Wheat, barley and oats were grown in undrained plastic buckets containing soil collected from upper, mid and lower slopes of a valley that was subject to winter waterlogging. Two weeks after planting, the water content for each soil was either maintained at 80 per cent of full water holding capacity or subjected to intermittent or continuous waterlogging for six weeks. In a second experiment, using lowerslope soil only, the same three cereals were subjected to similar waterlogging treatments commencing at two or six weeks after planting or at ear emergence. In this experiment the plants received either no nitrogen fertilizer or 100 kg nitrogen ha-1. Both soil composition and waterlogging had a significant effect on plant yield but the effect of waterlogging was much greater. Waterlogging reduced root growth and penetration, the production of tillers and fertile heads, and delayed ear emergence and plant maturation. Reduction in plant growth rate was measurable within three days from the onset of waterlogging. In the first experiment intermittent and continuous waterlogging reduced vegetative growth yield (mean of three cereals in three soils) by 37 per cent and 55 per cent respectively; and wheat grain yields by 40 per cent and 53 per cent respectively. However, there was no differential effect of the two waterlogging treatments on the grain yield of barley and oats, the mean reduction being 39 per cent for barley and 48 per cent for oats. In the second experiment waterlogging at the earliest growth stage resulted in the greatest reduction in root, herbage and grain yield. Waterlogging at ear emergence killed some tillers and roots and reduced the plants stability at maturity. Grain size was reduced in some treatments. Application of nitrogen fertilizer compensated, either partially or fully, for reduction in grain yield due to waterlogging treatments on all three cereals. Some reasons for yield reduction in the three species and the practical implications of the results are discussed.
A wheat crop was grown on a nitrogen-deficient sandy soil. Urea was supplied at rates of 0 (N0), 56 (N1), and 336 (N2) kg nitrogen/ha. In general, the relative growth rate (R) decreased with time. During the first half of the growing season R for N2 > N1 >> N0, but in August deficient plants recovered rapidly. The recovery was associated with the emergence of the second tiller, an increase in net assimilation rate and nitrogen uptake, and a decrease in nitrogen stress. There was no evidence that the recovery was due to an increase in mineralization of soil nitrogen. Nitrogen stress in treatment N0 decreased from 48% in July to 14% in September. By contrast, stress in treatment N1 increased rapidly from 6 to 23% during July and fell to 5% by September. Flower initiation in N0 plants was delayed 14 days and ear emergence was delayed 8 days compared with N1 plants. The grain yields of N0 and N1 plants were 30 and 60% respectively of those of N2 plants. The decrease was due mainly to differences in the number of heads per sq metre, although nitrogen deficiency also reduced the number of grains per head and the weight per grain. The duration of photosynthetic activity after anthesis was not affected by nitrogen treatments. Harvest index was highest on N0 treatment and similar on N1 and N2 treatments.
Nitrogen loss from sheep urine was measured on two soil types under different surface cover and moisture conditions at a location with high summer temperatures. Some of the factors influencing nitrogen loss were studied in pots and lysimeters. Grass plants utilized almost half the nitrogen applied in urine. Loss of nitrogen by volatilization and leaching was considerably less under a grass cover than on bare soil. When urine was applied during the hot summer months, there were large losses (50 per cent) of nitrogen even under a grass cover. During the summer, rewetting of urine patches to simulate rainfall increased the amount of nitrogen lost. Eighty per cent of the urine nitrogen was lost after three wettings. Frequency of wetting was more important than volume of water applied. Urine application markedly increased the pH of the soils over a long period.
Loss of nitrogen from maturing subterranean 'clover and soft brome plants was measured in both field and pot experiments. In subterranean clover, nitrogen loss commenced after seed setting. In soft brome grass, the loss began at flowering and continued until senescence. Nitrogen loss from herbage may be due to translocation to burrs and seeds, to leaching by rain and dew or to volatilization to the air. The practical implications of this nitrogen loss in terms of animal production are discussed.
Ammonium sulphate was applied to a grazed pasture on a duplex soil for five years at annual rates of nil (N0), 280 kg ha-1(N1) and 840 kg ha-1(N3). N0 became clover dominant and N3 became grass dominant but net increase in nitrogen in the top 10 cm of soil over five years was similar on N0, N1 and N3. Fertilizer nitrogen was rapidly lost after heavy rains in autumn. On all treatments, soil accumulated inorganic nitrogen in summer. A balance sheet for sulphur could not account for 48 per cent of sulphur in the top 10 cm on N0 (superphosphate only), 82 per cent on N1 and 90 per cent on N3. Ammonium sulphate decreased pH, exchangeable calcium, magnesium and potassium (but not sodium), and cation saturation, and increased exchangeable acidity. Apart from pH, these effects were confined to the top 10 cm of soil. The results show that in a sandy-surfaced soil ammonium sulphate is an inefficient source of nitrogen and sulphur because nitrogen and sulphur are readily lost.
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