The Effect of N Fertilizer Placement and Timing on Soil Profile Dynamics of Available Phosphorus and Exchangeable Potassium at Different Growth Stages of Spring Wheat ( Triticum aestivum L. cv. Spectrum) on Leached Chernozem

摘要

The maintenance of optimum levels of nitrogen (N), phosphorus (P) and potassium (K) in the soil encourages vigorous growth of spring wheat root systems, which in turn significantly enhances uptake of other nutrients in the soil. A study was carried out to determine the effect of N fertilizer placement and timing on accumulations and distributions of available P and exchangeable K in the soil profile at different phenological stages of the wheat crop at Krasnodar Agricultural Research Institute in Krasnodar County (45°5`N, 38°50`E, >400 m elev.) in Eastern Europe. The experiment was designed as Randomized Complete Block with four replicates, which were subjected to six N fertilizer treatments (T1-T6). Spring wheat was grown under rainfed conditions with treatments varying in N fertilizer placement and timing. N treatments (T2-T6) triggered an available P bulge of 2.7-6.4 mg 100 g-1 soil when compared with accumulations in the control plots during the first season. In the second season, available P build-ups of 2.7-6.3 mg 100 g-1 soil were observed. This was attributed to increased concentration of H+ ions associated with the use of ammoniacal fertilizers, which promoted the solubility of immobile P into relatively available forms. There was a bulge of available P content in the topsoil of about 0.1-2.8 mg 100 g-1 soil and 0.2-3.0 mg 100 g-1 soil compared with its content in the upper subsoil in the first and second seasons, respectively. This was largely due to the rapid fixation of P, which curtailed its mobility to lower layers of the soil. Peak build-ups of available P (11.0-17.3 mg 100 g-1 soil) and exchangeable K (19.1-28.1 mg 100g-1 soil) in the soil profile were observed at tillering stage before rapidly dwindling to as low as 9.0 and 16.6 mg 100 g-1 soil, respectively at anthesis in the first season of the study. In the second season this trend was largely distorted by excessive rainfall. This pattern was attributed to the peak P uptake by wheat plants between stem elongation and anthesis, which caused the P depletion in the soil profile. Comparatively high potassium requirement by wheat plants between seed germination and anthesis accounted for corresponding depletion of exchangeable K in the soil profile at anthesis. However, at milky ripe stage the content of exchangeable K exceeded that at tillering stage by as much as 8.9 mg 100 g-1 soil, which was attributed to the release of K by wheat plant roots into the soil towards the end of the vegetative period. Single basal application of N (T2) had the lowest accumulations of available P (9.4-16.1mg 100 g-1 soil) compared with other treatments in which N was applied. T3 plots in which N45P90K60 was applied as incorporated basal fertilizers before planting and N45 applied at tillering stage by broadcasting method, recorded the highest content of available P (19.9 mg 100 g-1 soil). The single split application of N (N45) as ammonium nitrate may have created a rather acidic environment, which is conducive for the temporary solubility of P coupled with a rather subdued growth of wheat plants and the related low uptake of P under dwindling supply of N in post-tillering period. The same trend was perpetuated in the accumulations of exchangeable K during the growth and development of the wheat crop. T3 plots had the highest content of K (13.0-20.6 mg 100 g-1 soil). This was largely attributed to the reduced plant growth associated with this treatment and the related subdued uptake of this nutrient by N-starved wheat plants in the post-tillering period.