Efficiency of split nitrogen fertilization with …mtt.fi/afs/pdf/afsf11_59.pdf59 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 11 (2002): 59–74. Efficiency of split nitrogen fertilization

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A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D

Vol. 11 (2002): 59–74.

Efficiency of split nitrogen fertilization with adjustedirrigation on potato

Paavo KuismaPotato Research Institute, Ruosuontie 156, FIN-16900 Lammi, Finland, e-mail: [email protected]

The split application of fertilizer nitrogen and adjusted irrigation were studied at Potato ResearchInstitute in Lammi in 1993–1995. Independent of year, variety or irrigation, the split application ofnitrogen had no benefit on yield or yield quality compared to a single dose at planting on the levels ofrecommended nitrogen fertilization. The highest yields were received with the highest dose 110 kgha–1 N, given all at planting, followed by 80, 50 + 30 + 30, 50 + 30 and 50 kg ha–1 N. The yieldswithout fertilizer nitrogen averaged 24.8 t ha–1. The adjusted irrigation increased tuber yields 17%.The utilization of fertilizer and organic soil nitrogen was good on potato crop if water supply wastaken care of. Some values of apparent fertilizer nitrogen recovery exceeded 1.0 thus suggesting thatthe moderate fertilization improved the ability of potato crop to utilize the natural nitrogen sources insoils.

Key words: irrigation, nitrogen, potatoes, Solanum tuberosum, fertilizers

© Agricultural and Food Science in FinlandManuscript received May 2001

Introduction

A sufficient level of soil nitrogen is needed foran abundant yield growth during the tuber bulk-ing (Burton 1989). However, high amounts offertilizer nitrogen in planting may cause tooabundant growth of the foliage in potato cropduring early phases of development, and conse-quently delay tuber initiation and senescence ofleaves (Harris 1978, Beukema and van der Zaag1979, Allen and Scott 1980, Linnér 1988). Ex-cessive nitrogen availability during the entiregrowing season, particularly near the end of tu-

ber bulking, delays tuber maturity (Ojala et al.1990).

The length of the growing season in Finnishpotato production areas is only 140–180 days,and delayed tuber formation and late senescenceare a risk for the yield and its quality. Thereforethe application of fertilizer nitrogen must alwaysbe in a balance between potential yield and cropsenescence (Varis 1972, Mustonen 1997). Highamount of fertilizer nitrogen applied as a singledose at planting is an environmental risk. Thenitrate nitrogen easily leaches into ground wa-ter in case of high rainfall or excessive irriga-tion on light, sandy soils (Harris 1978, Burton

mailto:[email protected]

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Kuisma, P. Split nitrogen fertilization with irrigation on potato

1989, Goffart and Guiot 1996) which are typi-cal in potato production in Finland.

The splitting of fertilization is suggested toavoid the unfavourable effects of excessive ni-trogen (Harris 1978, Burton 1989). In a splitapplication, only a part of the total fertilizer ni-trogen is given at planting, and the rest is ap-plied later in one or several doses during thegrowing period.

Many studies in Western Europe have report-ed significant yield increase and better qualityof potatoes owing to split nitrogen dressing (e.g.van Loon et al. 1987, Nitsch and Varis 1991),especially if early summers are rainy and risksto leaching are high (Linnér 1988, Goffart andGuiot 1996). Svensson et al. (1973) state thatthe benefits of nitrogen splitting are most obvi-ous under the conditions where the prerequisitesto growth are good and high yields are expect-ed. The profits of split application however, areminimal if leaching is no risk (Harris 1978).

With a split application the availability ofnitrogen in soil can be better matched to the cropuptake of nitrogen throughout the season. In thisway, late application could be adjusted to takeaccount of reductions in nitrogen requirement asa result of drought, frost, etc. Also higher ratesof mineralization than expected can be dealtwithout raising the amount of residual nitrogenafter harvest (Vos and Marshall 1993). Accord-ing to Beukema and van der Zaag (1979) in thesplit fertilization the additional nitrogen shouldbe applied not later than three weeks after theemergence.

The aim of this study was to test the effect ofsplit application of nitrogen with adjusted irri-

gation on potato yield in Finnish conditionswhere the recommendations of nitrogen fertili-zation are rather low, 40–100 kg ha–1. A partialaim was also to study if split fertilization en-sured a high yield on potato in the case of rapidearly development.

Material and methods

In the three year field experiment at Potato Re-search Institute in Lammi (61°06' N, 23°02' E,97 m above sea level) the irrigation and the splitapplication of fertilizer nitrogen on potato werestudied in 1993–1995. In 1993–1994, the studywas done with two cultivars: Fambo, early tablepotato, and Tanu, early starch potato. In 1995,only a middle late crisp variety Lady Rosetta wasused. The trials were located on sandy soils (Ta-ble 1) typical for potato production in Finland.

Irrigation and nitrogen fertilization were ar-ranged in a split-plot design with three complete-ly randomised blocks. In 1993–1994 the varie-ties located on two neighbouring trials. Irriga-tion was placed in the main plots:

A0 = unirrigatedA1 = irrigated

The irrigation was applied with nozzle boomwhen the soil moisture in top soil decreased to50% of plant available water capacity (Elonenet al. 1967), determined by the gypsum blocktechniques (Boyoucos 1950). Gypsum blockswere installed at the depth of 30 cm to the mid-

Table 1. Soil properties (0–30 cm) on trial fields.

Year Soil type pH Ca P K Mg Soilinorganic N

mg l–1 kg ha–1

at planting

1993 coarse sandy loam 5.9 1050 12.0 182 0831994 fine sandy loam 6.1 1210 07.3 185 111 321995 fine sandy loam 5.2 0888 13.0 162 063 25

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dle of bed on plots of nitrogen fertilization 80kg ha–1. Dates and amount of irrigation are pre-sented in Table 2.

The nitrogen fertilization in subplots wasdone with six combinations:

B0 = no nitrogenB1 = single dose 50 kg ha–1 N at plantingB2 = single dose 80 kg ha–1 N at plantingB3 = single dose 110 kg ha–1 N at plantingB4 = split fertilization; 50 kg ha–1 N at planting,

30 kg ha–1 N at hillingB5 = split fertilization; 50 kg ha–1 N at planting,

30 kg ha–1 N at hilling, 30 kg ha–1 N beforeblooming

At hilling, about 40 days after planting, thepotato crop reached the developmental stage of33–35 (the end of leaf expansion) (Hack et al.1993). Second application was about 20 dayslater, just before blooming. In both applicationsfertilizers were given as top dressings.

The base nitrogen 50 kg ha–1 N on split ferti-lization and all rates of single fertilization wereplaced in two rows about 5 cm beneath and 10 cmaside of seed tubers at planting with ammoniumnitrate limestone (Oulunsalpietari, Kemira AgroOy, Finland, N 27.5%). Split applications afterplanting were top dressed with ammonium ni-trate limestone in 1993 and with calcium nitrate(Kemira Agro Oy, Finland, N 15.5%) in 1994–1995. Phosphorus and potassium were broadcastbefore planting as superphosphate (P 9%) andpotassium sulphate (K 42%), respectively, ac-cording to information of soil analysis and re-quirements of varieties.

At planting an individual plot consisted offour rows each 10.0 m long and 80 cm apart.Potato yields were quantified by harvesting twomiddle rows of 8.0 m length from each plot. Atrial field was ploughed to the depth of 25 cm inthe previous autumn after the harvest of springbarley. A day before planting, the seed bed wastilled twice with a spring-tined harrow to thedepth of 17 cm. Seed presprouted for three weeksin light, was planted to the depth of 5 cm with asemiautomatic two-row planter on 12 May, 18May and 5 June in 1993, 1994 and 1995, respec-tively. The planting distance was 31 cm on bothvarieties in 1993. In 1994 Fambo was planted to31 cm and Tanu to 26 cm. In 1995 Lady Rosettawas planted to 28 cm. The size of certified seed(class A) was 30–45 mm, 35–50 mm and 30–40mm in 1993, 1994 and 1995, respectively. Trialswere earthed about 40 days after planting with atwo-row mouldboard ridger. Plant protection wasdone according to the local agricultural practice.Trials were harvested with a tractor carried one-row harvester on 28 August, 6–7 September and13 September in 1993, 1994 and 1995, respec-tively.

Crop analysis during growing season includ-ed observations of emergence time, occurrenceof crop diseases and determinations of growthstage on 45, 75 and 90 days after planting (DAP)using a modification of the growth stage identi-fication keys described by Hack et al. (1993).After the harvest, yields were graded into tubersizes on 10 mm differences which were summa-rized as size distribution 30–40 mm, 40–70 mm,and > 70 mm. Before grading a sample of about

Table 2. Dates and rates of irrigation.

1993 1994 1995Date mm Date mm Date mm

10 June 19 12–13 July 25 30 June 1516 June 15 15–16 July 30 18 July 1030 June 16 19 July 12 02 August 2209 July 22 21–22 July 2613 July 20 29 July 17

Total 92 1100 47

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5 kg tubers per plot was taken for specific grav-ity determinations and for analysis of externalquality. Specific gravity was converted to starchcontent according to the table used on officialvariety trials (Kangas 1998). After the determi-nation of specific gravity samples, treatmentswere joined over replicates for analysis of ex-ternal quality and in 1994–1995 also for analy-sis of total nitrogen content in tubers. Thus thedifferences between means of these analyseswere not studied statistically.

Classification of tuber defects described byKangas (1998) was used in analysis of externalquality. Marketable yield on cultivars Fambo andLady Rosetta was determined as a yield of 40–70 mm sized healthy tubers. The total nitrogencontent of tubers at final harvest was analysedin a commercial laboratory (Novalab Oy) withthe Kjeldahl method, the sample being 20 medi-um size tubers. The nitrate nitrogen content wasanalysed from 20 middle size tubers at PotatoResearch Institute using ion specific electrodesaccording to VTT Method 4207-84 describeddetailed by Ruippo and Alikärri (1989).

In 1994–1995 the soil inorganic nitrogen(NO3

–-N and NH4+-N) before planting, weekly

during the growing period between 40–75 DAPand after harvest were determined using the “soilbaggage”, developed by Kemira Agro Oy (1994).Before the planting one sample consisted oftwenty sub samples from the depth of 0–30 cm,

taken randomly on the trial field before the till-age. After the harvest, samples consisted of fivesub samples on every plot from the depth of 0–30 cm. At 40, 47, 54, 61, 68 and 75 DAP thesamples were collected as two whole bed pro-files on every plot.

Weather in May–September (Table 3) wasmeasured at Potato Research Institute. The longterm averages were taken from the Finnish Me-teorological Institute’s weather station at Hel-sinki University, Lammi Biological Station, lo-cated 4 km from the experiments.

The data were collected and calculated on aMS®Excel-Worksheet. The results were furtheranalyzed with a SPSS 10.01 -statistical packageusing the analysis of variance by split-plot de-sign on GLM-procedure where years, and varie-ties in trials 1993–1994 were analyzed as fixedfactors in series of experiments:

Yijklm = µ + Tj + Vk + TVjk + eij(1) + Il + TIjl + VIkl

+ TVIjkl + eijkl(2) + Nm + TNjm + VNkm

+ TVNjkm + INlm + TINjlm + VINklm

+ TVINjklm + eijklm(3)

where Yijklm is the response for the block i, trialyear j, variety k, irrigation l and nitrogen fertili-zation m; µ is the overall mean; B is the randomblock effect; T, V, I and N are the fixed effectsof trial year, variety, irrigation and nitrogen fer-tilization; TV,…,TVIN interactions of fixed ef-fects; and the random error terms eijk(1) =

Table 3. Weather conditions at Potato Research Institute in 1993–1995 and 30-year averages at Helsinki University, LammiBiological Research Station.

Mean temperature (°C) Precipitation (mm)

Month 1993 1994 1995 1961–1990 1993 1994 1995 1961–1990

May 12.6 07.6 07.2 08.8 003 041 098 038June 11.0 12.2 15.6 14.0 058 060 028 047July 14.8 19.0 13.5 16.7 118 004 056 074August 12.0 14.3 13.3 14.8 129 070 063 087September 04.8 09.4 07.8 09.7 032 105 043 068

Mean/Sum 11.1 12.5 11.5 12.8 340 280 288 314DD °C1) .967 1160. .914

1) DD °C = effective temperature sum (> + 5°C)

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T(V(B)), e ijkl(2) = T(V(IB)) and e ijklm

(3) =T(V(I(NB))) (Snedecor 1956).

In 1995 the analysis of variance was basedon the following mixed model for split-plot de-sign:

Yilm = µ + Bi + Il + eil(1) + Nm + INlm + eilm

(2)

where Yilm is the response for the block i, irriga-tion l and nitrogen fertilization m; µ is the over-all mean; B is the random block effect; I and Nare the fixed effects of irrigation and nitrogenfertilization; IN is the two-factor interaction offixed effects; and the random error terms eil

(1) =BI and eilm

(2) = I(NB). Summaries of varianceanalysis are presented in Tables 4 and 5. The sta-

tistical differences (P < 0.05) between groupmeans were further studied by Tukey’s HSD(Honestly Significant Difference) test.

Results and discussion

Crop developmentOn the emergence, the greatest differences werebetween years and varieties, as expected. May1993 was exceptionally warm, and potatoesplanted in the middle of May emerged in two

Table 4. Summaries of variance analysis on varieties Fambo and Tanu in 1993–1994.

Source of Emergence DAP45 DAP75 DAP90 Tuber Starch-% Starch < 40 mm 40–70 mm > 70 mmvariation yield yield

Year (Y) *** *** *** *** * Ns *** *** *** ***Variety (V) * Ns Ns *** *** *** Ns *** *** NsYxV * * * Ns Ns ° Ns *** *** *Irrigation (I) * Ns ° * *** ° *** *** *** NsYxI * * ° *** *** ° *** *** *** NsVxI Ns Ns ** * Ns Ns Ns ** *** °YxVxI ° Ns Ns * Ns Ns Ns ** ** NsNitrogen (N) *** *** *** *** *** *** *** *** *** ***YxN *** *** * Ns *** * *** ** *** NsVxN Ns * Ns ** ° * ** ** Ns NsIxN * Ns Ns * Ns *** Ns * * NsYxVxN * Ns Ns Ns ** Ns * *** ** *YxIxN Ns Ns Ns Ns *** Ns ** * ** NsVxIxN Ns Ns Ns Ns Ns Ns Ns * ** NsYxVxIxN ° Ns Ns Ns Ns Ns Ns * Ns Ns

DAP = days after planting; Ns = no significant difference; ° = 0.1 < P < 0.05; * = 0.05 < P < 0.01; ** = 0.01 < P < 0.001;*** = P < 0.001

Table 5. Summaries of variance analysis on variety Lady Rosetta in 1995.

Source of Emergence DAP45 DAP75 DAP90 Tuber Starch-% Starch < 40 mm 40–70 mm > 70 mmvariation yield yield

Irrigation (I) Ns Ns Ns Ns Ns Ns Ns Ns Ns **Nitrogen (N) Ns Ns Ns * *** *** *** Ns ° NsIxN Ns Ns Ns Ns Ns Ns Ns Ns Ns Ns

DAP = days after planting; Ns = no significant difference; ° = 0.1 < P < 0.05; * = 0.05 < P < 0.01; ** = 0.01 < P < 0.001;*** = P < 0.0

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and a half weeks. May 1994 was cooler than theaverage, and the emergence required about onemonth. In 1995 the trial was planted at the be-ginning of June, and variety Lady Rosettaemerged about five days faster than Fambo andTanu in 1993. The development of crop followedthe order of emergence in the early growth (45DAP). In 1995, the faster growth of Lady Ro-setta continued to the harvest (Table 6).

In 1993 the main plots of irrigation emergedabout 0.9 days faster than unirrigated plots. Thedifference was statistically significant (P < 0.05)although it had no practical meaning. Nitrogenfertilisation, interacted with year (P < 0.001),irrigation (P < 0.05) and year x variety (P < 0.05),also had highly significant influence (P < 0.001)on the emergence, the influence, however, hav-ing again no practical meaning.

Table 6. Crop development in 1993–1994.

Year Variety Fertilization 45 DAP 75 DAP 90 DAPkg /ha N

U I U I U I

1993 Fambo 0 36a 36a 77a 75abc 81a 83ab

50 36a 36a 81a 81a 83a 84a

80 37a 36a 78a 78ab 82a 84a

110 36a 36a 78a 71c 82a 81b

50 + 30 35a 36a 81a 81a 83a 84a

50 + 30+ 30 36a 35a 81a 74bc 82a 82ab

Mean 36A 36A 79A 77B 82A 83A

Tanu 0 34a 34a 74a 74ab 78b 78c

50 33a 34a 74a 78a 82a 84a

80 34a 35a 71a 78a 81a 83ab

110 35a 36a 71a 71b 81a 81b

50 + 30 34a 34a 74a 78a 82a 83ab

50 + 30+ 30 33a 35a 74a 77ab 81a 81b

Mean 34A 35A 73B 76A 81A 82A

1994 Fambo 0 25c 26b 72a 71a 82a 81a

50 30a 29a 74a 72a 84a 81a

80 27bc 27ab 73a 72a 82a 81a

110 28ab 28ab 72a 72a 82a 81a

50 + 30 28ab 29a 72a 72a 84a 82a

50 + 30+ 30 30a 29a 73a 72a 83a 81a

Mean 28A 28A 73A 72A 82A 81A

Tanu 0 26b 25c 74a 72a 82a 77bc

50 30a 30ab 76a 73a 84a 81a

80 30a 28b 75a 72a 82a 77bc

110 30a 30ab 75a 71a 82a 74d

50 + 30 30a 31a 75a 72a 84a 79ab

50 + 30+ 30 31a 31a 75a 72a 83a 75cd

Mean 30A 29A 75A 72B 83A 77B

DAP = days after planting; U = unirrigated; I = irrigatedWithin a single column group means followed by the same letter (with capital letters between irrigation treatments in eachDAP) are not statistically different at level P < 0.05 in Tukey’s HSD-test.

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The effect of irrigation on the developmentof the canopy was seen in 1994 when the irriga-tion significantly (P < 0.001) delayed the growthfrom 75 DAP, especially on cultivar Tanu. In1994 the last irrigation was given near the endof July. Ojala et al. (1990) reported that irriga-tion can slightly delay the maturity of potato cropif applied abundantly in the latter part of thegrowing season.

The effects of nitrogen fertilization variedduring the growing period and were dependenton year and variety. On Lady Rosetta in 1995,the only significant difference (P < 0.05), al-though practically meaningless, was detected at90 DAP on the development of potato canopydue to nitrogen fertilization. On cultivars Fam-bo and Tanu in 1993–1994 at early growth (45DAP), when the only differences between nitro-gen fertilization were from planting and no irri-gation was yet given, unfertilized potatoes grewsignificantly (P < 0.001) slower, other nitrogenrates being similar. The slow development ofunfertilized potato continued to the harvest. Theunfavourable effect of high nitrogen rate in plant-ing and the late split application came out dur-ing the summer, and was most visible at 75 DAPin 1993. Similar results were published by e.g.Harris (1978). The interaction between irriga-tion and nitrogen fertilization on the develop-ment of potato canopy was weak as Harris (1990)reported.

YieldThe results of this study showed that the splitapplication of nitrogen compared to a single doseat planting did not give any additional advan-tages on yield, size distribution, starch contentor external quality in Finnish conditions whenmoderate nitrogen rates were used (Figs. 1 and2). The highest yields were produced when thehighest rate 110 kg ha–1 N was given at planting.The same amount of nitrogen split into threeapplications resulted to about the same yield as80 kg ha–1 N in a single dose, and 80 kg ha–1 Nsplit into two applications gave the yield com-

parable to a single dose of 50 kg ha–1 N. Howev-er, the yield decrease due to split fertilizationwas clearly smaller than noticed in earlier stud-ies in 1980s at Potato Research Institute (Peru-nantutkimuslaitos 1985). In Michigan, USA,Joern and Vitosh (1995a) made the same con-clusion when studying the similar levels of ni-trogen fertilization for Russet Burbank potatoesunder the similar length of growing season as inFinland. Neither in conditions of Alaska, did thesplit fertilization influence the potato yield(Gavlak et al. 1993). Similarly e.g. Svensson etal. (1973), Carlsson (1977, 1988, 1995) andGoffart and Guiot (1996) report that the split ap-plication of fertilizer nitrogen does not differfrom the fertilization where all nitrogen is givenat planting when moderate rates of nitrogen areused. In Middle Sweden, Carlsson (1995) noticedyield decrease even up to 30% due to split ap-plication of nitrogen fertilization compared to aplacement of all fertilizers at planting.

Van Loon et al. (1987) in Holland noticed atthe end of eighties that a split application wasadvantageous only when the total rates of ferti-lizer nitrogen were high (> 200 kg ha–1 N).Sieczka et al. (1993) reported that there was noresponse to supplemental nitrogen during thegrowing period when ≥ 168 kg ha–1 N was givenat planting. MacLean (1984) in Atlantic Canadaalso noticed that no yield increase was reachedif the nitrogen rate exceeded 135 kg ha–1, andthe split application didn’t change the situation.In this study the highest rate of fertilizer nitro-gen was clearly smaller, only 110 kg ha–1 N, thanin the studies mentioned.

The irrigation affected significantly (P <0.001) the tuber yield in 1993–1994 on cultivarsFambo and Tanu (Table 4). The influence was,however, dependent on the year. In 1993, Julyand August were rainy and cool, but in 1994 Julywas extremely dry and hot. Consequently allyield increase due to irrigation was acquired in1994 when the increase of tuber yield on irrigat-ed plots was 42% (Table 7). In 1995, June andJuly were slightly drier, but clearly cooler thannormally and irrigation had no effect on the tu-ber yield of variety Lady Rosetta. The signifi-

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Fig. 1. Effects of irrigation and nitrogen fertilization on yields and starch content of varieties Fambo in1993–1994 and Lady Rosetta in 1995.

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Fig. 2. Effects of irrigation and nitrogen fertilization on starch production of variety Tanu in 1993–1994.

cant interaction of irrigation and nitrogen ferti-lization was seen in the way that differences be-tween fertilized plots were smaller compared tothe unirrigated crop (Figs. 1 and 2). Especiallyyields on split fertilization approached yieldsproduced with the same amount of nitrogen asin a single dose at planting. Similar results arereported by e.g. Guarda et al. (1990, 1993, 1996).

Compared to the tuber yields the variationwas much smaller on starch content or externalquality of tubers. Consequently starch yields and

marketable 40–70 mm yields were most relatedto total tuber yield (Figs. 1 and 2). As expectedthe highest rates of fertilizer nitrogen producedlower starch content. The effect was stronger onirrigated potato and nearly same wether a singledose or a split application was used. Carlsson(1977) and Walther (1984) reported that the splitapplication of nitrogen had influence on neitherthe starch nor dry matter content of tubers.

The irrigation had no influence on starch con-tent. Ojala et al. (1990) also summarized that

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irrigation didn’t necessary decrease the starchcontent of potato. At Potato Research Institutein 1989–1991, the irrigation even increased thestarch content of potato in years with dry earlysummers and early start of irrigation (Kuisma1998).

Due to small and irregular, although statisti-cally significant differences caused by a largerportion of small tubers (< 40 mm) on unferti-lized potatoes, tuber size distribution and exter-nal quality are worth examining only in relationto irrigation. The effect of irrigation on tuber sizedistribution was strongly affected by the trialyear. In 1993 and 1995 the irrigation had no ef-fect on tuber size. In the dry summer of 1994,the irrigation increased the proportion of tubers40–70 mm at the expense of those of a small size< 40 mm on variety Tanu. On variety Fambo theinfluence of irrigation on tuber size was similarbut clearly smaller than on variety Tanu (Ta-ble 7).

On the average, the variation on quality de-fects due to fertilization was small and unevenfor any conclusions. Thus they are ignored infurther examination. The studies in Norway haveshown that split application of nitrogen has noeffect on the external quality of tubers (Hveem

forsøksgard 1991). Hunnius et al. (1972) alsonoticed that the split application of fertilizer ni-trogen didn’t affect tuber defects.

The clearest influence of irrigation was seenon common scab. Irrigation decreased the pro-portion of scabby tubers by 60% in 1993 whenJune was very dry. Tuber shape was also slight-ly better on irrigated crop (Table 7).

Nitrogen balanceSoil nitrogen mineralisation is approximately50 kg ha–1 annually in southern Finland (Lindenet al. 1992). In this study soil inorganic nitrogenin 0–30 cm layer before planting was 25 kg ha–1

N and 32 kg ha–1 N in 1994 and 1995, respec-tively. Values are on the level reported in Fin-land by Leppänen and Esala (1995).

In the dry summer of 1994, soil inorganicnitrogen fluctuated independently of variety,water supply or even of supply of fertilizer ni-trogen up to middle of July (Fig. 3). The highestvalues were unreliable because the analysismethod was no more accurate when soil inor-ganic nitrogen exceeded ca. 150 kg ha–1 N. Fromthe latter part of July the effects of drought could

Table 7. Effects of irrigation on tuber yield, size distribution and external quality in 1993–1995.

Tuber yield Tuber size (mm) % of tubers

Year Variety Irrigationt ha–1

< 40 40–70 > 70 Healthy Scab Shape Greendefects

1993 Fambo Unirrigated 38.2a 11a 87a 2a 64 16 04 08Irrigated 37.4a 13a 84a 3a 65 08 02 11

Tanu Unirrigated 31.4a 18a 78a 4a 48 29 11 06Irrigated 32.2a 19a 77a 3a 63 08 06 05

1994 Fambo Unirrigated 34.0b 13a 85b 2a 81 00 00 02Irrigated 48.9a 06b 91a 3a 82 00 00 02

Tanu Unirrigated 28.2b 61a 39b 0a 89 00 00 01Irrigated 42.2a 35b 65a 0a 85 02 00 02

1995 Lady Rosetta Unirrigated 41.5a 09a 90a 1a 42 04 02 02Irrigated 43.8a 08a 90a 2a 54 02 01 02

Within a single column group, means followed by the same letter are not statistically different at level P < 0.05 in Tukey’sHSD-test

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be seen, and the soil inorganic nitrogen stayedhigh on an unirrigated field up to the harvest.On irrigated plots the soil inorganic nitrogendecreased below 20 kg ha–1 N at the beginningof August. At the harvest, the residual nitrogenin soil was about 10 kg ha–1 N, independently ofnitrogen fertilization. In 1995 on cultivar LadyRosetta, the changes of soil nitrogen were much

more even than on varieties Fambo and Tanu in1994. The effects of irrigation also were mini-mal, and the soil inorganic nitrogen was onlyslightly over 10 kg ha–1 N on the unirrigatedplots.

The level of tuber nitrate nitrogen contentwas in general very low, only 5 and 12 mg kg–1

tuber fresh weight on Fambo in 1994 and on

Fig. 3. Effect of irrigation and nitrogen fertilization on soil inorganic nitrogen in 1994–1995.

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Lady Rosetta in 1995, respectively. However,higher values of residual soil nitrogen at harveston unirrigated treatments resulted in a tuber ni-trate content which was about triple comparedto irrigated potatoes, and clearly higher due tohigh rates of fertilizer nitrogen or split applica-tion (Table 8). Johnson et al. (1996) also report-ed that on suboptimal nitrogen application a fullirrigation resulted to lower nitrate concentrationon tubers after harvest. On the irrigated field,the low residual nitrogen in soil after harvestshows that the potato crop can utilize nitrogen

effectively in good soil moisture conditions (Fig.4) as e.g. Carlsson (1977) and Johnson et al.(1996) reported.

Independently of cultivar or irrigation, thenitrogen uptake of tubers was 46–62 kg ha–1 onunfertilized plots in 1994–1995. On fertilizedtreatments nitrogen uptake of tuber yield wasrelated to given fertilization. Nitrogen uptake oftubers was clearly higher than the rate of ferti-lizer nitrogen (Fig. 4).

One kilogramme of nitrogen in tuber yieldgave 301 kg ha–1 tubers on fertilized plots whenirrigation was not used, and 347 kg ha–1 on irri-gated potatoes. These were rather independenton used nitrogen fertilization and on the similarlevel as Joern and Vitosh (1995a) reported inMichigan. On the unfertilized plots, efficiencyof nitrogen was clearly higher; 1 kg of tuber ni-trogen produced a tuber yield, which was 412kg ha–1 on irrigated plots and 353 kg ha–1 whenirrigation was not used.

The apparent nitrogen recovery (ANR =(N-yieldN-fertilized–N-yieldN-0)/fertilizer-N) (Novoaand Loomis 1981) of tuber yield in 1994 was onthe average 0.68 and 0.58 on unirrigated Famboand Tanu, respectively. When irrigation was usedANR was 0.99 and 0.79. On Lady Rosetta in1995, ANR was independent of irrigation andslightly higher than on irrigated Fambo in 1994(Table 9). On unirrigated potato in 1994, ANRwas about on the same level as Guarda et al.

Table 8. Effects of irrigation and nitrogen fertilization onnitrate content of tubers (NO3-N mg kg–1 tuber fresh weight)in 1994–1995.

Fertilization Fambo Lady RosettaIrrigation kg N/ha 1994 1995

Unirrigated 0 2 0950 6 0780 8 02110 190 2350+ 30 5 1650+ 30 +30 9 62

Irrigated 0 1 0250 1 0480 2 12110 1 1050+ 30 1 1550+ 30 +30 8 10

Table 9. Apparent nitrogen recovery in 1994 and 1995.

1994 1995

Irrigation N kg ha–1 Fambo Tanu Lady Rosetta

Unirrigated 50 0.83 0.58 1.1480 0.88 0.64 1.02110 0.46 0.61 1.0450 + 30 0.72 0.57 0.8550 + 30+ 30 0.50 0.50 0.94

Irrigated 50 1.48 0.97 1.2080 1.01 0.74 1.22110 0.89 0.73 1.0050 + 30 0.78 0.72 0.8850 + 30+ 30 0.77 0.81 0.92

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Fig. 4. The balance sheet of nitrogen in 1994.

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(1993) noticed in Italy, but slightly higher thanMolfetta et al. (1993) reported. In Michigan,USA, Joern and Vitosh (1995b) received a con-siderably lower recovery of fertilizer nitrogen.In their studies the tuber yield contained only37% and the whole crop altogether 52% of ap-plied nitrogen, independently of the nitrogen rateor application time. They also found 27% of fer-tilizer nitrogen to be in the soil 0–120 cm afterharvest. Vos (1997) reported ANR-values 0.4–0.8 on whole plant basis. Guarda et al. (1996)reported that irrigation improved remarkably theefficiency of nitrogen fertilization in early pota-to. In this study the split application of nitrogendidn’t improve the efficiency of fertilizer nitro-gen as Westermann et al. (1988) reported.

Conclusions

The study showed that in general the split appli-cation of nitrogen fertilization isn’t beneficial onpotato in Finnish conditions when the recom-mended levels of fertilizer nitrogen are used. Themost interesting result was to discover how veryefficiently the potato crop utilises nitrogensources in soil. From an environmental aspectthe study proved that the use of fertilizer nitro-gen according to general recommendations withirrigation, is not only a yield maximizing andmost cost-effective but also a environmentallysafe way to grow potato.

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SELOSTUSJaetun typpilannoituksen käyttömahdollisuudet perunalle

Paavo KuismaPerunantutkimuslaitos

noituksen jakamisesta ei ole hyötyä, kun pitäydytäänlannoitussuositusten mukaisissa typpimäärissä. Kes-kimäärin parhaat sadot saatiin, kun kaikki lannoite-typpi annettiin istutuksen yhteydessä. Tutkimus toimyös esille, että peruna käyttää tehokkaasti hyväk-seen kasvukauden aikana maasta mobilisoituvan ty-pen, jos vesitaloudesta pidetään hyvää huolta esimer-kiksi sadetuksella. Tästä osoituksena olivat kuivanheinäkuun 1994 noston jälkeen sadettamattomassamaassa olleet yli 100 kg/ha typpimäärät. Sadetetus-sa maassa ja kosteampana kesänä 1995 maasta löy-tyi liukoista typpeä noston jälkeen noin kolmannesniistä määristä, mitä keväällä oli istutusvaiheessa.

Parhaimmillaan typpilannoituksen näennäinenhyväksikäyttö sadossa oli yli yhden osoittaen, ettälannoitetun perunakasvuston kyky ottaa käyttöönsämaan luontaisia typpivaroja oli itse asiassa parempikuin ilman typpeä viljellyn perunan. Ilman typpilan-noitusta perunasadossa pellosta poistuvat typpimää-rät olivat 46–62 kg N/ha, ja lannoitetussa perunassakeskimäärin 76–176 kg N/ha.

Suurten typpimäärien antaminen lannoituksessa yh-dellä kertaa on riskialtista. Runsas lannoitetyppi kas-vukauden alussa voi viivästyttää perunan kehitystäniin, että kasvustot eivät ehdi tuleentua riittävästi en-nen sadonkorjuuta. Osa keväällä istutuksessa anne-tusta lannoitetypestä saattaa myös huuhtoutua poh-javesiin runsaan sateen tai sadetuksen seurauksena,sillä perunaa viljellään yleensä keveillä, helposti lä-päisevillä maalajeilla. Keskieurooppalaisissa ja myösruotsalaisissa tutkimuksissa suurten lannoitetyppi-määrien jakamisesta useampaan lannoituskertaan onsaatu hyviä tuloksia. Jaettua typpilannoitusta käyte-täänkin jossain määrin käytännön viljelyssä.

Perunantutkimuslaitoksella tutkittiin vuosina1993–1995 kenttäkokeessa sadetukseen liitettynä pe-runan typpilannoitustarvetta ja suurimpien lannoite-määrien jakomahdollisuuksia suomalaisessa perunan-tuotannossa. Muuten viljelytekniikassa pyrittiin hyö-dyntämään mahdollisimman hyvin kaikki perunankasvua aikaistavat toimet.

Tutkimus osoitti, että kasvuoloissamme typpilan-

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