Long-term land quality studies on a toposequellce, presenting a continuum of edaphic and hydrologic conditions from dry land to hydromorphic valley bottom opened interesting possibilities for sequential testing of drought tolerance of rice varieties.Variations in the soil nitrogen cycle according to the location in the toposequence may be used for further fundamental and applied research.The phenomena of iron deficiency in tile higher part of this specific toposequence and of iron and/or managanese toxicity in the lower part can be used in screening varieties resistant or tolerant to iron deficiency and to iron and/or manganese toxicity.A very close relationship was found between tile occurrence and severity of blast (Pyricularla oryzae) and the dynamics of the moisture status of the soil; tile rice on the upland part was severely affected after drought stress. Blast affected some varieties much more than others in the dry land part of the toposequence. The principles of sequential testing are also valid for other crops on which research is. now proceeding. Sequential testing of crops can be used for other conditions where one or several environmental parameters change over short distances.
The response of maize (Zea mays L. Cv. TZB) to liming and the retention and leaching of Ca and other nutrient ions were studied in the greenhouse using an acid Ultisol from southern Nigeria. The soil was limed to various pH levels and maize was grown for 10 weeks. The largest growth response occurred when the soil pH was raised from 4.3 to 5.0 by liming (1.3 metric ton/ha). The highest yield was obtained when the soil pH was between 5.5 to 6.0.Substantial downward movement of Ca from the limed surface layer (0–15 cm) was noted, but these Ca ions were mostly adsorbed in the unlimed subsurface layer. The vertical distribution of exchangeable Ca in the limed soils followed a similar trend with pH, but was inversely related to exchangeable Al.Leaching losses of applied nutrient ions (Ca, Mg, K, NO3) during the 10‐week cropping period were relatively small. There was a significant increase in nitrate content in leachates when soil was limed above pH 7.0. Only 1% of the applied Ca was in the leachate while about 80% of the applied Ca was retained in exchangeable form when the soil was limed at a rate of 2 metric ton/ha.
Three low‐yielding Luzon soils, Quingua clay loam, Bantog loam, and Umingan loam, on which plants did not respond sufficiently to N, P, or K, were tested in pot studies with three levels of B, S, and Zn in a 34 factorial design to show the response on the yield and nutrient content of rice (Oryza sativa L.). None of the soils was deficient in B, as additions of B resulted in decreased yield and the development of foliar symptoms resembling those of B toxicity. Plants in two soils responded to the addition of S; those in Quingua clay loam gave a greater response than those in Bantog loam. Addition of Zn did not increase the yield of plants in any of the three soils. However, young plants growing in Umingan loam exhibited symptoms resembling those of Zn deficiency in treatment Zn0. The level of each nutrient in the straw was increased by the addition of that nutrient to the soil. The most pronounced effect was on the S level in the plant when S was applied to the soil. The linear or quadratic equation to calculate this relationship is presented for each nutrient. The highest level of B applied to the soil, treatment B2, significantly increased the level of S in the straw compared to treatment B0 but it did not increase yield.
Depletion of soil solution P from zones around plant roots indicates that diffusive flux of P from the soil solution to the plant root is inadequate to keep pace with removal by plant roots. Consequently, the concentration and/or concentration gradients of solution P maintained under conditions of repeated removal over time are an important soil property that is a factor in determining the quantity of P diffusing to root surfaces. This study was conducted to determine the P release characteristics of five soils differing in P retention properties and pretreated with quantities of P equivalent to their sorption maxima.Five‐gram samples were sequentially extracted with 3.5‐ml aliquots of 10−3N CaCl2 for 350, 15‐min cycles. Quantities of extracted P were 15 to 30% of the added P. Concentrations maintained during repeated extractions revealed three distinct phases of release presumably associated with different forms of soil P. Release persisted through approximately 25, 125, and 150 extraction cycles for the initial, intermediate, and final phases, respectively. For different soils, concentrations of P ranged from 0.5 to 1.1 µg/ml in initial phases and 0.02 to 0.08 µg/ml in final release phases. Though pretreatment of soils with P was based on retention properties, relative extractability and concentrations in solution during release phases appeared to be a characteristic of each soil. During the intermediate release phase, P concentration was very high and decreased rapidly for one soil, was moderately high and stable for another soil, and was very low and hardly discernible from the final phase for a third soil.
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