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
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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
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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).
Are nitrogen and phosphorous fertilisers over applied in North-Yorkshire agricultural soils?
April 30, 2015
17
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.