An easy-to-use simulation model was developed with the aim of improving fertilizer practice when crop residues are incorporated instead of removed. It was tested against data from a well-monitored N fertilizer experiment in which three successive brassica crops were grown followed by barley. Experimental findings included: (a) that fertilizer-N greatly increased yield of 3 crops without increasing residual soil mineral-N at harvest unless supply exceeded crop demand; (b) that, by contrast, fertilizer-N increased both yield of and residual soil mineral-N left by the remaining crop throughout the range of applications; and (c) that at each harvest the apparent disappearance of fertilizer-N by immobilization and other processes was almost proportional to fertilizer-N. These phenomena were simulated by the model. Overall the model gave estimates of soil mineral-N, plant weight and O/ o N in the crop for each crop that were either in close agreement with or linearly related to the measured values. Deviations from this pattern are shown to result almost entirely from experimental error. In addition the model gave simulations of the time course of soil mineral-N and soil water that were in good agreement with measurement.Simulations with the model indicate that appreciable benefits from residue incorporation of crops will only be obtained when fertilizer-N is also applied, unless plant masses at harvest are small.
The effects on succeeding crops of nitrogen in residues returned
to the soil of brassica vegetable crops
(Brassica oleracea) were studied on a nitrogen-retentive silt
loam soil at Horticulture Research
International, Kirton, Lincolnshire, UK. A sequence of four crops was
started in 1988 and again in
1989. In the first sequence, two successive cauliflower crops
(Brassica oleracea cv. botrytis L.), crops
1 and 2 in the first year, were followed by Brussels sprouts
(Brassica oleracea cv. Gemmifera D.C.),
crop 3, in the second and spring barley (Hordeum vulgare),
crop 4, in the third year. The second
sequence, started in spring 1989, was on an area adjacent to the first,
but with spring wheat (Triticum aestivum) as crop 4. The
sites followed an unfertilized, 1-year grass ley (1987) or spring barley
(1988)
with 73 and 107 kg N/ha soil mineral nitrogen (SMN, NH4+NO3)
in the 0–90 cm soil profile at the
start of each sequence. The marketable yield of the first cauliflowers
in both sequences increased with
fertilizer nitrogen up to 240 or 300 kg/ha. The response of the
second cauliflower crop to fresh N
declined with increasing amounts of SMN (0–90 cm) at planting, with
no response when SMN
exceeded c. 270 kg N/ha. Crop 3 apparently responded to
fresh N in sequence 1 but not in sequence
2 even though SMN at planting ranged up to c. 400 kg N/ha.
This difference in response was largely
explained by the amount and distribution of SMN in the 0–30 and
30–60 cm layers when the sprouts
were planted. Soil mineral N to 90 cm when the cereals were sown was
only c. 100 kg N/ha, which
did not reflect the large amounts of N applied to the previous
crops. In spite of this, barley yield
without any fresh fertilizer N did vary with the amounts of N applied
to the previous crops. The
results showed that SMN was a useful predictor of fertilizer response in
some, but not all, situations.
To use SMN more generally requires interpretation using dynamic simulation
models.
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