unsightly and unappetizing scene to

consumers.

Different researchers have various ideas as to when to

start irrigating potatoes. Exponents of pre-planting irriga-

tion (72) claim that such a timing is of advantage in

reducing clotting, improving soil structure, and eventually

leading to increased yield. They advise that water should

also be applied to the field soon after planting, to hasten

seed germination. Werner (93), however, does point out the

necessity for adequate moisture in the early season in order

to produce large plants with large leaf area necessary for a

good crop of tubers. Large tops have to be produced as early

as possible so that much of the tuber crop can be produced

during the cool weather of early summer. Large leaf area is

necessary for the manufacture of carbohydrates needed to

produce a big crop of tubers. If growth is checked early in

the season by moisture deficits, plant size will be reduced

and yield will be correspondingly low. It is desirable,

therefore, to have soil sufficiently moist at all times to

assure growth at a continuous rate.

Other workers (26, 29, 33, 60, 61} examining morpho-

logical development stages of the potatoes, have found that

10

water consumption in potatoes is highest at the time the crop

is fifty days old, or at flowering, stolon or tuber formation

stage, depending on the planting date. During this stage,

the potato is particularly sensitive to inadequate moisture

supply. Similarly, Garay (29) demonstrated significantly

higher yields with irrigation begun some fifty days after

planting than with irrigation started seventy days after

planting. Fuehring (27) has even observed that tuber yields

increased with increase in number of irrigations during

tuberization and tuber growth. Moisture stress applied to

potato plants during early tuber set period increased the

percentage of malformed tubers having pointed stem ends,

bottle necks, and dumb-bell shapes, although total yield and

grade of tubers was not significantly affected.

Evaluating the soil moisture content before switching on

irrigation, Ivins and Milthorpe (33) felt that best results

were obtained if irrigation water was applied when 50% of the

available soil moisture had been used from the top 12 or 18

inches of soil. In so doing, the soil moisture content was

brought back to field capacity or close enough, for optimum

crop performance. Working on the same subject, Nyc (64) and

others (66) pointed out that when the optimum soil moisture

content is 75-80%, potatoes do not require irrigating as total

yield, grade or tuber quality were not affected.

Irrigation needs of potatoes can be predicted by

11

moisture tensiometer readings, when the first symptoms of

moisture stress are observed, or when indicated by a soil

moisture budget involving estimated evapotranspiration.

Comparing the above methods Dubetz and Krogman (20) and

Krogman and Torfason (44) have confirmed that the moisture

tensiometer method is the best of the three while evapotrans-

piration method is better than moisture stress procedure.

Although irrigation is a very useful practice, it is

not desirable to continue to irrigate when its utility value

starts to decline. One should stop irrigating as soon as

potatoes are mature. Irrigating beyond this point would

cause soil around the tubers to be wet, a situation which may

induce poor tuber shapes and/or raised lenticel formation on

the tubers. This, no doubt, helps to lower the quality of

the tubers.

Lakes, dams, wells, streams, and springs have been in

the past employed as sources of irrigation water. Although

there have been no comparative studies of the above sources,

it is however known that while trickle irrigation system can

cope with saline water, sprinkler irrigation or any of the

other systems cannot. Sprinkling with saline water causes

leaf burn and reduced production, even though drip irrigation

with same water causes no problem (6).

12

Nitrogen Fertilizer

Wilcox and Hoff (96) demonstrated that nitrogen fertili-

zation of potatoes for early summer harvest yield and quality

is determined by the number and size of tubers produced. The

effect varies between localities and is probably due to

differences in the growing season, temperatures, and light

intensity. Gerdes et al. (32) and other workers (15, 72) claim

that applied nitrogen promoted early potato top growth through

increased branching, leaf number, and expansion. Potassium

and phosphorus are known to enhance leaf expansion while

phosphorus also aids in increased meristematic activity (72).

Consequently, greater foliage is produced which could mean

more photosynthetic area and activity leading to more carbo-

hydrate manufacture, and culminating in greater translocations

to the primary sink, tubers. Mineral nutrients are also

known to affect the rate of dry matter production, chiefly by

changing leaf area (15). As a result, the total dry weight

yields of crops with different nutrient supply are closely

related to the size of the leaf area, the seat of active

photosynthesis and the basis for yield increases.

Nitrogen has been shown to increase the number of tubers

set by allowing for the survival of tuber initiates and

increasing the average tuber size by enhancing their rapid

development (21, 34). Also, high rates of mineral fertilizer~'

13

especially nitrogen have been shown to increase crop moisture

content (22).

While some researchers (24, 49, 58, 59, 66, 73, 91, 95,

96) attribute N fertilization to increased scabbiness, scurf,

percentage of cracked tubers, and lowering of dry matter

content of tubers, others (3, 12, 27, 30, 45, 58, 59, 66, 87,

91, 95) have found that nitrogen not only increases total

tuber yields and the percentage of grade one tubers but also

increases the nitrogen content of both foliage and tubers.

Foliar nitrogen has been established to reflect the amount of

applied nitrogen. The total nitrogen content of fresh tubers

has also been shown to be directly and linearly related to

the rate of nitrogen applied. Since these findings reflect

ultimate yield increases, many growers tend to apply luxury

levels of nitrogen. Unfortunately, however, not only do they

waste nitrogen by so doing, they also create ideal conditions

for tuber malformations, and lowering of specific gravity,

chip quality, and storage life of the tubers produced. In

addition, excess nitrogen can delay tuber initiation and can

unduly prolong leaf area duration, thereby delaying crop

maturity (59, 88).

Soltanpour (83) looking closely at the nutritional

requirements of potato at different stages of development has

established that maximum uptake of nitrogen occurred 60 to

90 days after planting; that nitrogen should be plentiful at

14

this stage of growth and not be depleted for several weeks;

that maximum uptake by tubers occurred later in the season

and that nearly all of this nitrogen is translocated from

the vines. Investigating the effects of whole versus split

nitrogen application Dimitov (17), working with alluvial

meadow chernozen soil, has shown that all nitrogen applied

at planting accelerated growth and tuber formation more than

a split application. He also stated that these were more

true of early than late cultivars. Svensson et al. (87),

while not rejecting the foregoing concept, state that split

applications with late supplementary dressings helped to

increase the uniformity of tuber size, a necessary quality

factor to consumers. Furthermore, split nitrogen application

has been encouraged with croppings under irrigation or

whenever a long season cultivar is involved, with or without

irrigation. It has also been claimed that delay in

harvesting early summer potatoes increases the effects of

applied nitrogen by producing higher yield (1, 72). While

some reports (12, 91) accept that no significant differences

are obtained whether fertilizers are banded or broadcast,

Painter and Augustin (66) point out that banding fertilizer

produces more tubers having growth cracks, culls, and reduced

yield of number ones when compared with broadcasting. Dif-

fering from the foregoing view, Vitosh (91) reports that banded

and side-dressed nitrogen were more efficient than broadcast

15

nitrogen plowed down prior to planting.

Although the amount of fertilizer may well depend on

soil type and its fertility status coupled with operating

environmental factors, many growers tend to apply more nitro-

gen to early (spring} potatoes than to late (fall} potatoes

because this element is usually less available early in the

season. Evaluating the economic limits of nitrogen

fertilization of potatoes, Gerdes et al. (32} have arrived

at the economic rate of 150 kg N/ha. Such a nitrogen level

produced greater leaf persistence, increased total yields

with higher dry matter content, and increased yield of number

one potatoes, as opposed to lower or higher levels of

nitrogen. In a similar but separate research (12}, economic

level of nitrogen rate for potato production has been shown

to be 180 kg N/ha.

Nitrogen fertilizer sources are variable, hence, nitro-

gen can be supplied to potatoes in a variety of forms,

including ammonium nitrate, urea, potassium nitrate,

ammonium sulphate, ureaform, calcium nitrate, anhydrous

ammonia, and sodium nitrate. Some sources have proved more

advantageous than others. Vitosh (91} has shown that equal

yields were obtained with all nitrogen sources and that

specific gravity was not affected, but that continued use of

these nitrogen fertilizers would create soil pH problems

which would ultimately affect tuber yield and specific gravity.

16

Lorenz and Johnson (47), in a comparative study of nitrogen

source effect on potato production on a slightly alkaline

coarse-textured soil in California, have reported that

ammonium sulphate was much better than calcium or sodium

nitrate for growing of potatoes. While some reports (77)

have stated that potatoes, pineapples, and young stages of

some cereals grow better on ammonium, others (10) point out

that the ammonium ion would be detrimental to the growth and

development of the potato plant and so conditions which

favor the persistence of nitrogen in this form should be

avoided.

Some nitrogen sources no doubt do tend to have apparent

intermediate advantages over others, but the end product may

well be the same. Slow release fertilizers as sulphur coated

urea, for instance, are too costly to be applied to any

economic advantage since the ultimate yield does not balance

out the high costs.

Irrigation vs. Nitrogen

Irrigation and nitrogen fertilization separately or in

combination are accepted practices that enhance potato

production. In recent years, nitrogen fertilizers have been

added as a variable to irrigation trials. Irrigation and

nitrogen complement each other and, in fact, applications of

low rates of nitrogen fertilizer in irrigation water lead to

17

increased yield. Fertilizing with nitrogen under irrigated

conditions increases the reliability of the crops by enabling

them to utilize more nitrogen, especially with long season

cultivars, and leads to significant yield increases and

percentage of large sized tubers when compared with

fertilized but unirrigated plots (1, 32, 59, 81). Abel's

data (1) show that that in the dry regime, yields of

potatoes receiving 336 kg N/ha were not significantly higher

than those receiving only 56 kg N/ha under irrigation condi-

tions. Irrigation, therefore, increases the efficiency of

applied nitrogen. Also, fertilizing with 150 kg N/ha

increased the dry matter content of the tubers from 18.1% to

22.1% while lower or higher fertilizer rates produced no

significant difference (90). Mosley's data (58) show that

under unirrigated conditions, nitrogen rates above 112 kg/ha

are not economical while under irrigated conditions, up to

twice the above rate or even more is still economical.

Irrigating with nutrient solution has been credited with the

advantage that the crop under such a system is being pro-

vided with its nutritional requirements at the time of need.

Nitrate fertilizer, in such circumstances, would leach

directly into the soil and would be almost immediately

utilized via the roots. Evaluating the nitrate-nitrogen con-

centration of tissues of potatoes grown and fertilized under

irrigation conditions, Carter and Bosma (12) have reported

18

that the nitrate-nitrogen concentration of the tissues were

directly related to the level of applied nitrogen. However,

when more water was applied at each irrigation, nitrate-

nitrogen concentration of the tubers was less than at the

lower level of applied water at each nitrogen rate. The

nitrate-nitrogen concentration was even further reduced by

the highest irrigation water level or by the application of

nitrogen fertilizer in the slowly available form.

The loss of nitrogen is related to the amount of water

percolating through the soil. High rainfall or excess

irrigation early in the season when evapotranspiration

(evaporation from soil plus transpiration from plants) is

minimal can lead to loss of considerable amounts of nitrogen.

However, if much of the nitrogen has been leached below the

top two feet of soil, a side-dress application of nitrogen

applied through the irrigation system may be very beneficial.

Vitosh (91) has found that 125 kg N/ha applied at planting

time was nearly sufficient for maximum production with the

normal irrigation treatment. However, with excess irrigation,

an additional 168 kg/ha of side-dress nitrogen would be

required to obtain the same yield (92) .

19

MATERIALS AND METHODS

The experimental site was the Iowa State University

Horticulture Experimental Station, some ten miles northeast

of Ames, Iowa. The soil type is Clarion-Nicollet loam with

high water-holding capacity of 5 to 10%. A split plot design

replicated four times was used, with treatments randomized

within plots and subplots. The main plot consisted of three

irrigation levels: check (natural conditions), sprinkler

irrigation, and trickle irrigation. The subplots were

three nitrogen fertilizer rates, 67, 135, ~nd 202 kg/ha.

At planting 112 kg/ha of phosphate (P 2o5 ) and 112 kg/ha of

potash (K 20) was broadcast and incorporated into the

experimental plots. The objective was to raise the soil

level of these nutrient elements which tested low in available

phosphorus (28 kg P/ha) and available potassium {162 kg K/ha).

The soil pH was 7.05.

Certified seed of the long season Kennebec potato cultivar

was procurred. Tuber seed pieces were cut to two ounces.

This was achieved by using whole tubers of about this size

and larger tubers cut to two-ounce sizes. Small, whole

tubers saved labor for cutting, were less likely to rot in

the ground, would not lose viability through bleeding and

20

drying, thereby giving better stand establishment. Tubers

large enough to be cut in half were cut lengthwise. Tubers

large enough for three, two-ounce pieces were cut, one piece

off the stem end, while the remaining portion was split

off the stem end, while the remaining portion was split

lengthwise. When it was necessary for a tuber to be cut

into four pieces, the first cut was lengthwise

followed by a crosswise one. This system of cutting was

adopted because block-shaped pieces provide the minimum

area of cut surface of a given weight of a seed piece.

Less cut surface is desirable to avoid soil rot organisms.

Preservation of the seed piece is important because sprout

development is dependent upon it, and the young plant

draws upon it for essential food material until it is a

foot or more tall (94). Each seed piece has to have at

least one eye (or bud) and two eyes was judged to be

convenient, to assure sprouting. The prepared sets were

then dusted with captan, a fungicide, to guard against

rot organisms that could reduce sprouting vigor.

The seeds were planted on April 28, 1976 when the

temperature had warmed up considerably (Table 1) . Each row

was 6.1 m long with seed pieces at 30.5 em apart in the

21

row resulting in about twenty plants per row. Row spacing

was 1.2 m. The depth of planting was about 7.5 em. There

were three rows per plot, but only the middle row was used

for data collection and analysis. Weeds were controlled

by cultivation and a pre-emergence application of Linuron

(Lorox) two weeks after planting (May 11, 1976). Cultivation

was discontinued as soon as the plants came into full bloom

and the rows were closing up. At this time plants were

hilled.

By June 1, 1976, crop establishment was excellent and

two-thirds of the nitrogen rates were side-dressed using

ammonium nitrate (34-0-0) approximately three inches to

the side of each row and two to three inches deep. Before

the stand filled in (June 23), the final one-third nitrogen

application was side-dressed. In June a weekly, general,

spray program of insecticides and fungicides was applied

for the rest of the growing season.

April was a wet month and although May was not as wet

(Table 1) , the soil moisture content was still adequate for

crop growth. By late June (after the full fertilizer

rate had been applied), there was still a scarcity of

22

Table 1. Monthly rainfall and average temperatures during growing season of 1976 a

Temperature (degrees F) Rainfall (inches)

Deviation Deviation from from

1976 Normal 1976 Normal

April 53.7 +4.3 5.66 +2.50

May 59.0 -1.5 2.97 -1.52

June 69.2 -0.3 5.74 -0.03

July 74.7 +1.1 1.10 -2.33

Aug. 71.4 -0.6 0.25 -3.38

Sept. 66.0 +1. 9 o.oo -3.28

a Data reported from weather station, Ames, 8WSW.

rainfall. The plants were attaining full foilage and were

flowering profusely. Tuberization had started. Maximum

evapotranspiration losses had started to occur and would

continue through July to August. Irrigation was therefore

necessary from the end of June through the remainder of

the season.

The trickle irrigation system, controlled by an auto-

matic electric time clock and solenoid valves, was set

to water the field evenly for ninety minutes each day,

starting about noon when the temperature for the day

was nearing its maximum. Irrigating at this time had a

23

cooling effect on the soil. The trickle system afforded

application of water to the row crops through small orifices

or emitters spaced 30.5 em and designed to discharge water at

the rate of 0.38 gallon per minute per 100 feet (or 30.5 m) row.

Water was delivered to the orifices through plastic main and

submains generally laid on soil surface. The rate of discharge

was determined by the size of the orifice and the pressure in

the pipelines. The complete trickle system included a 200-

mesh screen for removing suspended particles from the water.

Under the foregoing conditions, one inch of water was applied

per week per acre. This worked out to be 30,000 gallons of

irrigation water per week per acre of 283,590 l/ha. 1 The

sprinkler irrigation system supplied the same amount

of water per week as the trickle irrigation system,

but was applied once a week. The sprinkler system was

switched on about noon when the temperature for the day was

approaching maximum. Cooling of both the potato plants and

the entire crop land was achieved. This effect could have been

marked if the sprinkler system was switched on daily as was the

case with Peterson and Weigle (68). Irrigating about noon,

contributed to the cool microclimate the potato requires for

optimal performance. Under such situations, the

stomates remainder open for longer

11 gallon/acre = 9.453 liters/hectare.

24

durations of time, thereby probably enhancing photosynthetic

activity and promoting yield.

The plots continued receiving irrigation water at the

prescribed rates until the tubers reached maturity. Irri-

gating beyond this stage was no longer economical. Irriga-

tion was withheld after the first week of September. Tubers

are of better cooking quality and are in better condition

for harvesting if there was a slight moisture deficiency

at harvest.

Leaf samples were taken on July 6, July 20, and August

3, 1976. These sampling dates coincided with 69, 83, and 97

days after emergence. Characteristically, leaf samples were

taken at bloom on July 6. This, together with the biweekly

sampling interval, were consistent with the sampling

procedures of earlier workers {30, 67, 76). The plants were

at about the 8-leaf stage at first sampling time.

The first, whole {compound), mature, fully expanded

leaf, generally judged to be the fourth or fifth

from the shoot apex, was selected at random from

five plants throughout each plot of some twenty plants.

Altogether, five leaf samples from five different plants

25

were taken per plot. The samples were placed in paper bags,

dried at 70°C in a forced air oven for 48 hours, and ground

to pass through a 20-inch mesh screen (sieve} in a Wiley

mill. The total nitrogen concentration of the samples was

then determined by the micro-Kjeldal method.

Harvest took place on September 29, 1976, 154 days

after planting. This period was consistent with other

researches carried out with the long season Kennebec potato

cultivar. At that time the tubers were mature and had

attained their optimum starch content. More than 50% of

the leaves had dried off and the stems had begun to do so.

As the experiment was situated on a loam soil, there was

fear of rain which would hamper harvest due to prolonged

wet soil. After digging they were manually graded into the

conventional U.S.A.'s, U.S.B.'s, and culls by passing them

through screens. Potato tubers designated as U.S.A. are

those that are greater than 1 7/8 inches (4.76 em} in

diameter. The U.S.B. class are those tubers that are less

than 1 7/8 inches (4.76 em} in diameter. The culls were

made up of undersized tubers and larger tubers that were

malformed, had secondary growths and cracks, or that were

highly infected by hollow heart or scab. Sunburn was

not included as one of the diagnostic characters of the

culls.

26

The specific gravity of the tubers

was determined to compare the percentage solids of the

different treatments. Since the determination of solids by

the laboratory method based on the loss of moisture after

drying was not fast enough and had proved a costly exercise,

an alternative way, the specific gravity method, was adopted

to obtain a good estimate of total solids. The procedure

requires the weight of sample in air and then in water.

Hydrometer method uses direct comparison of weights in air

and water. Tubers for specific gravity determination were

representative of all locations in each plot; they were

random samples with respect to size U.S.A., and were strictly

clean (no soil sticking on them) . Such measures were taken

because soil on tubers would tend to raise the specific

gravity reading while hollow heart disease, for example,

would tend to lower it. The potato hydrometer method,

devised by the American Potato Chip Institute, gives a

direct specific gravity reading. Exactly eight pounds of

potato tubers were weighed in air and placed in a basket (of

known weight) suspended below the hydrometer. The assembly

with the potatoes was then placed in a container of water,

deep enough to float it. The specific gravity was then read

directly from the scale on the hydrometer.

27

RESULTS AND DISCUSSION

For all the traits measured, the interaction between

irrigation methods and nitrogen rates was not significant.

This indicates that potatoes would respond to different

nitrogen rates the same way with irrigation water application

by sprinkler or trickle method or without irrigation.

Similarly, potatoes would respond to different irrigation

methods independent of fertilization {Table 2).

Variations in stand count were not significantly

different {Table 3}. Variations in stand count from one plot

to the other were, if anything, minor. Differences in vege-

tative and/or tuber yield traits cannot, therefore, be

attributed to plant population. Neither the irrigation

types nor the nitrogen fertilizer rates influenced

standability at any stage of plant development.

Irrigation

Yield

The analysis of variance for yield traits are consigned

in Table 2. Although the total tuber yield of the different

irrigation treatments was not significantly different the

trickle irrigated plots tended to yield better than the

sprinkler irrigated ones {Fig. 1 and Table 4}.

The total tuber weight was then separated into U.S.A.

and U.S.B. grades, and cull weight {Table 4}. Though not

28

Table 2. Analysis of variance for yield traits (kg/ha) of Kennebec potato grown at the Horticultural Experimental Station, Ames, Iowa, 1976

Source d.f. M.S. a

Irrigation 2 231147636

Blocks 3 33060337

Error A 6 54235869

Nitrogen 2 2413855

I * NR 4 1549741

Error B 18 26760931

a Total tuber weight.

b u.s.A. c u.s.B.

PROB F

0.0704

0.9136

0.9905

d Total marketable weight.

eTotal marketable percent.

f Cull weight.

gSpecific gravity.

*

M.S. b PROB F

22850360 0.5437

42888040.4

25285181.4

6649744.9 0.6378

3896075.2 0.8896

14163257.2

Significant difference at 5% level of probability.

M.S. c PROB F

* 3577171.4 0.0332

7201467.5

563635.6

2324299.2 0.1953

101969.3 0.9851

1303273.7

29

M.S. d PROB F M.S. e PROB F M.S. f PROB F M.S. g PROB F

* 42342982.5 0.3341 207.354 0.1496 78467090.2 0.0423 0,000047 0.0743

20613451.2 88.228 12848014.9 0.000008

31940119.2 78.311 13984422.2 0.000011

1718731.8 0.9019 14.079 0.6559 2461007.9 0.6488 0.000017 0.1685

3094067.1 0.9407 48.043 0.2432 6960276.3 0.3156 0.000004 o. 7199

16641621 32.008 5452744.7 0.000008

Table 3. Analysis of variance for four plant vegetative traits of Kennebec potato grown at three levels of irrigation and three levels of Nitrogen fertil-izer at the Horticultural Experimental Station, Ames, Iowa, 1976

Source d. f. a M.S. PROB F b M.S. PROB F c M.S. PROB F d M.S. PROB F

Irrigation 2 3.0278 0.2398 0.8424 0.0747 0.6516 0.1056 0.2401 0.6159

Blocks 3 4.5185 0.2645 0.7958 0.2450

Error A 6 1.6574 0.2045 0.1950 0.4511

* ** Nitrogen 2 0.1944 0.7622 0.4455 0.0339 0.4431 0.1155 1.7930 0.0014

I * NR 4 1.6111 0.0958 0.0521 0.7546 0.06424 0.8403 0.0400 0.9181

Error B 18 0.6944 0.1094 0.1828 0.1759

aStandability.

bLeaf Nitrogen on July 6.

cLeaf Nitrogen on July 20.

dLeaf Nitrogen on August 3.

* Significant differences at 5% level of probability.

** Significant differences at 1% level of probability.

w 0

Table 4. Planned comparison for six tuber yield traits of Kennebec potato summed across three levels of fertilizer nitrogen at the Horticultural Experi-mental Station in Ames, Iowa, 1976

kg/ha Irrigation

Method Total Wt. U.S.A. U.S.B. Total Cull Wt. Sp. Gr. Mktable Wt.

Check 28564 18538 3318 21828 6731 1. 086

Sprinkler 32270 18843 3989 22832 9438 1.083

Trickle 37308 21066 4399 25465 11843 1.082

* * Check vs. Irrigation n.s. n.s. -- n.s. -- n.s.

Sprinkler vs. Trickle n.s. n.s. n.s. n.s. n.s. n.s.

* Significant difference by t-test at 5% level of probability.

w .......

40,00

30,000

28

20,000

10,000

Check

32

KEY

21828

Total yield

22832

Sprinkler Irrigation Method

308

Trickle

Marketable yield

25465

Figure 1. Tuber yields (total and marketable) of Kennebec potato grown under three irrigation levels at the Horticultural Experimental Station, Ames, Iowa, 1976

40,000

35,000

rd ..c: "' til ~

30,000 .. "0 .--1 Q)

·r-1 ~

25,000

20,000

Check

33

KEY ()-() Total tuber yield fr--..6 Marketable yield <>--0 Cull yield

10,000

8,000

6,000

Sprinkler Trickle Irrigation Method

rd ..c: "' b1 ~

... +l ..c: b1

·r-1 Q) ~

r-1 r-1 ::l u

Figure 2. Effect of different irrigation methods on total yield, marketable yield, and cull weight of potato tubers

34

significantly different, the weight of U.S.A. tubers was

highest under trickle irrigation and least under conditions

of no irrigation.

With respect to U.S.B. tubers, trickle irrigated plots

increased size B tubers 33% over the non-irrigated controls.

All other treatments were not significant.

The yield of culls seemed to follow the same pattern as

the yield of U.S.B. tubers. Trickle irrigation increased

cull weight 76% over the check and 26% over sprinkler

irrigation (Table 4, Fig. 2). Definitely, therefore, it is

not just irrigation but rather trickle irrigation that is

affecting these two tuber yield traits.

Similar to the findings of Krogman and Torfason in

southern Alberta, Canada (44), the yield and quality of

potatoes differed little whether sprinkler or trickle

irrigated. In addition, there were no significant yield

differences between the sprinkler irrigated plots and those

left under rain-fed conditions. It does appear, therefore,

that altering the aerial environment of the plants for a few

hours per week in addition to replenishing soil moisture by

sprinkling was inconsequential.

Even though sprinkler cooling once a week probably

lowered the air temperature and evidently lowered leaf tissue

temperature during the hot weather, yield differences among

treatments means were small, when compared to check and

35

trickle irrigated plots. Although yields under sprinkler

irrigating once a week were neither superior to those

obtained with trickle frequencies and amounts, nor to those

from check plots, the trend, however, preferentially indi-

cates trickle, sprinkler, and check.

Peterson and Weigle (68) working at Muscatine, Iowa

reported that sprinkler cooling applied daily between 11 a.m.

d 4 h th t t b 85oF, an p.m. w enever e empera ure rose a ove

increased yield and quality of potatoes in two years when

temperatures were near or above normal but produced no

increase in one year when temperatures were below normal.

Also, Sanders and Nylund (79) reported that in Minnesota, low

volume sprinkling increased yields over non-irrigated plots

only when temperature stress was rated high. Krogman and

Torfason (44) working in southern Alberta, Canada, found

that high temperature stress on the aerial parts of the plant

was not a limiting factor in potato production as long as

soil moisture was in good supply. Compared with the fore-

going data from the present research indicate that the

temperatures during the growing season were near or above

normal (Table 1) , that there was no incidence of moisture

stress (as observed from the aerial parts of the plants) and

that the soil moisture continued to be in good supply. These

nonwithstanding the yield and quality gains of the sprinkler

irrigated plots were not significantly different from those

36

of the plots left under rain-fed conditions only. This is

probably so because of the high night temperatures in Iowa

compared to those of other potato growing regions.

Large plants, warm summer and wet soil surface contri-

bute to high consumptive use of water. These conditions

occur during the middle of the growing season of potatoes.

If the soil moisture in the root zone is depleted to near

wilting point, malformed tubers may result. Total yield may

decrease, coupled with decrease in yield of No. 1 tubers.

As potatoes approach maturity, lower moisture levels will

not harm yield or grade; however, soil moisture should not

be depleted to the point where tubers may start dehydrating.

More research needs to be conducted to determine the minimum

soil moisture level during the maturity of the potato crop.

Soil moisture level during the final days of the potato

growing season is an important factor to be considered by

potato growers as the amount of water in the root zone at

maturity can affect the yield and quality of the crop.

The water holding capacity of the soil is the water

held in the soil between wilting point and field capacity.

Most agricultural soils have a water-holding capacity

between 0.25 and 2.5 inches per foot of soil and Werner (93)

has shown that potatoes procure more than 65% of their

water requirement from the top foot of soil and about 25%

from the second foot. So, as the potato plant grows, the

37

total amount of water available to it increases as the roots

penetrate deeper into the soil. Also, Painter and Augustin

(66) have shown that yield and grade of tubers did not vary

significantly where supplementary irrigation water was

applied when available soil moisture reached 65, 75, or 85%.

The specific gravity values are shown in Table 4. When

compared, the tubers from the trickle irrigated plots gave

the lowest specific gravity values while those from the check

plots gave the highest. The sprinkler irrigated treatment

yielded tubers with specific gravity values intermediate

between the trickle irrigated and the control plots.

Although these specific gravity differences were not

significant, they however did depict a trend, namely, that

the higher the soil moisture level, the more the tendency

to lower the specific gravity of the tubers produced.

These findings tend to agree with those of Sanders and

co-workers (79) who had established that tubers from

mist irrigated treatment tended to be lower in dry matter

content than those from the non-irrigated controls. It

seems probable that the higher levels of soil moisture

during the season resulted in reduced dry matter content.

Similarly, working on sweet potatoes at Baton Rouge,

Louisiana, Roysell and co-workers (75) had found that as

soil moisture levels increased, there was a significant

38

decrease in percent dry matter content of the sweet potato

roots produced.

However, Krogman and Torfason (44) researching at

Alberta, Canada reported significant differences between the

specific gravity values of tubers from potato plots sub-

jected to conventional irrigation of every seven days or so

during July and August and only when the soil moisture

suction exceeded a certain value, and those irrigated daily

to offset the evapotranspiration of the previous 24 hours.

The different methods of scheduling of irrigation by these

workers is worthy of note. It needs to be stated, too, that

the specific gravity values of potato tubers obtained in

Canada under similar situations as the present study were

relatively higher (1.095) when compared to values from the

present work (Table 4) possibly because of the northernly

location of Canada.

It should also be noted that Peterson and Weigle (68)

have reported dry matter increases when potatoes grown in

sand were mist-irrigated.

39

Leaf Nitrogen

Plant analysis is rapidly becoming a meaningful tool

for better understanding of plant nutrition. Leaves were

sampled and analyzed for their nitrogen content. This was

done with a view to ellucidating whether or not nitrogen

supply was adequate to satisfy crop growth rate. Also as

irrigation was a main treatment in this trial, the possi-

bility of nitrogen leaching loss would not be ruled out. If

nitrogen supply was found to be inadequate, then there would

be time enough to apply supplementary nitrogen to the

potatoes to utilize during the same growing season.

The different irrigation methods did not significantly

affect percent leaf nitrogen at all stages of plant growth

and development (Table 3) . These results were at variance

for the three irrigation treatments and showed no definite

trend or pattern with the different irrigation levels.

However, there was a tendency for percent leaf nitrogen to

increase to a peak at mid-season of crop growth and there-

after to decrease (Table 3, Fig. 3). These findings are

similar to those of Sanders et al. (78) who

stated that leaves from different irrigation treatments

did not differ consistently in their levels of total nitrogen

and other nutrients; that the total nitrogen content of the

leaves declined after attaining a peak, no matter the irriga-

tiP

.. s:: (]) tTl 0 ~ +J ·r-1 z 4-l liS (])

....:1

5.5

5.0

4.5

4.0

0 July 6

40

July 20

Sampling Date Aug. 3

Sprinkler

Check

Trickle

Figure 3. Effect of irrigation method on leaf nitrogen content taken at different stages of development of Kennebec potato plants

41

tion method and that this event occurred as the plants

matured.

Nitrogen Rate

The analysis of variance for the tuber traits as they

were affected by nitrogen fertilization is shown in Table 2.

Kennebec potatoes did not respond to nitrogen fertilizer

application (Fig. 4). However, the studies indicate a trend

toward increased yields with increasing nitrogen rates up to

202 kg N/ha. These data are consistent with findings of

Mosley (58) who worked on a sandy loam soil in Ohio but

under non-irrigated conditions. It does appear therefore

that the clarion-nicolet loam soil on which the present study

was carried out was inherently rich enough in nitrogen to

the extent of being able to support a crop of potatoes

without recourse to supplementary heavy nitrogen application.

Maximum production of U.S.A. tubers and total

marketable tubers occurred when ammonium nitrate was applied

at 135 kg N/ha (Table 5). At this rate of fertilization

also, the cull weight was the least. Although the highest

nitrogen level, 202 kg N/ha yielded the greatest amount of

U.S.B. tubers, the second nitrogen rate (135 kg N/ha) would

still be preferred since with that rate of application the

cull weight was kept down relative to total marketable

weight. The total marketable percentage was also highest by

40,000

30,000

20,00

10,00

67

42

79 32539

23690

135

Total yield

33223

203

Nitrogen Rate (kg/ha)

KEY

Marketable yield

Figure 4. Tuber yields (total and marketable) of Kennebec potato

Table 5. Effects of Different Nitrogen Levels on Yield, Grade, Tuber Size, and Sp. Gr. of Kennebec Potatoes, Ames, Iowa, 1976

Total N-Rate Total Mktable kg/ha Total Wt. U.S.IA U.S.IB Mktable Wt. Percentage Culls Sp.Gr.

kg/ha

67 32379 18875 4084 22955 71 9419 1. 083

135 32539 20312 3400 23690 73 8849 1.084

202 33223 19259 4221 23480 71 9743 1. 082

LSD.05 n.s. n.s. n.s. n.s. n.s.

~

w

44

fertilizing at 135 kg N/ha. It may, therefore, neither be

pertinent nor economical to apply nitrogen fertilizer under

the conditions of this experiment or similar, at any rates

greater than 135 kg N/ha. This figure is close enough to

the 112 kg N/ha obtained under non-irrigated conditions by

Mosley (58) and 180 kg N/ha found by Carter and Bosma (12),

thereby making allowance for losses that may occur under

irrigated conditions--leaching, denitrification, etc.

Hence, the level of nitrogen required to produce a good crop

of potatoes is usually applied, adding a little extra for

possible losses that might occur. Also, the Idaho Fertilizer

Guide recommends a nitrogen rate of 135 kg N/ha (120

lbs/acre) for potato production.

The rate of nitrogen to be used may at times depend on

the grade of potatoes desired by the grower. If the grower

has a good market for very large (U.S.A.) tubers it seems

as if he has to strike a comfortable mean between the two

higher rates. If, however, he desires only U.S.B., he

undoubtedly has to go in for the 202 kg N/ha rate and/or

reduce the spacing between plants.

The data showing the response of Kennebec potatoes to

different nitrogen treatments do show that fertilizing with

nitrogen for maximum yield may result in production of low

grade tubers with undesirable characteristics for the fresh

market or processing.

45

Increased nitrogen rates did not show any consistent

pattern with respect to tuber specific gravity. The varia-

tions were, in fact, not significant (Table 5) although a

slight decline appears to be evident at 202 kg N/ha. Earlier

workers, Carter and Bosma (12), Mazur and Rzasa (49) and

Painter and Augustin (66) have however, established that

increased nitrogen rates decreased tuber specific gravity.

It is to be noted that these foregoing workers not only

sited their experiments on a different soil type (silt loam)

from the present studies, but also applied much higher rates

of nitrogen fertilizer. In addition, Toshkhodzhaev (89) has

come up with a definite rate of 150 kg N/ha increasing tuber

dry matter content from 18.1% to 22.1%. It is to be noted

that from the data obtained from the present study (Table 5)

tubers obtained from nitrogen fertilizer rate of 135 kg N/ha

(which is close enough to 150 kg N/ha) gave a higher specific

gravity value when compared to a lower or higher nitrogen rate.

Working on chernozenic soil (Cavendish sandy loam) in

Canada and without irrigation, Dubetz and Bole (19) had

shown that in their study specific gravity was not influenced

by the relatively high levels of nitrogen and phosphorus.

Nitrogen fertilization has some influence on percent

leaf nitrogen. Leaf nitrogen content reflects the amount of

applied nitrogen. Higher nitrogen rates increased percent

leaf nitrogen. These findings are consistent with the

46

results of earlier workers, Gardner and Jones (30), Mosley

(58), Painter and Augustin (66), and Soltanpour (83).

However, these influences appear to be limited to the early

and late stages of plant development (Fig. 5, Table 3).

Percent leaf nitrogen was relatively low early in the

season, recorded a peak at about mid-season, and decreased

rapidly as the season progressed and the plants matured

(Table 3) . From early to mid-season, potato builds up the

nitrogen contents of its leaves only to translocate these

later to enhance the development of the tuber initiates. The

same fertilizer rate (135 kg N/ha) gave the highest total

marketable tuber weight (Table 5) and the greatest leaf

nitrogen content. It does appear, therefore, that applying

nitrogen fertilizer at any rate greater than 135 kg N/ha,

under the conditions of the present study should not be

encouraged. Also, with the lowest level of applied nitrogen,

percent leaf nitrogen content declined rapidly over the

season. This result is similar to the findings of Gardner

and Jones (30).

The application of 135 kg N/ha resulted in what may be

regarded as adequate percent leaf nitrogen throughout the

season, since a higher rate of nitrogen did not increase

leaf nitrogen. However, Gardner and Jones (30) working on

a similar subject at Idaho arrived at a figure of

202 kg N/ha. This higher figure could be probably

47

attributed to Idaho soils being more deficient in nitrogen

than Ames soils or more nitrogen being lost on account of

poor management, denitrification, leaching, etc.

Percent leaf nitrogen at early season through bloom

(peak vegetative state) contributes to correlating leaf

sampling dates and total yield. Hence, early and mid-season

samplings are considered more accurate for predicting the

nitrogen status of the potato plant (30, 58). Not only are

they more reliable for predicting yield response to nitrogen

fertilization, but they will often allow time for applying

additional nitrogen fertilizer for use by the current crop,

if the need is detected at these early sampling dates. If

the total nitrogen content falls below a sufficiency level,

an application of nitrogen is needed in order to avoid reduc-

tions in yield. With care and experience, percent leaf

nitrogen can be used as a guide to more efficient use of

nitrogen fertilizers in potato production.

In growing potatoes for chip manufacture, emphasis

should be placed on cultural practices that produce tubers

of maximum solid content. High solid content is especially

important in growing potatoes for chips because the yield of

chips is in direct proportion to their solids content.

Also, chips made from potatoes of high solids content absorb

less frying oil than those made from potatoes of low solids

content. Tubers with high sugar content in place of starch

5.5

OP 5.0 ..

s:: Q) tJl 0 l-1 +J ·r-1 z 4-l 4.5 cO Q)

....:!

4.0

0 July 6

48

July 20 Sampling Date

Aug. 3

202 kg N/ha

135 kg N/ha

67 kg N/ha

Figure 5. Effect of Nitrogen rates on leaf nitrogen content taken at different stages of development of Kennebec potato plants.

49

are indicative of physiological immaturity and such tubers

produce dark-colored chips. Light-colored chips are much

more preferred by consumers. Potato growers can similarly

have a range of purposes for their potatoes; these include

baking, mashing, cooking, or boiling. Some are even much

more concerned with the production of certified seeds.

Depending on the grower's particular purpose, he will be able

to draw from the array of established cultural practices to

suit his needs.

Looking closely at the need for irrigating potatoes

from the crop standpoint, one would easily appreciate that

potato, Solanum tuberosum L. is a herbaceous dicotyledonous

annual, though sometimes regarded as a potential perennial

because of its ability to reproduce vegetatively by means of

tubers. With its above-ground stem herbaceous and erect in

the early stages of development and later becoming spreading

and prostrate or semi-prostrate, the potato canopy contri-

butes to effective ground cover so that transpiration and

not evaporation is more of the case in point. Also, the

herbaceous succulent and fleshy nature of this crop aids in

water conservation. Furthermore, observations during the

course of the experiment showed that the potato plants never

exhibited any signs of wilting whether temporary or

permanent, the relative high temperatures and the intense

sunshine notwithstanding. This goes to show that the crops

50

did not suffer from any moisture stress. The foregoing

observations were true of all the irrigation treatments. It

might be worthwhile, however, to add that mist or dew served

as a source of moisture to the crop plants and that these

were not measured. It would appear also that the plants

were able to procure enough moisture from the early season

rainfall to satisfy the yield levels obtained from the

Clarion-Nicolet loam soil on which the experiment was sited.

A close look at the nature of the soil on which the

experiment was carried out might perhaps be able to

ellucidate some other factors concerning irrigation and

fertilization of potatoes. The principal soil series was

Clarion-Nicolet resulting in a loam texture. Clarion is

internally well-drained and Nicolet somewhat poorly. The

experimental plots were able to retain a high percentage of

moisture from the early season rainfall so much so that the

effect of irrigation was not very pronounced. Poor drainage

also raised problems of nitrogen losses by denitrification.

Irrigation of potatoes could be encouraged on sandy

loam soils. It may not be that economical to irrigate

potatoes grown on heavy soils with high clay content.

However, the present work indicates that if need be at all,

potatoes grown on heavy soils should be trickle and not

sprinkler irrigated. In any event, one has to weigh the

economics of applying such an expensive but efficient system

51

to such a low value crop as the potato. It does, however,

not stand to be questioned that trickle irrigation has been

profitably employed to other high value vegetable crops,

fruit trees, and sugar cane.

Well-drained sandy loam soils produce crops with higher

solids tha~ heavy clay or poorly drained soils. Total solids

are higher in potatoes grown on mineral soils than on ~uck or

peat soils. Soils of low fertility (as a result of no

fertilization or low rates of fertilization) often yield

small crops, though their quality may be high. However,

current agricultural competition does not permit growers

with low yield to remain in business.

52

SUMMARY AND CONCLUSION

Irrigation could be a crucial input to potato production.

Since water is crucial to the physiological processes of crops

and considering that plant life evolved in an aqueous medium,

this is not surprising. Irrigation is an important cultural

practice and can be controlled by the potato grower.

Knowledge of good irrigation practices is important as soil

moisture levels can affect yield, grade, and resistance of

tubers to bruising and certain diseases. It is important

that the potato grower be aware of the relationship of water

to his crop and soil, so as to obtain the best economic

return. Although yield and water use efficiency of trickle

irrigated potatoes were not significantly greater than those

of the check and sprinkler irrigated plots probably because

of the soil type on which the trial was conducted, similar

trials conducted on sandy and sandy loam soils have shown

these differences to be significant. Further work needs to

be done to determine the most efficient method of water

application for various types of soil in order to gain most

from irrigation.

With increased concern over pollution and our environ-

ment, it becomes necessary to control the amount of nitrogen

fertilizer application to agricultural crops, as nitrogen is

the nutrient that is most suspect in contributing to

pollution. This is so because the increased use of nitrogen

53

in the past few years, and in most cases in amounts greatly

in excess of crop needs as one strives to maximize yields.

Secondly, nitrogen is so highly mobile in the soil, moving

through the profile in nitrate form alongside with water

and accumulating and concentrating between the top 12 to 18

inches of soil by the end of the growing season.

Kennebec potatoes grown in a loamy soil in Ames

requires 135 kg N/ha to produce a desirable yield of tubers.

However, if the grower prefers U.S.B. tubers at the

expense of U.S.A. and total marketable tubers, then

applying 202 kg N/ha would be appropriate. The amounts of

nitrogen in excess of the 135 kg N/ha adequate rate or

actual needs of the potato did not contribute to increased

yield or quality of potato tubers, If anything, they could

be detrimental by contributing to pollution hazard and/or

slightly lowering tuber specific gravity.

Relatively low yield levels were obtained from this

study. This is probably not unusual for Iowa where the night

temperatures are relatively high. If the trial were sited

where the night temperatures were cooler, more nitrogen and

more soil moisture would have been utilized by the potatoes

and yield levels would have been relatively high.

In Iowa, potatoes are raised in Muscatine for early

summer harvest, so as to earn high market values. To this

end, only early-maturing or short-season cultivars (example

54

Norland, Superior) are grown. Muscatine soil is typically

sandy. It not only warms up faster than other soil types

in spring but also is an ideal soil for potato culture.

These situations make high yield expectations of growers to

come true. On the average the grower of long season Kennebec

in Iowa is not reaping much profit unless his cropland is in

northern Iowa on peat or muck land.

As this study was carried out for only one season, a

repeat of it is desirahle in order to confirm the results so

far obtained. In doing so, it rnay be necessary to

incorporate both early- and late-maturing cultivars.

Since potato is the "bread of the poor" and the demand

for it is elastic, it would be worthwhile to carry out a

number of studies with both short and long season cultivars

on different soil types, utilizing various irrigation methods

to establish the best culture for the potato crop under a

given set of conditions.

55

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64

ACKNOWLEDGEMENTS

The author wishes to express sincere gratitude to Dr.

H. G. Taber for his guidance throughout this work and in

the preparation of this manuscript.

Appreciation is expressed to Dr. D. F. Cox (for his

assistance with the statistical work} and to the other

members of the committee.

Special thanks is expressed to J. Osi Mozie esq., for

his encouragement during the college career of the author

and to Hashem M. Soliman for his encouragement during this

study.

Finally, I wish to express thanks to my wife, Ngozi,

for her enduring courage, understanding, and moral support

during this study.