Dissertation - Are fertilizers over-applied in North Yorkshire agricultural soils

Dissertation is submitted in part fulfilment of the B.Sc. in Environmental Science, Environment Department, University of York.

Are nitrogen and phosphorous fertilisers over applied in North-Yorkshire agricultural soils? A study of nitrogen and phosphorous retention in agricultural soils growing Triticum aestivum.

Charles T H Wain (Y8184901)

4/30/2015

Academic Supervisor: Prof. M Hodson Word Count: 7497

Are nitrogen and phosphorous fertilisers over applied in North-Yorkshire agricultural soils?

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Acknowledgements

I wish to place on record extreme gratitude to Steve Fothergill and Tom Unsworth for access

to the farms and fertiliser records and use of land for completing this study. I would also like

to thank the Environment Department, University of York and all the staff who have helped

in the completion of this work, with special mention to Prof. Mark Hodson, Rebecca Sutton

and Deborah Sharpe.

Declaration

I, Charles T H Wain, declare that the work submitted in this dissertation is the result of my

own work and investigation and all the sources I have used have been indicated by means

of completed references.

Signed: Charles T H Wain

Date: 31/04/2015


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Abstract

Food production is an ever present and increasingly common problem to meet which is

exacerbated with increasing population. The use of fertilisers to increase yields is well

documented as are economic and environmental issues associated with application

inefficiencies. This study aims to determine if there is a problem with this in North Yorkshire.

The study used soil from three farms in North Yorkshire to grow T. aestivum under

greenhouse conditions and the soils and plant material was analysed to determine the

transfer and availability of nitrogen and phosphorous within the close plant pot system. The

budgets were determined by comparing this amount of N or P lost from the system to that

of the addition by means of artificial fertiliser application. It was found that N is over-applied

in soils growing T. aestivum in North Yorkshire, P was found to be over-applied in one of the

three soils compared in this study.

1. Introduction

1.1 Food production and scarcity

Global population has been rising since c. 1350 at the end of the great famine and Black

Death and is estimated to reach 8 billion by 2025 (Tollefson, 2011). Land is relatively finite

and this puts added pressure on food production for two main reasons; a growing

population increases national food demand and this increasing population uses ever more

land on which to live giving rise to competition for agricultural land with other land uses

such as housing. This point is illustrated by the fact that between 1890 and 1951 3.6 million

acres of arable land was removed in the UK (Holderness, 1985) during the same period the

population has seen great increases.

Land prices for agriculture have nearly doubled from 2002-2008 to £14,154 (Living

Countryside, n.d.). This increase in price puts added economic pressures on farmers to

achieve the maximum yields from their land as it increases the fixed costs that eat into the

profits from crop production. The near future aims for the UK’s agricultural industry is to

increase production whilst reducing quantity of land used (Beddington, 2010). Not only is

achieving an increase in per area yield needed, it is needed whilst lowering water use on the

land and also lowering greenhouse gas (GHG) emissions to ensure environmental protection

and increase agricultural production. Around 70% of the world’s water use is attributable to

agriculture (FAO Global Perspective Studies Unit, 2007) and the IPCC (2007) report

highlights that agriculture is responsible for 10 – 12% of global GHG emissions.


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The GHG emissions that are causing climate change are responsible for the recent

stagnation in common wheat or Triticum aestivum yields in Europe (Moore and Lobell,

2015). It is estimated that from 1989 to 2014 changes to European rainfall and temperature

levels is responsible for a 2.5 % decrease in T. aestivum yields (Moore and Lobell, 2015). A

decrease in yield of T. aestivum could lead to an increase in production methods that

enhance GHG emissions, which in turn would lower yields further trapping the industry in a

negative feedback loop. It is for these reasons that environmental degradation has to be

considered whilst tackling the food production problem.

The Malthusian view that agricultural output will not meet demand for a growing population

(Malthus, 1798) is proven to be incorrect thus far in history. Work by Campbell (1979)

showing that between 1955 and 1979 there was per head increase in food supply globally

contradicts Malthus. The role of a present day farm however is not just to produce food, it is

be run as a profitable business by maximising output and minimizing costs to ensure

maximum profit. This profit is directly relatable to the output of the farm in terms of crop

yield.

The yield of a T. aestivum can be expressed in terms of the following components

(Monteith, 1977; Hay and Walker, 1989):

Y = Q × F × ε × H, (1)

where Q is the quantity of solar radiation, F is the fraction of the Q intercepted by the

canopy, ε is the radiation use efficiency of the canopy and H is the harvest index. The solar

radiation and the amount of this used by the canopy is out of the control of the farmers.

Harvest indices are the percentage of biomass available to contribute directly towards the

usable yield namely the grain in the case of T. aestivum (Harrison et al., 1969). H and ε are

therefore the variable components of the yield of crops and are controlled by a range of

factors governing the fertility of the soil; soil properties, water availability nutrient availability

(specifically nitrogen (N)) (for the purpose of this study N refers to available nitrogen in the

forms NH4 and NO3 (due to the nature of NO2 rapidly continuing nitrification and becoming

NO3, NO3 is used as a collective term throughout this report)) (Hay, 1999).

1.2 The role of fertilisers

Fertilisers act as supplements to soil fertility, they replace nutrient deficits in soils to ensure

optimum plant growth and to achieve optimum yields in an agricultural setting. It is the


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intrinsic nature of farming that the nutrients used in crop growth are stored in the plant

during growth and subsequently taken off site at the time of harvest. The excessive use of

fertilisers however has the potential to cause significant harm to ecosystem health, if not

managed appropriately. It is predicted that a doubling of fertiliser and pesticide use globally

from 2000 – 2050 will cause two to three times more eutrophication of aquatic ecosystems

(Tilman et al., 2001). It is worth noting however that had global cereal yields remained at

1961 levels in 2004, it would have required an additional 1.4 billion hectares to achieve the

same quantity of cereal production (Cassman and Wood, 2005). There is a balancing act

between achieving high levels of yield of agricultural crops with fertiliser use and ensuring

pollution of aquatic ecosystems and species in the surrounding area is not exacerbated, it is

the role of policy makers working with farmers to ensure the best is achieved for both the

farmers and the natural environment.

Fertiliser use in Britain has been falling slightly in recent decades, 31% decline in N use and

42% for both potash and phosphate between 1983 and 2013 (DEFRA, 2014). N application

has fallen by 31% overall, but has remained relatively stable with application to tillage crops

with only a slight decline over the same period (DEFRA, 2014) (Fig. 1).

Figure 1: Overall fertiliser use (kg ha-1

) on all crops and grasses, Great Britain 1983 - 2013

Cresser et al. (1993) state that on average for the UK each kg of N added to a soil will

increase the yield of T. aestivum by 24 kg until application of N and output begin to plateau,

the level at which this curve plateaus is dependent on the physical, chemical and biological

makeup of the soil in question. Ignoring labour costs the increased grain yield can reach c.

eight times the value of the applied N. It is statistics like this that create the problem in

complacency regarding the over application of fertilisers in agriculture.


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1.3 Nutrient retention in agricultural soils

The retention of nutrients in the soil available for uptake is the determining factor in the

success of fertiliser application programmes. Excessive amounts of fertiliser run the risk of

polluting nearby aquatic systems through leeching and groundwater transfers and is also

economically futile to waste fertiliser in this manner. However, if too little fertiliser is added

the farmer is running a risk of not reaching the potential yield for that harvest and not

maximising profit to be made from the land. The balance between too much and too little

creates an ‘economic optimum’ or most economically desirable application quantity which

can form the cornerstone of fertiliser management (Cresser et al., 1993). The economic

optimum however, has many problems associated with it and should be treated with

caution. It is overly simple, and Sutherland et al. (1986) shows that there are significant

differences between the expected and observed yields when following the economic

optimum model. This difference can be in part due to the ever changing nature of soils.

N and phosphorous (P) are the two nutrients that will be examined in this study as they are

the most commonly applied fertiliser globally (Cresser et al., 1993). Their use in crop

development is directly related to producing the largest yield (Eq. 1).

N cycling is mainly regulated by biotic processes, with microorganisms playing a central role

in the availability of N in the soils (Cresser et al., 1993) and as such is extremely dependant

on temperature and moisture in the soil. Due to the high variability of temperature and

moisture the amount of available N in soils varies as such making predictions about the

required amount from fertiliser addition are made difficult as a result. This also explains the

high variance of the values (Table 1) of losses of N from agro-ecosystems.

Table 1: Estimated global losses (Tg) of N from agro-ecosystems (from Rosswall and Paustian (1984)).

Sources of N loss Tg of N lost

Harvested products 30

Leaching losses 2

Erosion 2-20

Denitrification 1-44

Ammonia Volatilisation 13-23

N occurs in both the anionic and cationic forms in soils. Ammonium is held on cation-

exchange sites, and can be fixed with minerals such as vermiculite. Nitrite, a phytotoxic


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species is highly mobile and readily oxidises to nitrate, and as such can be considered

negligible in the soil. Nitrate isn’t substantially adsorbed by minerals at any pH, and as such

is highly available for both removal via leaching, erosion and denitrification and also for

uptake during plant growth (Cresser et al., 1993). Consequentially, N availability depends on

the rate of conversion from organic to inorganic N. The optimum acidity for this process,

known as mineralisation is between pH 6-8 (Cresser et al., 1993).

The effect of N on T. aestivum yields is to increase the green area index (ratio of canopy

area in relation to ground area) (Sylvester-Bradley and Kindred, 2009) which promotes a

higher amount of radiation being intercepted, increasing F (Eq. 1). Disease is a limiting

factor to the yield of T. aestivum commonly reducing the canopy duration, lowering the

uptake of radiation needed for photosynthesis (Barry et al., 2010). P in the soil can reduce

the effects of disease of this kind (Zahibi et al., 2011) increasing yield as a result. A large

proportion of P however usually becomes rapidly unavailable due to immobilisation in the

soil (Zahibi et al., 2011) however the presence of bacteria such as Rhizobacter are shown to

improve the retention of P in the soil and as such increase the positive effects of N on yield

(Khalid et al., 2004).

P occurs in anionic forms in soil and is highly prone to adsorbing, especially in acidic

conditions. As pH increases from very acidic conditions to near neutrality the more available

P is in the soil (Cresser et al., 1993). Due to the nature of P and the characteristics

governing its availability in soils it can be highly variable independently across both space in

a field and time. pH, clay content and organic matter (OM) content can make applied P

almost equivalent to available P in some areas of a field, and it can be extremely difficult to

build up concentrations in other areas of the same field (Lambert et al., 2007).

Nutrient availability is a governing factor in the yield potential of a crop in a certain location;

the way in which nutrients become available for plant growth is through uptake through the

medium of the soil solution (Sparks, 2003). Soil solution is defined as the aqueous liquid

phase of the soil and its solutes (Glossary of Soil Science Terms, 1997). Dependant on the

concentration of the ion in solution, the ion can be simultaneously available for plant uptake

and also loss from the system. If the concentration in the plant is already high, it is more

likely to be lost from the system via groundwater leeching, evaporation, volitization or

precipitation rather than by uptake to the plant (Sparks, 2003). It is for this reason that

water content is important in the uptake and efficient use of fertilisers.


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1.4 Study Aims

This study investigates the application rates of N and P fertilisers to three agricultural soils in

North Yorkshire and addresses the hypotheses outlined in the following paragraphs.

The predominant focus of the study was attaining whether or not N and P fertiliser are over-

applied to the three agricultural farms.

The yield of the crop and the retention of the nutrient in the different systems are also of

importance to the results of this study as it can outline an overview to the fate of the

fertiliser for the different soil and whether it contributes directly to the yield of the crop or is

inefficient. The hypotheses to be tested to address these questions are if a high percentage

of N and P will be lost from the system becoming unavailable for enhancing plant growth.

Yields of T. aestivum will be significantly different at the different farms on account of the

differing fertiliser application practises also. The work presented here will determine the best

traits and farming habits at each site, both in terms of chemical alterations to the soil

through fertiliser application and through the physical structure of the soil. It will build a box

model which can be applied to each farm using the data obtained to determine the

efficiency of fertiliser application practises. The development of the application efficiency

equation is outlined when discussing the treatment of the data and its appropriate analysis

in methods section.

2. Methods

2.1 Experimental design

The soils used for the experiment were from three rural farms in North Yorkshire c. 11 – 14

km north of the city of York; Thornton-le-Clay, Sheriff Hutton and Skewesby (54.0792 N, -

0.9595 W; 54.0913 N, -1.0034 W; 54.1338 N, -0.10396 W respectively). The soil types are

described below (Table. 2) annual rainfall is c. 800 ml, and temperature between 8.5 – 10

°C (Met Office, 2015).

Table 2 - Overview of soil characteristics from soil maps; provided by UK Soil Observatory hosted by British Geological Survey (n.d.).

Thornton-le-Clay Sheriff Hutton Skewesby

NSRI soilscapes Slowly permeable seasonally wet slightly acid but base rich loamey and clayey soils

Slowly permeable seasonally wet slightly acid but base rich loamey and clayey soils

Freely draining slightly acid loamey soils

UKSO soil group loamey soil loamey clayey soil silty loamey soil


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The soil was sampled from the sites in early June 2014 towards the end of the T. aestivum

growing season. The time of last fertiliser application was late April 2014, the amounts and

type of fertiliser applied was provided by the respective farmers. The soil was collected from

the upper 30 centimeters of the top soil to ensure maximum content of the most recently

applied fertiliser were in the samples collected.

The experiment used sacrificial sampling method to be sampled at three time points; at the

time of germination (01/09/14), after twenty-nine days of growth (29/09/14) and fifty-eight

days of growth (27/10/14). The experiment was conducted in greenhouse conditions (21

°C) to enhance the rate of growth as time was a limiting factor of the experiment.

The soil from each farm was thoroughly mixed and c. 750 g were added to each of the

twelve plant pots (depth 28 cm, width 12 cm), four for each sampling time point, including

one control without T. aestivum seeds present. This soil was mixed again to ensure even

aeration and four T. aestivum seeds were planted in each pot at a depth of 1.5 cm.

The pots are placed in a plastic saucer to allow excess water to drain into the sauce, it

creates a closed system as the water cannot escape and is available for further uptake

through capillary action of the soil when required (Rowell, 1994). Water was applied to the

pot experiment every two days at an amount of 5.4 cm3 day-1 keeping the amount

consistent with average rainfall in Yorkshire (c. 800 ml annually).

The tops of the plants were cut at soil level, they were weighed at the time of harvested,

then dried in a paper bag at 70 °C until the mass became stable and the final mass was

recorded (Rowell, 1994). This enables the measurement of dry mass of plant material.

The roots are then removed from the soil as best as possible, and the roots are discarded.

The entirety of the soil is removed from the pot experiment and air dried and 2 mm sieved

(Rowell, 1994).

2.2 pH and soil moisture content

pH meter was calibrated using buffer solutions of pH 4 and pH 7 as the soils were acidic and

fell within this range as outlined in Rowell (1994). 10 g of soil had added to it 25 ml of

ultrapure water, this solution was shaken for 20 minutes on an end over end shaker and the

pH was taken of the solution. The addition of water changes the pH of the solution creates

inaccuracy, but the data obtained is still very useful for drawing comparisons between soils.


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Soil moisture content was determined by drying 10 g of air dried soil in an oven at 105 °C

overnight. The final weight is recorded and the difference between the original weight and

this is the amount of water driven off (Rowell, 1994).

2.3 Plant N and P

Plant P content is found by methods of a nitric acid digest of plant material as outlined by

Merrington (n.d., unpublished literature). 0.5 g of crushed and dried plant material is

digested for three hours at 60 °C and a further six hours at 110 °C in 10 ml of concentrated

nitric acid. This solution is made up to 100ml with ultrapure water. The solution was

analysed with an autoanalyser.

N in the plant material is found using a similar method to plant P. Concentrated sulphuric

acid is used rather than nitric acid (Rowell, 1994).

2.4 Soil N and P

P in the soil is measured by extracting 5 g of soil with 100 ml of 0.5 M sodium bicarbonate

and polyacrylamide solution which is buffered to pH 8.5 with sodium hydroxide. The method

is known as Olsen’s method. The solution is shaken on an end over end shaker for thirty

minutes and filtered using Whatman No. 125 paper and analysed on a colorimeter (Rowell,

2004).

Soil N is measured in a similar way, 10 g of soil is extracted with 100 ml of 1M potassium

chloride, this is shaken for sixty minutes and filtered in the same way, this is analysed by

using a colorimeter.

2.5 Data analysis and quality control

Accuracy and precision of the results were an important consideration throughout the

experimental design and implementation stages of the study. Re-runs of randomly selected

extraction solutions were conducted to test for instrumental accuracy. It was found to only

have slight variance in the output results, +/- 0.08 µg L-1 which when converted into a kg

ha-1 concentration of soil of g kg-1 of plant material was found to be unimportant. To test for

precision of the results a calibration curve was constructed using solutions of known

concentrations, the difference between these results and the real concentration indicates the

level of associated precision which was found to be R2 > 0.95 meaning there is a high level

of associated precision with the findings of this study.


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Due to the nature of the data being collected only having N=9 for each variable the

determination of outliers were difficult to detect with a high level of confidence. This

however isn’t especially important to the findings of the study when the quality control

assurances were found to be over 95 %. The means for all variables were found, and given

+/- one standard error as a unit of uncertainty. ANOVA’s were performed to detect

differences between the means of different soils and sampling time points, and correlations

were statistically determined by use of Pearson’s product moment correlation coefficient.

To address the question regarding the over-application of fertilisers an equation was

developed to determine the amount of N/P retained in the system and a comparison to the

amount of fertiliser added to the soil.

(Total N/P plant + soil day 58 ÷ Total N/P plant + soil day 0) x 100 = Retained N/P % (2)

Fertiliser N/P - Retained N/P = N/P budget (3)

3. Results

3.1 N and P concentrations in the soil and plant material

Soil from Thornton-le-Clay shows a decrease in soil N concentration as time progresses from

34.32 kg ha-1 at 0 days to 8.38 kg ha -1 at 58 days. Although there is a small dataset (N=9)

there is no overlap of standard error between the means at each time point. The first

sampling time point has a large standard deviation of 12.44, however despite this it can still

be concluded that there is a significant difference between the means at 0, 29 and 58 days

respectively (f = 9.29, p = 0.015). P concentrations increased between 0 and 29 days from

136.51 kg ha -1 to 152.61 kg ha -1 and were lower at 58 days than both of the previous two

sampling points. Standard error is very low with a cumulative value of 13.01, F and p values

indicate that the means are significantly different from one another (F = 300.26, p < 0.001).

Soil from Skewesby also shows a decrease in soil N concentration as time progresses from

57.49 kg ha -1 at 0 days to 8.30 kg ha -1 at 58 days. A large F value (F = 20.50, p = 0.002)

indicates that the means are significantly different. It is worth noting however that there is a

fairly high level of uncertainty associated with this as the cumulative standard deviation is

23.56. Concentrations of P in the soil at Skewesby show the same as Thornton-le-Clay soil P

concentration results; an increase in mean concentration between 0 (61.96 kg ha -1) and 29

days (73.83 kg ha -1) and the sample taken at 58 days being much lower the first two

concentrations with P concentration being just 25.89 kg ha -1 (F = 208.91, p < 0.001).


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N concentrations at Sheriff Hutton differed from both Thornton-le-Clay and Skewesby in the

fact that there is no significant change in the concentration in the soil between 0 and 29

days sampling points, there is however a very large difference between the first two

sampling time points 45.33 and 46.68 kg ha-1 respectively and the sampling time after 58

days 10.28 (F = 38.15, p < 0.001). P concentrations in the soil from Sheriff Hutton show no

statistical difference between the sampling points at day 0 and day 29 as the standard

errors overlap, although the means do show a subtle decrease from 44.48 mg kg-1 to 40.12

mg kg-1. The concentration after 58 days (12.71 kg ha -1) is much lower than both of the

previous sampling times (F = 35.16, p < 0.001).

Table 3 - Mean concentrations of N and P in both the soil and plant material reported +/- 1 standard error, F and P values are given for Thornton-le-Clay, Skewesby and Sheriff Hutton (N=9 per soil/plant).

There is no real difference between the means of N concentration in the soil at Skewesby

(36.41 kg ha -1) and Sheriff Hutton (34.10 kg ha -1) as the standard error of the means

overlaps, however Thornton-le-Clay (21.56 kg ha-1) was shown to be significantly lower in

terms of mean N concentration than the other two soils.

P concentrations are considerably higher than those of N in all soils; Thornton-le-Clay

(118.95 kg ha -1) has the highest mean concentration and is significantly higher than the

soils of Skewesby and Sheriff Hutton, 53.89 and 32.43 kg ha -1 respectively. There was no

statistical difference between Skewesby and Sheriff Hutton as the mean P results had very

large standard error associated with them. The large standard error is caused due to the

large variation of P concentration between the three different sampling points and the low

number of samples (N=9 per soil).


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Thornton-le-Clay N concentrations in the plant material show an increase from 0 to 29 days

and further increase to the 58 day sampling point with concentrations of 4.85, 7.83 and

10.87 mg kg-1 respectively, there is a significant difference between the means at the

different sampling time points (F = 9.287, p = 0.015). P concentrations in the plant material

grown in the same soil, show an increase between 0 (13.26 mg kg-1) and 58 days (20.93 mg

kg-1), with a slight drop to 12.42 mg kg-1 after 29 days. The means are all significantly

different with a very high F value of 146.39 (p <0.001).

Skewesby shows an increase in N concentration in the plant material at all time points with

concentrations of 9.60, 16.33 and 19.67 mg kg-1 for 0, 29 and 58 days respectively. There

was a significant difference between all the means (F = 297.07, p < 0.001). P

concentrations in the same plant material stayed relatively stable, although there is a real

difference between the means due to the precision of the results obtained (F = 57.97, p <

0.001). An increase from 9.70 mg kg-1 to 12.83 mg kg-1 at 0 days and 58 days respectively.

Sherriff Hutton showed an increase in both N and P concentrations between increases in the

time of the sampling points. N increasing from 9.37 mg kg-1 to 18.83 mg kg-1 between 0 and

58 days, with a significant difference between all means (F = 68.90, p < 0.001). P

concentrations increased from 7.23 mg kg-1 at 0 days to 7.55 and 9.67 mg kg-1 at 29 and 58

days respectively, with a significant difference between the means (F = 104.48, p < 0.001).

Thornton-le-Clay has the lowest concentration of N in the plant material with a mean

concentration of 7.86 mg kg-1, Skewesby and Sheriff Hutton were nearly twice the mean

concentration with values of 15.20 and 13.86 mg kg-1 respectively. There is a significant

difference between the means of these values as there is no overlap of standard error

between any of different plant materials. Sheriff Hutton had the lowest concentration of P in

the plant material however (8.15 mg kg-1) with Thornton-le-Clay having the highest

concentration and Skewesby the second highest with 15.54 and 10.47 mg kg-1 respectively.

3.2 Mass of plant material

Thornton-le-Clay was the soil in which the plants with the largest mean plant material mass

(21.05 g), Skewesby had the second highest mean mass and Sheriff Hutton the lowest

(15.69 and 13.94 g respectively). There is a significant different between the means of the

three systems (F = 39.07, p < 0.001).


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Figure 2 - Mean mass (grams) of the plants after 58 days with soils from Thornton-le-Clay, Skewesby and Sheriff Hutton (N=9).

3.3 Total N and P plant uptake

Plants grown in soil from Skewesby have the highest total N and P uptake (3.09 g N x10-4

and 1.48 g P x10-3) with Thornton-le-Clay having the lowest total uptake of N and P (2.28 g

N x10-4 and 1.09 g P x10-3). Sheriff Hutton has uptake values in-between that of Thornton-

le-Clay and Skewesby being 2.62 g N x10-4 and 1.26 g P x10-3. There is a real difference

between the means for both quantities of N (F = 10.60, p= 0.11) and P (F = 10.60, p=

0.11) taken up by the plant.


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Figure 3 - N uptake in plants from the three farms (g of N x10

-4) samples taken of day 58 after germination (N=9).

Figure 4 - P uptake in plants from the three farms (g of N x10

-3) samples taken of day 58 after germination (N=9).

3.4 Water content of air dry soil and pH

Sheriff Hutton has the largest water content of air dried soil at 2.79 % which is significantly

larger than both Skewesby and Thornton-le-Clay at just 0.98 and 0.61 % respectively.


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Although Thornton-le-Clay has a lower water content it is not significantly different from that

of Skewesby +/- 1 standard error overlap showing that there is no clear difference between

the mean values.

All the pH values are significantly different from one another all moderately acidic soils, but

with little difference between them Thornton-le-Clay is the most acidic soil (pH 5.34) and

Skewesby is the least acidic with a pH of 5.78.

Table 4 - pH and water content (%) of air dry soil for the three different soils with appropriate F and p values (N=26).

Thornton-le-

Clay

Skewesby Sheriff Hutton F and p values

Water content

(%)

0.61 +/- 0.11 0.98 +/- 0.28 2.79 +/- 0.46 F = 13.594, p <

0.001

pH 5.34 +/- 0.04 5.78 +/- 0.05 5.51 +/- 0.09 F = 11.47, p <

0.001

3.5 N and P percentage retention

Retention % is calculated in equation 2.

N retention does not have sufficient statistic evidence to prove any difference between the

means of the different systems N retention (F = 0.59, p = 0.58).

Figure 5 - N retention (%) in the three different soils (N=9).


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P retention in the soil shows a significant difference between the three systems however

Thornton-le-Clay has the highest retention % of 59.22 %, Skewesby has 54.01 % and

Sheriff Hutton just 43.47 % (F = 16.92, p = 0.003).

Figure 6 - P retention (%) in the three different soils (N=9).

When comparing P retention and the mass of the crop after 58 days of sampling, it was

found to resemble a fairly strong positive correlation. As the mass of the crop increases so

does the retention of P (%) in the soil. Pearson’s 1 tailed bivariate correlation test was

conducted on the data and found a strong positive correlation (r = 0.796, p = 0.050).


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Figure 7 - % P retention and mass of crop (grams) sampled at 58 days (N=9).

3.6 Fertiliser application practices

Fertilisers were applied at the same time for each of the three soils in late April, c. two

weeks before sampling took place. Sheriff Hutton was the only soil have P applied (60 kg ha-

1). Skewesby had the most N applied to the soil (136 kg ha-1), Sheriff Hutton was also a

large quantity (120 kg ha-1) and Thornton-le-Clay a considerable amount less (34.5 kg ha-1).

Table 5 - Application of N and P fertilisers to Thornton-le-Clay, Skewesby and Sheriff Hutton in April 2014.

Farm

Fertiliser Thornton-le-Clay Skewesby Sheriff Hutton

P (kg ha-1) 60

N (kg ha-1) 34.5 120 136

3.7 Soil N and P budgets and comparison with yield

N and P soil budgets are attained by the use of equation 3, a positive value for the budget

meaning that more of either N or P has been applied to the soil than has been lost. In all

three soils N is over-applied; P is only applied at Skewesby, but is over-applied when done

so. There is a negative relationship between the budget and the yield attained, the more

over-applied a soil is with fertiliser the lower the yield achieved is. Thornton-le-Clay has the


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lowest rate of over-application of N and the largest yield, Sheriff Hutton over-applies N by

100.95 kg ha-1 and achieves the lowest yield of the three farms.

Table 6 - N and P budgets (kg ha-1

) and yield (g) for Thornton-le-Clay, Skewesby and Sheriff Hutton soils.

Soil N Budget (kg ha-1) P Budget (kg ha-1) Yield (g)

Thornton-le-Clay 8.56

-68.79 21.05

Skewesby 70.55

23.93 15.69

Sheriff Hutton 100.95 -31.77 13.94

4. Discussion

As shown in the results Skewesby and Sheriff Hutton are both over-applied regarding N, and

Skewesby is also over-applied in terms of P. Thornton-le-Clay gained the largest yield and

also has the best N and P budgets in terms of efficiency of the three farms analysed being

only slightly over-applied in terms of N and under-applied in terms of P. This section aims to

address the reasons for this and the effects of these application traits, discuss the limitations

of the study and to provide recommendations for further research and management

strategies for the farms in question.

The benchmark for available N is 75 kg ha-1 according to the HGCA (2008) but at 0 days soil

concentrations are 32.32, 57.49 and 45.33 kg ha-1 for Thornton-le-Clay, Skewesby and

Sheriff Hutton respectively. These are far below the benchmark for recommended levels set

by the HGCA and even further below what would be expected based on the fertiliser

application levels (Table 5). Iserman (1990) states that across Western Europe have N

utilisation rates of c. 59 – 72 %, as such Thornton-le-Clay is well in exceedance of this

utilisation ratio, whereas both Skewesby and Sheriff Hutton fall well below this.

As this study was carried out under controlled rather than natural conditions, certain

limitations to the application of the results have to be considered. The artificially high

temperature can create greater than normal nutrient uptake into the crop, especially with N

(HGCA, 2008) some studies even found temperature variation to have greater effects on

nutrient uptake (Guo et al., 2008).

The rate of nutrient uptake changes dependant on growth stage of T. aestivum. According

to HGCA (2008) the average uptake is c. 279 kg ha-1 of N over the entire growing cycle, with

40 % of this value being the first node and flag leaf stage which is only a period of five

weeks (HGCA, 2008). As this study used only the first c. 1400 thermal hours of crop cycle


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19

(harvest usually occurs at c. 3100 thermal hours) nutrient uptake levels aren’t fully

representative of the entire growing cycle, but can be used as a comparison between the

three different soils nonetheless.

Due to the design of the experiment, root development was limited due to the physical

constraints of the plant pot T. aestivum were grown in. Root growth of T. aestivum is slow

during the foundation phase, very rapid during the construction phase of growth and

decreases during the production phase when senescence of the plant occurs (HGCA, 2008).

The root growth potential is governed by the soil type and the pore space within that soil.

Intermediate soil compaction provides optimum conditions for root stability and

development and as such nutrient uptake (Arvidsson, 1999). Too much compaction in the

soil hampers development of the root system and too little compaction lowers the root to

soil contact rates, lowering nutrient uptake to the plant (Arvidsson, 1999). Thornton-le-Clay

had the most established root system by field observation, whereas Skewesby and Sheriff

Hutton were not as advanced. This could be due to the nature of the soils, with Skewesby

being less prone to compaction, and Sheriff Hutton being composed of high clay content

soil. This could be a predominant reason for yield attained by Thornton-le-Clay despite

having lower nutrient concentrations than the other two soils.

Increased depth and coverage of tillage is suggested as a way to increase nutrient retention

and also increase root development potential in clayey and loamey soils (Ulen and

Jakobsson, 2005). Sowing early on also increases the potential for root development to be

increased, as the soil has more time from tillage to germination to develop a deeper root

network and to avoid suffering from effects of compaction (HGCA, 2008).

Water content of air dried soil also indicates the resistance to water flow, with the larger the

water content a good representation of the size of the resistance to flow (Ashman and Puri,

2002). Sheriff Hutton had a substantially larger water retention 2.79 % in comparison to

0.61 % and 0.98 % for Thornton-le-Clay and Skewesby respectively. A large resistance to

flow as demonstrated in Sheriff Hutton indicates a soil less susceptible to leeching of

nutrients. This is the case for N in Sheriff Hutton soil, but not for P. This however may be

caused by the reaching of saturation of the soil towards 58 days by overwatering as the soil

had higher clay content than the soils from Thornton-le-Clay and Skewesby retained more

water and were more prone to saturation. It is shown that the majority of the P lost was

between 29 days and 58 days (Table 3) which is caused by the saturation of the soil at this


April 30, 2015

20

time noted as a field observation. This may also be a cause for the lower yield in Sheriff

Hutton soil.

The three soils studied here had a range of pH from 5.34 – 5.78, this in comparison to the

target pH of 6.5 for T. aestivum growth is lower than optimum (Goulding and Annis, 1998).

For the soils to achieve this optimum pH for wheat growth it would require the application of

between 7398 kg ha-1 and 5156 kg ha-1 of lime for Thornton-le-Clay and Skewesby

respectively with Sheriff Hutton being between these values. T. aestivum grows best

between pH 6 and pH 7, and as such none of the soils analysed are at the optimum pH for

wheat growth (Vitosh, 1998). Growing wheat in soils lower than this can cause manganese

deficiencies, and lowered uptake of P, and a reduce rate of N mineralisation in the soil

(Cresser et al., 1993). Over-application of N to the soils creates a lower pH (Ju et al., 2007)

as more hydrogen ions are displaced from the soil surface into the soil solution, enhancing

the pH problem in the soils.

It is shown by Stephen et al. (2005) N fertiliser application time and mode are not as

important for the crops yield as quantity, as their 2003 study showed no significant

differences between yields when varying mode of application and timings. If application time

and mode and unimportant to the yield of T. aestivum then the soil structure, quantity of N

application and water retention properties of the soil are the determining factors that affect

the retention and use of N in the soil to maximise yield.

The use of P in wheat growth is most needed for tillering and root development (Hamid and

Sarwar, 1977). The role of P as a fertiliser in this study is therefore slightly limited as

tillering phase of the growth cycle was in its infancy and not taken to completion. Due to the

fact that root content where a large quantity of P resides was not measured also means that

the value for P lost in the soil will be larger than it actually is, and as such the budget

doesn’t take into account the nutrients stored in the roots that will be available to the next

crop, the budget will therefore have a lower value for P than is accurate.

Alongside the economic inefficiencies of over-application of fertilisers, the over-use also

brings problems associated with environmental degradation. N and P in the soil not used for

plant growth or tightly bound to soil ionic sites and instead held in the soil solution are

prone to leeching from the soil into groundwater and other water resources (Cresser et al.,

1993). The problems associated with N and P in water supplies are well documented and

include eutrophication and severe oxygen depletion and loss of productivity, for a detail


April 30, 2015

21

breakdown of the processes that cause the breakdown of waterbody productivity see

Andrews et al. (2004).

N and P coastal influxes from riverine systems are dominated from non-point sources, and

the largest contributor of N to the North Atlantic sea is from Northern English watershed

regions draining into the North Sea (Howarth et al., 1996), such as the Holderness Coast

and the River Humber from which groundwater under the three farms sampled eventually

flows via route of the River Derwent. The problem of eutrophication of lakes streams and

rivers is primarily caused by N and P agriculture inputs and high levels of rainfall (Bechmann

et al., 2005).

The results of this study highlight the over-application of both N and in some cases P to

agricultural soils in North Yorkshire, however it is incorrect to make the assumption that this

is a direct attributing cause of eutrophication in North Yorkshire and the high levels of lost

fertiliser cannot be said to be influencing eutrophication without conducting a point-source

pollution study into the eventual fate of fertilisers in the agricultural soils used in this study.

The application from a crop management perspective however does show a trend of over-

application and insufficient use of other management techniques to improve yield such as

applying best management practices, and using advice of an agronomist to achieve the

maximum yield without causing damage to the surrounding area (Roberts, 2009).

5. Conclusion

Addressing the main finding of this study is the budget between fertiliser application and the

loss rates from the soil, through plant uptake and loss from the soils via changes to the form

of N and P in their respective cycles. N was found to be over-applied in terms of this budget

in all three farms. Thornton-le-Clay was the only farm to over-apply P to its soil, it was

however the only farm to have applied P so far that growing season.

There is a significant difference in the yields of the farms after 58 days of growth under

artificial heat in the greenhouse conditions, Thornton-le-Clay achieving the highest yield with

Skewesby and Sheriff Hutton c. 4 g lighter on average, this is a significant difference

between the three soils (F = 39.07, p < 0.001). Although Thornton-le-Clay has the lowest

soil concentrations of N and P, the low water content of air dry soil shows that the retention

in the soil will be high as it is less likely to leech to the soil solution in great quantities as

there is less solution in the soil. The most acidic pH was also recorded in the soil from

Thornton-le-Clay meaning that the soil solution has a high concentration of H+ ions, this


April 30, 2015

22

implies larger ions such as NH4+ will be held on the exchange sites and unavailable for

leeching, improving the nutrient retention properties in the soil.

Skewesby had large concentrations of both N and P at day 0 sampling point, this is due to

the high levels of both applied via fertiliser. There was a significant loss of the nutrients

from the soil between 29 days and 58 days, this however was in part due to large uptake by

the T. aestivum but despite this there was still are net losses to be found overall. There

were fairly low levels of water retention to be found 0.98 % as such implying low levels of

leeching from the system. Retention in the soil was low for N, although no statistical

differences were found between the three farms, however P retention was significantly

higher c. 53 %. The yield was lower than what would have been expected from the nutrient

concentrations in the soil and the amount taken up by the crop. The diminished yield can be

attributed to water content and soil structure limiting the development of the crop.

Sheriff Hutton had substantial amounts of N and P in the soil. Retention of the nutrients in

the system however was shown to be poor, with large amounts of N and P lost from the

system. Low yields were measured in the crops grown in soil from Sheriff Hutton, the was a

large water content, which could be due to large pore space, the large pore space also

decreases soil to root contact and reduces the potential uptake by the roots. Results of this

soil however have to be interpreted with some degree of caution, the soil become water

logged in the last 7 – 12 days of the study and values recorded at day 58 may have been

altered from what would have been observed in an open field system as opposed to the

closed system recreated in this study.

Further research into the same farms using longer term studies analysing the nutrient

retention in the field over the whole growing season, and making longer term comparisons

would be the logical recommendation for further study in this author’s opinion. Root analysis

may be of great importance also for determining the amount of nutrients unavailable to

plant growth during the same season, but that would be made available in future growth

cycles after decomposition. It would be important to know this to be able to make accurate

recommendations about future fertiliser needs for the soil to achieve maximum yields.

Another possible study would be into the effect on yield varying fertiliser application would

have. This study conducted acts a preliminary investigation into the application practices of

the farms, and conclude that they are over-applied.


April 30, 2015

23

This further research recommendations and a point-source study into the eventual fate of

the applied fertilisers would give an overall picture into the state of agricultural fertiliser

practices in North Yorkshire. The recommendations for the farms at present is to lower

fertiliser application and soil sample at the start of growing and the end and determine more

accurately the fertiliser budget for the soils.

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Dissertation - Are fertilizers over-applied in North Yorkshire agricultural soils