Development and testing of a model for predicting tillage
effects on nitrate leaching from cracked clay soils
A.M. Matthews
a, A.C. Armstrong
a,*, P.B. Leeds-Harrison
b, G.L. Harris
a, J.A. Catt
caADAS Gleadthorpe, Meden Vale, Mans®eld, Notts NG20 9PF, UK
bDepartment of Agricultural Water Management, School of Agriculture, Food and Environment,
Cran®eld University, Silsoe, Bedfordshire MK45 4DT, UK
cIACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK
Accepted 23 March 1999
Abstract
Both water movement and nitrate leaching in structured soils are strongly in¯uenced by the nature of the macro-porosity. That macro-porosity can however also be manipulated by choice of tillage operations. In order to investigate the potential impacts of tillage on rates of nitrate leaching from structured soils, a model speci®c to these soils, CRACK-NP was developed. The model, its application and validation for an experimental site on a heavy clay soil (Verti-Eutric Gleysoil) at Brimstone Farm, Oxfordshire, UK, is described. The model considers the soil as a series of aggregates whose size is also the spacing of the macro-porosity. Water and solutes move in the macro-pores, but within the peds they move only by diffusion, internal in®ltration and root uptake (evaporation). The model re¯ects the in¯uence of diffusion limitation in the release of solutes to by-passing water. The model was then used to investigate the in¯uence of variable ped spacings which were created by variations in tillage practices. The results both from the model and from the ®eld data demonstrated that ®ner soil structures, which have larger surface contact areas and shorter diffusion path lengths, present greater opportunities for interaction between peds and the water moving around them, and so release more nitrates through the drainage waters.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Nitrate leaching models; Cracking clay soils; Tillage effects; Macro-porosity; Diffusion limitation
1. Introduction
The effects of tillage are perhaps most marked for clay soils where the formation of aggregates and inter-aggregate spaces enables both water movement and the penetration of plant roots. In clay soils, the
domi-nant hydrological ¯ow mechanism is the movement of water through the inter-ped spaces, normally termed macro-pores, and the ¯ow process is normally known as macro-pore or by-pass ¯ow (Bouma, 1981; Beven and Germann, 1982). Because tillage alters the nature and distribution of aggregates and the spaces between them, it can have a major in¯uence on soil hydrology (Armstrong and Harris, 1996), and consequently the pattern and amounts of solutes being leached (Addis-cott and Dexter, 1994). Although some models have
*Corresponding author. Tel.: 44-1623-844-331; fax:
44-1623-844-472.
E-mail address: adrian.armstrong@adas.co.uk (A.C. Armstrong).
considered the effect of tillage on the movement of water and solutes through clay soils (e.g., Leeds-Harrison et al., 1992), few have considered its impact on the nitrogen budget of a site.
This paper reports a modelling study which con-siders the in¯uence of tillage practices on the leaching of nitrate from structured clay soils. Because conven-tional leaching models do not perform well for heavily cracked soils (Armstrong et al., 1995), this has required the development of a model that explicitly addresses the issue of water movement in such soils. Leaching models range from very simple functional models, which are often based on empirical relation-ships, to complex mechanistic models, which expli-citly consider the physical processes of water and solute movement but often require large parameter sets (e.g., Addiscott and Wagenet, 1985, for a review and classi®cation of leaching models). However, to facilitate both the representation of tillage effects and also to address the by-pass ¯ow in clay soils, it is important that a model has a complete, detailed and ¯exible description of soil structure, and this is pos-sible only by the use of relatively complex models. In this paper, we describe the characterisation of tillage effects within the nitrate leaching model CRACK-NP. The ability of the model to reproduce differences in short-term leaching behaviour, whilst maintaining a realistic nitrogen budget over a longer period will be tested against data collected from the Brimstone Farm experimental site in Oxfordshire, UK.
2. Field context
Detailed data sets are required for the development and application of models in order to supply the
necessary input parameters, and to provide the data for the validation of simulation results (Armstrong et al., 1996). The data used for this study were provided by the Brimstone Farm experiment, described by Cannell et al. (1984), Catt (1991), and Harris et al. (1993). The site was established in 1978 at Coleshill, near Faringdon in Oxfordshire on a heavy clay soil of the Denchworth series, a Verti-Eutric Gleysol (FAO classi®cation) typical of much of the lowland clay in arable use in UK (Goss et al., 1988), and some basic physical properties are summarised in Table 1. This site has been intensively monitored for most aspects of site agronomy and hydrology, and so provides an ideal source of data and parameters for the validation of leaching models.
The site has 20 plot-scale lysimeters, each of which is 0.2 ha in area and therefore large enough for normal agricultural operations to be undertaken. The initial phase of experimentation (1978±1988) examined the effects of different soil management practices on the loss of water and nitrate from these plots. Half the plots were conventionally ploughed to a depth of 20 cm and the other half were prepared by direct drilling. Of the 10 plots in each tillage treatment, half were drained by mole-and-pipe systems and half were left undrained. The two treatments (tillage and drai-nage) were the subject of a full factorial design with ®vefold replication. Measurements available from the drained plots include half-hourly rainfall, water table and drain¯ow readings and nitrate concentrations in the drains sampled during ¯ow events. In addition, full measurements were made of the crop conditions, and many detailed studies of soil water and the nitrogen economy of the site were undertaken (Catt, 1991).
Large vertical cracks tend to occur during the summer months as the soil dries and these can provide
Table 1
Typical soil properties for each horizon (percent for soil components) (information from Cannell et al., 1984)
Horizon (depth (cm))
Ap (0±20) Bg1 (20±35) Bg2 (35±62) Bgk (62±89) Bcgk (89±112)
Sand (60mm to 2 mm) 7 6 3 2 1
Silt (2±60mm) 39 39 37 36 37
Clay (<2mm) 54 56 60 62 62
Organic carbon 3.3 0.8 0.7 0.6 0.6
CaCO3equivalent <0.1 <0.1 0.2 12.3 21.6
rapid pathways for the transport of water and solutes to the mole drains (Goss et al., 1993). This situation changes in winter when the soil wets up, causing the cracks to close so that a larger proportion of the incoming water is lost as surface runoff. On the ploughed plots, the connectivity of the macro-pores is interrupted at the base of the plough layer, resulting in lower ¯ow rates and a less peaky hydrograph than for direct drilled plots (Harris et al., 1993).
The differences in soil structure produced by ploughing and direct drilling are important when considering the movement of nitrate. Over winter the majority of nitrate comes from mineralisation of soil organic matter and diffusion of nitrate into the macro-pore water is more rapid in the case of smaller aggregates (Addiscott et al., 1983). In the ploughed situation, there is plenty of opportunity for diffusion to take place due to the slow movement of water through the topsoil and the small mean aggregate size. By contrast, in direct drilled soils water moves rapidly to the drains as by-pass ¯ow, and the amount of nitrate leached is limited by the rate at which it can diffuse from the large aggregates (Goss et al., 1988, 1990). In spring, after fertiliser has been applied, this rapid by-pass ¯ow carries nitrate from fertiliser on the soil surface preferentially to the drains in direct drilled soil, resulting in greater losses of nitrate than for ploughed soil (Goss et al., 1990, 1993).
3. Model development
A number of models of differing types and com-plexities were considered for testing against data from Brimstone Farm. Because of the overriding in¯uence of macro-pores on the movement of water and con-sequently nitrate through the soil, models which do not take macro-pore ¯ow into account typically fail to reproduce the behaviour observed at the site. One model which was considered to be appropriate was CRACK (Jarvis and Leeds-Harrison, 1987a,b; Arm-strong et al., 1997).
The CRACK model was speci®cally designed to simulate water movement in a cracked clay soil and so contains an explicit description of the arrangement of soil aggregates. Jarvis (1989a) described the addition of a conserved solute to the model. In CRACK, down-ward movement of water and solute through the soil
matrix is assumed to be negligible, with the macro-pores forming the dominant ¯ow pathway. The aggre-gates act as temporary stores of water and solute by lateral transfer from the macro-pores. This treatment of the soil structure is highly applicable to the situation observed at Brimstone Farm. This advantage, how-ever, is bought at the price of loss of generality: because it ignores water ¯ow in the matrix, this model can only be used for such highly structured soils. The CRACK model also has the advantage that different tillage treatments can be modelled by varying the size of the aggregates in each layer. For these reasons, CRACK was chosen in this study to simulate water ¯ow and nitrate leaching at the Brimstone Farm site.
3.1. Model description
In CRACK, the soil is represented as layers of cubic aggregates separated by planar cracks (Fig. 1). The critical parameter which determines the structure in each layer is the crack spacing; large values of crack spacing give large ped sizes and a correspondingly coarse structure while a ®ne structure can be obtained by keeping the crack spacing small.
In the model, soil structure parameters are depen-dent on the water content of the soil. The volume of cracks is a function of the difference between the initial water content in each layer and ®eld capacity. In all situations, individual peds are assumed to shrink
according to a swelling function described by the slope of line between void ratio and volumetric water content. CRACK assumes that a certain amount of ®ssuring still exists in a fully swollen soil at its ®eld capacity water content (the stable crack volume).
In®ltration into peds at the soil surface is calculated using Philip's in®ltration equation (Philip, 1957), and macro-pore ¯ow starts when the rainfall intensity exceeds the in®ltration capacity of the surface aggre-gates. Water uptake into peds can then take place through the internally wetted surfaces, at a rate gov-erned by the same in®ltration equation. However, even during prolonged rainfall or irrigation, only a small proportion of the total ped surface area is wetted, because of point source inputs at the surface and/or pore necking within the soil. Observations by Bouma and Dekker (1978) using dye tracers suggest that only 5% of the internal surfaces carry water at any one time. The height of the water table in the cracks is controlled by the balance between the input at the soil surface and the loss due to ped water uptake within the soil and drain out¯ow. The rate of rise or fall is also a function of the crack volume in the layer in which the water table is located. If the water table rises to the surface, any further rainfall input is immediately lost as surface runoff.
Simulation of the transport of a conserved solute was introduced in the 1989 version of the model (Jarvis, 1989a). Solute movement occurs by mass ¯ow in the macro-pore water or by diffusion into and out of the soil peds. Mass transport is calculated from the amount of water ¯owing in the cracks and the known solute concentration in the water. Diffusion is calcu-lated using the method of Addiscott (1982), whereby each ped is divided into concentric segments of equal volume and Fick's Law is used to give the rate of solute diffusion between them.
A simple description of the development of an annual crop is included in the model. Both crop height and root depth increase at a constant rate after emergence up to a maximum value when the crop reaches its maximum leaf area. Crop height is zero both before emergence and after harvest, whilst root depth is set to an effective minimum to allow for an estimation of bare soil evaporation. Crop water uptake is based on the concept of a weighted stress index (Jarvis, 1989b) which effectively allows the crop to react to stress in one part of the root system
by increasing uptake from other parts where condi-tions are more favourable.
3.2. Inclusion of nitrogen routines Ð the CRACK-NP model
To model the over-winter nitrogen balance of the soil it was necessary to include the mineralisation of soil organic nitrogen and nitri®cation of ammonium to produce nitrate within the existing CRACK model. The resulting model is known as CRACK-NP. The model uses a simple representation of the over-winter nitrogen budget, derived from the SACFARM model of Addiscott and Whitmore (1987). Nitrogen is pre-sent in all layers of the soil, and is removed by leaching and crop uptake, and is gained by miner-alisation and nitri®cation of ammonium. No account is taken of denitri®cation. The model is thus applicable only to the prediction of over-wintering losses of nitrate, and cannot (as yet) be used for the detailed examination of nitrate fate over multiple years.
The processes of mineralisation and nitri®cation are governed by equations used in the SACFARM nitro-gen budget model of Addiscott and Whitmore (1987). The rate of mineralisation (km) is given by
kmk20eÿB T
ÿ1ÿ293ÿ1
where k20 is the mineralisation rate at 208C in the
topsoil (mg/m3),Tthe absolute soil temperature andB
a constant. This rate is assumed constant in the topsoil but decreases exponentially with depth in the subsoil. The rate of nitri®cation (kn) is given by
knaAgmbAgm 1=2
whereAmis the ammonium concentration anda,band
gare constants described by Addiscott and Whitmore
(1987). This rate is adjusted for volumetric water content and soil temperature.
The model also calculates the amount of nitrogen taken up by the crop. In keeping with the simple crop model used in CRACK, the total nitrogen in the crop is assumed to increase linearly with crop height. This de®nes the daily amount of nitrogen the crop needs to take up from the soil. The model then calculates the amount of nitrogen in each soil layer that is root-accessible, assuming that the root distribution decreases exponentially with depth (Gerwitz and Page, 1974). The nitrogen required is then extracted from each soil layer in proportion to the amount available, and is partitioned between nitrate and ammonium pro rata.
The remaining component of the nitrogen balance is loss of nitrogen by denitri®cation. This can be sig-ni®cant over the winter period when the soil is wet and anaerobic conditions exist and also in spring following the application of fertiliser nitrogen to the soil surface (Colbourn et al., 1984). At present, however, denitri-®cation is not included in the CRACK-NP model.
3.3. Model operation
The CRACK-NP model is written inFORTRAN and
will run satisfactorily on a 486 PC with coprocessor. Rainfall and potential evapotranspiration are required as inputs to the model. If these are supplied on a daily basis, a separate program is used which estimates hourly data. This uses a regression relationship between daily rainfall totals and peak rainfall rates to de®ne a triangular hyetograph with the estimated peak intensity and the same rainfall amount. Hourly evapotranspiration is distributed as a sine wave during the hours of daylight (Armstrong et al., 1994). The user must also specify a value for the internal model time step bearing in mind that numerical instabilities will occur if too large values are chosen. A basic time step of the order of 0.1 h is generally appropriate. For days with no rainfall, the speci®ed time step is multi-plied by a factor of 10, to speed-up execution times.
4. Model validation
A ®rst test of the CRACK model attempted to reproduce the short-term behaviour of water and solute at Brimstone Farm. This test was conducted by comparing model results with those observed from
a single plot (ploughed and drained) for a 15 day period in December 1989. Plot 6 was used as it was considered to have the most representative drainage treatment (Pepper, personal communication). The model used the 1989 version of the program (Jarvis, 1989a), in which the solute was considered to be non-reactive and conserved. It was considered that nitrate could be modelled as if it were a conserved solute for a short period in winter, during which soil temperatures were generally low and both mineralisation and nitri-®cation rates would also be low. The parameters used to derive the results were, as far as was possible, physically measured in the ®eld, and no retrospective calibration or ®tting was undertaken. Mean ped size (and hence also mean crack spacing) was measured using a combination of detailed soil pro®le description (using standard surveying techniques) and the in®ltra-tion of dilute Plaster of Paris to investigate the in®l-tration locations. In order to maintain the integrity of the experimental areas, these were undertaken in the discard areas, and so relate to the site in general and not to any one speci®c plot. The values of the mean crack spacing that were adopted from the combination of direct measurement and in®ltration investigation were 10 cm for the topsoil (0±20 cm) and 20 cm in the layer beneath (20±40 cm).
The results of this test show a good ®t for both the hydrology and the pattern of solute movement (Fig. 2). Both the drain discharge and depth to the water table in the macro-pores are reproduced very well by the model, so there is no doubt that the model has repro-duced the main features of the hydrology of the site. The simultaneous prediction of several components of the model, while using ®eld measured and not ®tted parameters, indicates that the model is indeed appro-priate to the ®eld situation. It is often possible to get a model to ®t the behaviour of one variable by adjusting parameters, but rarely possible to do so with multiple variables as we have here. The results also predict the nitrate concentrations in the drain discharges with a fair degree of accuracy. Although the model fails to predict the initially very high nitrate concentrations, this is considered to be due to the lack of speci®c ®eld data to specify the initial conditions, and does not represent a failure of the model.
Fig. 2. Results of running CRACK-NP using the data from Brimstone Farm Plot 6 (ploughed and drained) for a short period in winter 1989.
ability of CRACK to reproduce this behaviour is taken as excellent con®rmation of the validity of the model. Model results for an entire winter (1985±1986) are shown in Fig. 3, alongside data from Plot 7 at Brim-stone. The parameters used to generate this run are listed in Appendix A. Parameters derived from ®eld measurement are not adjusted, so there is no retro-spective ``calibration'' of the model. Although there were no hourly measurements of water table depth available for the simulation period, the modelled water tables show how CRACK-NP generates drainage in response to a build up of the water table in the soil macro-pores. It can be seen that the timing and magnitude of these drain¯ows agree quite well with the observed values for most of the simulation period. The nitrate concentration results show a larger difference between the model results and the ®eld data than do the drain¯ow results. The measured values show an initial ¯ush of nitrate from the soil pro®le with the ®rst winter drain¯ows. This depletes the store of readily available nitrate, and leaching thereafter continues at a lower but more steady con-centration. The model, on the other hand, suggests that the nitrate leaches at a fairly steady concentration which is reduced by dilution effects during drain¯ow events, and shows no ¯ush of nitrate at the start. The lack of a denitri®cation component in the model could also partly explain why the modelled nitrate concen-trations are higher than the observed for the latter half of the simulation period.
5. Use of the model to identify the effects of tillage
CRACK-NP can be used to suggest the effects of different tillage treatments by changing the values for the crack spacing in the top 20 cm of the soil pro®le, whilst keeping other parameters constant. By selecting suitable values for crack spacing, the observed differ-ences in topsoil structure between ploughed and direct drilled plots can be represented in the model. Although not directly measured, many visual descrip-tions of the ped sizes as related to the alternative tillage operations were available, and these were used in combination with the measured values for the discard areas, to give estimates of the possible range of ped sizes that might be anticipated as a consequence of different styles of tillage.
The sensitivity of CRACK-NP to changes in the value of the crack spacing parameter, and hence to ped size and diffusion path length, was tested by running the model for the period September 1985 through to early February 1986 at Brimstone Farm for several values of crack spacing. Over this period, N transfor-mations become important and it is necessary to provide an estimate of the soil mineral-N at the start of the model run. No measured soil mineral-N values were available, so an estimate of the residue left by the previous crop (oilseed rape, Brassica spp.) was made from the known performance of this cop and soil type. The value used was 160 kg N/ha split between nitrate-N and ammonium-nitrate-N in each model layer as shown in Appendix A.
The results (Table 2) show that an increase the size of the crack spacing results in a small increase in total drain¯ow over the simulation period but gives a much more marked decrease in the total amount of nitrate leached. This is due to the reduction in the contact area between macro-pore water and ped surfaces and the more rapid transport of water through the macro-pores as the size is increased, resulting in less opportunity for diffusion of nitrate from the peds into the macro-pore water.
The model predictions were then compared against measured data for the two tillage treatments, with crack spacings in the topsoil of 10 and 20 cm used in the model to represent ploughing and direct drilling, respectively. The comparison between measured values and model results are shown in Table 3. The model produces good estimates of the total drain¯ow over the period, agreeing with the validation runs, which suggested that the model could reproduce the hydrology of the site. The results for total nitrate
Table 2
leached also show good agreement with the observed data and re¯ect the differences between the tillage treatments. The inclusion of N-transformations into the model was important in achieving these results, with the model predicting 50 kg N/ha of mineralisa-tion and 15 kg N/ha of crop nitrogen uptake over the simulation period, giving a net input of 35 kg N/ha from the soil organic-N pool. The estimate for nitri-®cation of ammonium-N to nitrate-N was 70 kg N/ha. These results do not take into account denitri®ca-tion as this process was not included in the model. Denitri®cation losses at Brimstone Farm have been estimated at 3 kg N/ha per year for drained, ploughed plots and 8 kg N/ha per year for drained, direct drilled plots (Catt, 1991). However, it is dif®cult to estimate what proportion of these losses would have occurred over the simulation period.
6. Discussion
It has been shown that by choosing representative values of crack spacing in the CRACK-NP model, the differences in leaching of nitrate-N that are observed between soil which has been ploughed and direct drilled can be reproduced. However, although at pre-sent CRACK-NP gives good estimates of the total amounts of leaching, it does not completely reproduce the patterns of leaching which are observed. In parti-cular, the ¯ush of nitrate which occurs with the ®rst winter drain¯ow (Rose et al., 1991) is poorly simu-lated by the model. This is most likely due to the nitrate distribution in the model aggregates not corre-sponding to the situation in the ®eld when this ®rst drain¯ow occurs. In a theoretical analysis of the effect of soil aggregation on water and solute transport, Youngs and Leeds-Harrison (1990) suggested that during periods when both the macro-pore and micro-pore regions are unsaturated, evaporation from
the aggregate surfaces causes nitrate in the aggregate to be drawn outwards and accumulate there. This is likely to have occurred over the autumn period in the ®eld, leaving nitrate at or near the aggregate surfaces which could then have been readily leached with the ®rst ¯ush of water. At present CRACK-NP cannot reproduce this effect as evaporation is assumed to extract water from the whole aggregate, not just the surface, resulting in an even distribution of nitrate across the ped. However, further development of the model will allow this process to be taken into account.
Acknowledgements
The experimental site at Brimstone Farm is a joint ADAS/RES experiment. The ®nancial support for the site from the Ministry of Agriculture Fisheries and Food, and for the modelling from the AFRC/NERC Joint Initiative on Pollutant Transport in Soils and Rocks is gratefully acknowledged.
Appendix A. Parameter ®le for plot 7 (Fig. 3)
04/09/85 Start date
06/02/86 End date
0.1 Time interval (h)
8 No. of layers
0.6 Drain depth (m)
2.0 Drain spacing (m)
0.2 Plough depth (m)
0.55 Initial depth to water table (m)
2 Tortuosity factor
12 Ped sorptivity at WP (mm/h^0.5)
40 Hydraulic conductivity (mm/h)
50 Critical soil water depletion (%)
5 Critical soil air content (%)
1.12 Bulk density topsoil (kg/m3)
Table 3
Comparison between CRACK-NP predictions and observed data for the simulation period 4 September 1985 to 6 February 1986a
Tillage Crack spacing (cm) Drainflow (mm) Nitrate-N leached (kg N/ha)
Measured Simulated Measured Simulated
Plough 10 147.2 142.1 33.33 30.79
Direct drill 20 145.9 144.6 26.61 26.72
1.40 Bulk density subsoil (kg/m3)
20/01/85 Emergence date
20/06/86 Maximum leaf area date
20/08/86 Harvest date
0.1 Initial root depth (m)
1.0 Maximum root depth (m)
60 Root distribution factor
0.2 Root adaptability factor
5 Interception capacity (mm)
1.5 Correction factor (interception)
0.0 Nitrate concentration in rain (kg/m3)
10 No. of segments
1.0E-10 Diffusion coefficient (m2/s)
0.5 Impedance factor
34 Crop N at max leaf area (kg/ha)
0.24 Mineralisation rate (mg/mm per day)
0.004 Decrease of min/mm below plough
depth
0.32 Nitrification rate factor
0.1363 Nitrification rate factor
1 Ammonium-N inaccessible to nitrifiers
246 Maximum ammonium-N for
nitrifica-tion
1 Number of fertiliser-N applications
30/04/86 Application date (day/month/year)
130 Application amount (kg N/ha)
50 Percentage NH4-N in application
Addiscott, T.M., Whitmore, A.P., 1987. Computer simulation of changes in soil mineral nitrogen and crop nitrogen during autumn, winter and spring. J. Agric. Sci. 109, 141±157. Armstrong, A.C., Addiscott, T.M., Leeds-Harrison, P.B., 1995.
Methods for modelling solute movement in structured soils. In: Trudgill, S.T. (Ed.), Solute Movement in Catchment Systems. Wiley, Chichester, UK, pp. 133±161.
Armstrong, A.C., Harris, G.L., 1996. Movement of water and solutes from agricultural land, the effects of arti®cial drainage. In: Anderson, M.G., Brooks, S.M. (Eds.), Advances in Hillslope Processes, Vol. 1. Wiley, Chichester, UK, 1996, pp. 187±211. Armstrong, A.C., Matthews, A.M., Portwood, A.M., Addiscott,
T.M., Leeds-Harrison, P.B., 1994. Modelling the effects of climate change on the hydrology and water quality of structured soils. In: Rounsevell, M.D.A., Loveland, P.J. (Eds.), Soil Responses to Climate Change, NATO ASI Series, Vol. 123. Springer, Berlin.
Armstrong, A.C., Matthews, A.M., Portwood, A.M., Jarvis, N.J., Leeds-Harrison, P.B., 1997. CRACK-NP a model to predict the
1 2 3 4 5 6 7 8 Layer
0.05 0.05 0.1 0.1 0.1 0.2 0.2 0.2 Layer thicknesses (m)
55 55 55 55 55 55 50 50 Total porosity (%)
48 48 48 52 52 50 46 46 Field capacity (%)
42 42 42 46 46 45 43 43 Initial water content
5 5 5 2 2 1 1 1 Stable drainable porosity (%)
27 27 27 35 35 35 30 30 Wilting point (%)
0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 Crack spacing (m)
0 0 0 0.5 0.5 0.8 1.0 1.0 Shrinkage characteristic
10 10 20 20 20 40 40 40 Initial total N in layer (kg/ha)
8.4 8.4 16.8 16.8 16.8 33.6 33.6 33.6 Initial total nitrate in layer (kg/ha)
1.6 1.6 3.2 3.2 3.2 6.4 6.4 6.4 Initial ammonia in layer (kg/ha)
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