In Britain, Boyd and Dermot (1964) concluded from 124 fertilizer exDeriments with potatoes at different sites that responses to N were more dependent on seasonal variation than on soil. Greenwood et al. (1974) found that maximum yields of lettuce were achieved at 50 kg/ha in one year and at 300 kg/ha in another even though the experiments were carried out on adjacent sites. Working with spring wheat, McDowall (1972; (1961) reported that by heading, all of the dry matter and plant N has been taken up by well fertilized wheat. They suggested that the N in the grain was derived primarily by translocation from leaves, stems and chaff rather than from further absorption from the soil. Spratt and Gasser (1970) found that drought caused the extra N derived from the fertilizer to remain in the straw at harvest. Sosulski et al. (1966 have suggested that N fertility is the principal factor <:ontrolling the protein in grain, while soil mr:isture was more important in controllingJ yields; but Hutcheon and Rennie (1960)
Effects of length of cultivation, soil texture, and the presence of a water table on soil nitrogen was investigated in semiarid southwestern Saskatchewan. Cores from as deep as 8 m were taken in 1974 from fields that had been sampled in 1939 when they were broken, and in 1953 after they had been cropped for 14 yr to a wheat–summer-fallow rotation. Virgin soils were almost devoid of nitrate. After cropping for 14–18 yr, nitrate accumulated in the subsoil. After 35 yr, nitrate was still accumulating in the Sceptre clay, but the nitrate bulge had disappeared from the Wood Mountain loam leaving the nitrate uniformly distributed in the subsoil. Only 43% of the nitrate that was present in the Wood Mountain profile after 14 yr of cultivation was still present after 35 yr. This loss could be due to subsoil nitrate being leached beyond the sampling depth, or the result of periodic bidirectional movement of nitrate. Nitrate accumulated at two depths in the Sceptre clay. The accumulation at the lower depth was ascribed to precipitation entering the clay through large cracks before wetting the subsoil, and the upper accumulation to infiltration through the soil surface. In the first 14 yr of cultivation, total nitrogen was lost from the Wood Mountain loam at an average rate of 1.8%/yr (relative to the virgin soil); in the following 21 yr the rate was 0.65%/yr.
kg P/ha applied. The NaHCO3-extractable phosphorus level of the i00 kg P/ha treatment after 8 yr of cropping was reduced to about 8 kg P/ha which is considered to be inadequate for crop production. Soils treated with 200 and 400 kg P/ha contained high levels of NaHCO3-extractable P (20-54 kg P/ha) and little response in yield to additional P would be expected in the near future. Annual variation in yield and phosphorus uptake by the crops were affected by water supply. In years of high water supply, yield and phosphorus uptake were generally higher than in years of low water supply.
The effects of three rates of N fertilizer and nine fortnightly dates of initial harvest on the N, P, Ca, Mg and K concentrations and the K/(Ca + Mg) ratio of Russian wild ryegrass (Elymus junceus Fisch.), Altai wild ryegrass (Elymus angustus Trin.) and crested wheatgrass (Agropyron desertorum Fisch. Schult.) were studied on irrigated land for 2 yr. The N, P, K and K/(Ca + Mg) ratios declined with increasing maturity. The N, P, and K concentrations tended to increase with N fertilization, while N fertilization appeared to have little influence on percent Ca, percent Mg and the K/(Ca + Mg) ratio. Altai wild ryegrass had a higher N, P, Ca, Mg, K and K/(Ca + Mg) ratio than RWR which, in turn, was higher than CWG. Crested wheatgrass utilized prior to early June, RWR utilized prior to late June, and AWR utilized prior to early July had the suggested minimum level for lactating beef cows of 1.60% N. Only samples cut during May equalled the suggested minimum of 0.32% P. Supplementation with N and P may be required with rations or pastures using these grasses in June or later. The percent K was adequate on all sampling dates. The K/(Ca + Mg) ratio for CWG did not exceed the suggested maximum of 2.2, but the ratio for AWR and RWR was above this maximum for May, June and part of July, indicating there may be some danger of grass tetany with these grasses during the May-June period and supplementation with Mg and/or Ca may be required for good performance.
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