T. M. Addiscott

Computer simulation of changes in soil mineral nitrogen and crop nitrogen during autumn, winter and spring

The computer model described simulates changes in soil mineral nitrogen and crop uptake of nitrogen by computing on a daily basis the amounts of N leached, mineralized, nitrified and taken up by the crop. Denitrification is not included at present. The leaching submodel divides the soil into layers, each of which contains mobile and immobile water. It needs points from the soil moisture characteristic, measured directly or derived from soil survey data; it also needs daily rainfall and evaporation. The mineralization and nitrification submodel assumes pseudo-zero order kinetics and depends on the net mineralization rate in the topsoil and the daily soil temperature and moisture content, the latter being computed in the leaching submodel. The crop N uptake and dry-matter production submodel is a simple function driven by degree days of soil temperature and needs in addition only the sowing date and the date the soil returns to field capacity, the latter again being computed in the leaching submodel. A sensitivity analysis was made, showing the effects of 30% changes in the input variables on the simulated amounts of soil mineral N and crop N present in spring when decisions on N fertilizer rates have to be made. Soil mineral N was influenced most by changes in rainfall, soil water content, mineralization rate and soil temperature, whilst crop N was affected most by changes in soil temperature, rainfall and sowing date. The model has so far been applied only to winter wheat growing through autumn, winter and spring but it should be adaptable to other crops and to a full season. The model was validated by comparing its simulations with measurements of soil mineral N, dry matter and the amounts of N taken up by winter wheat in experiments made at seven sites during 5 years. The simulations were assessed graphically and with the aid of several statistical summaries of the goodness of fit. The agreement was generally very good; over all years 72% of all simulations of soil mineral N to 90 cm depth were within 20 kg N/ha of the soil measurements; also 78% of the simulations of crop nitrogen uptake were within 15 kg N/ha and 63% of the simulated yields of dry matter were within 25 g/m2 of the amounts measured. All correlation coefficients were large, positive, and highly significant, and on average no statistically significant differences were found between simulation and measurement either for soil mineral N or for crop N uptake.

Partitioning losses of nitrogen fertilizer between leaching and denitrification

When 15 N is used to trace the fate of N fertilizer applied in spring to winter wheat crops, some is not recovered in the crop or the soil and has to be presumed lost. In 13 experiments made from 1980 to 1983 on three widely differing soils, these losses ranged from 1 to 35%. We partitioned them between leaching and denitrification by using models to estimate the loss by leaching, talcing into account the N absorbed by the crops, and subtracting this loss from the total loss to obtain the apparent percentage loss by denitrification, L DN . An analysis of variance showed that L DN increased significantly with the quantity of N applied, so the study considered LDN values for a standard N application of 150 kg/ha subsequently. Regressions showed that L DN was better related to the wetness of the soil during the 3 weeks after fertilizer application than to the corresponding amount of rain, as would be expected for denitrification. Values of L DN could not, however, be satisfactorily related to soil temperature, probably because the range of temperatures was too narrow. The apparent losses by denitrification were, on average, nearly twice as large as those by leaching, but the ratio varied greatly between experiments.

Winter leaching of nitrate from autumn-applied calcium nitrate, ammonium sulphate, urea and sulphur-coated urea in bare soil

In a flinty clay loam at Rothamsted, nitrate concentrations in 0–13 and 13–26 cm layers of plots given all N-sources at 100 kgN/ha in early October were very small by mid-January. Incorporating the fertilizer in the first 13 cm slightly accelerated this loss. Sulphur-coated urea (SCU) maintained smaller nitrate concentrations than other sources in both layers. By early March SCU plots alone had slightly larger nitrate concentrations than the controls in the 0–26 cm layer, whilst in the 26–52 cm layer all N plots had slightly larger concentrations than the controls. Spring barley, given no more N and harvested green at ear emergence, took more N from all N plots than from the controls, most from ammonium sulphate and least from urea and SCU, but differenoes between sources were not significant. The nitrate loss had a negligible effect on soil pH and exchangeable cations. Calcium nitrate leaching data were used to test the equation of Burns (1975) and other simple equations which considered the effects of successive percolations in a two-layer system assuming that the layers either could or could not become temporarily oversaturated. All the equations underestimated leaching unless the most inaccessible soil water was left out of the calculations and gave best results when only gravitational water was taken into acoount.

Potassium in soils under different cropping systems: 3. Non-exchangeable potassium in soils from long-term experiments at Rothamsted and Woburn

Soils from long-term experiments at Rothamsted and Woburn were cropped for very long periods (up to 5½ years) with ryegrass in pots. Measurements of the potassium taken up by the ryegrass that was not exchangeable to ammonium acetate and the kinetics of its release both suggested two categories of non-exchangeable K. Of these, the first to be released was closely related to the initial exchangeable K, whilst the second, though partly related to the initial exchangeable K was also influenced by the clay percentage. Release of both categories may have been controlled by diffusion, because both showed good relationships between the quantity released and time. It is suggested that the first category may be K ‘trapped’ when K fertilizer added in the field decreased the interlamellar spaces of vermiculite layers in clay particles, whilst the second may simply be the ‘native’ K (described by others) present in clay and other minerals in the soil. Resowing the soils (without drying them) during the later stages of K. uptake suggested that the ability of the old ryegrass to absorb K was not a factor limiting K uptake even after long growth. When the ryegrass ceased to grow, the mean K potentials in the exhausted soils were close to the ‘uptake potential’ for ryegrass derived earlier by considering K uptakes from soils in relation to the quantity/potential relationships of the soils. Drying and rewetting the exhausted soils released K; the amount was influenced in one group of soils by the exchangeable K in the moist exhausted soil and in another group by the clay percentage.

The potassium Q/l relationships of soils given different K manuring

The potassium quantity/intensity ( Q/I ) relationships, which relate change in exchangeable K content ( Q ) to change in activity ratio were measured in soil samples from manuring experiments at Rothamsted and Woburn. Within each experiment, Q/I curves for different K-manuring treatments were super-imposable on each other and on the curve relating exchangeable K to I o , the activity ratio at which the soil neither gains nor loses K. The distances on the Q axis between the curves were equal to the differences in exchangeable K. The buffer capacity, dQ/dI , was related to the K saturation of the cation exchange capacity (CEC) by the equations (Q and CEC in m-equiv/100 g) Broadbalk and Hoosfield soil, and for Barnfield soils ( b = 3·08; m = 1·0) and for Woburn market garden soils ( b =2·41; m = 0·6) but for soils from other Woburn experiments, dQ/dl did not vary significantly with Q/CEC.

Potassium in soils under different cropping systems:2. The effects of cropping systems on the retention by the soils of added K not used by crops

The K balance, the difference between K added as fertilizer or farmyard manure (FYM) and K removed by the crops, was calculated for soils from the Classical and Ley-Arable experiments at Rothamsted and for the Woburn Ley-Arable experiment, for the duration of each experiment. Linear regressions on K balance accounted for 78% of the variation in exchangeable K (K e ) and for 83% in K uptake by ryegrass (K P ) in the Classical experiments, for 56 and60% respectively in the Ley-Arable experiments at Rothamsted, and for 39 and 6% in the Woburn Ley-Arable experiment. Regressions of K e and K p on K balance suggested that, in the Rothamsted Ley-Arable experiments, rather more than half of the K balance remained extractable by ryegrass from the plots with a rotation of crops, and apparently all of the K balance from those under continuous grass. About one-fifth of the K balance remained extractable by ryegrass from the soils in the Rothamsted Classical experiments and soils given FYM retained K slightly better than other soils. With all soils about half the K extractable by ryegrass was exchangeable to ammonium acetate. The plots with FYM or under continuous grass contain more organic matter than other plots in the same experiments. The following possible effects of increasing the organic matter content of the soils were investigated by calculating the multiple regressions of K, and KB on K balance with either percentage of organic C, total CEC, or organic CEC: (1) loss of K decreased by increasing the water retention and lessening leaching; (2) improved K retention by increasing the total cation exchange capacity (CEC) available for K absorption; (3) improved K retention by a mechanism arising from the different selectivities of clay and organic matter for K relative to Ca. In the Classical experiments, where organic matter usually increases because of FYM additions, effect (2) seems the most probable, perhaps because the K given in the FYM was already absorbed by organic exchange sites. In the Ley–Arable experiments, where the K was given mainly as soluble K fertilizer and the organic matter develops mainly under grass, effects (1) or (3) seemed to operate, probably simultaneously. The Woburn Ley-Arable experiment had no continuous grass plots, the soils differed little in organic matter content and no deductions could be made.