PhD Theses

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Browsing PhD Theses by Author "Mongi, Hussein O"

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    Lime, Nitrogen and Phosphorus effects on some chemical characteristics of an oxisol, leaf elemental content and yield in upland rice and maize at Morogoro
    (University of Dar es Salaam, 1974) Mongi, Hussein O
    The effect of lime, at varied N and P fertility, on some chemical characteristics of a moderately acid Oxisol (initial pH 5.6; average LR 10 tons/ha), leaf nutrient content, some growth, yield and yield components in upland rice cv. “Salama” and maize cv. “Ilonga composite” were evaluated during 1970-73 in a field study to further examine the nature of crop response to lime under tropical conditions at Morogoro, Tanzania. The lime rates were 0, 7.5, 10.0 and 12.5 tons agricultural lime/ha (mean ECCE* 90%); N rates were 0, 100, 200 and 300kg/N/ha as ammonium sulphate; and P rates were 0, 40, 80 and 120kg/ha as triple superphosphate. Lime was applied in November, 1970; the N and P rates were applied annually during 1970-73 except that P was not applied on the rice crop during 1972/73. Lime decreased exchangeable AI, Mn and Fe; this effect was complemented by P but counteracted by applied N. Exchangeable AI was not affected by lime in the absence of added N. Base saturation, electrical conductivity, N mineralization, Ca2+ and Ca-P were increased with liming along with increase in soil pH. Levels of AI-P and Fe-P were generally slightly increased by lime under rice cropping but under maize cropping AI-P increased slightly while, although Fe-P was reduced, there was a significant increase in FE-P at the 10 tons lime rate. Increases, the P effect being more pronounced. Exchangeable K under rice cropping was increased by lime at all applied N rates except in the absence of N where reductions occurred up to the 10 tons lime level but increased at 12.5 tons lime rate. Under maize cropping exchangeable K was increased by lime with the 10 tons lime rate recording the highest K levels at virtually all N and P rates. Exchangeable Mg was reduced considerably by lime only in the absence of added N under rice cropping and only up to the 10 tons lime level, the negative effect of lime being annulled by applied N. However, mean Mg levels were lowest and highest respectively under rice and maize at the 10 tons lime rate. Exchangeable Na under rice cropping was reduced by lime only up to the7.5 tons lime rate, beyond which increases occurred, with a peak at the 10 tons lime level. Grain yield in upland rice and maize increased with liming, with optimum yield obtained around the lime requirement rate (10 tons/ha) except for the second cropping year where optimum maize grain yields were obtained by liming with 7.5 tons/ha. Liming enhanced crop response to applied N and P, the respective optimum N and P rates being 100 and 40kg/kg/ha. Rice straw and grain yields, and maize silage yield were depressed at the 10 tons lime rate due to likely nutrient imbalances resulting from reduced soil N availability and probable increased uptake of N03-N. The optimum lime, N and P rates were also associated with low mean root-lodging indices, optimum plant population, grain/shelled-cob ratio and lower vegetative material production in maize as indicated by the silage data, and with optimum grain straw weight and grain yield ratio in upland rice. Liming beyond the lime requirement level generally reduced yields at low N and P fertility, the reduction being associated with generally decreased leaf concentrations of N, P, K, Ca, Mg, Zn, Cu, B, Fe, Mn, Mo, Na and Al. Reduced upland rice straw and grain yields at 10 tons lime rate were associated with increased leaf concentrations of Al, Fe, Ca and B and reduced leaf K, N and Cu concentrations. Reduced silage yields at the 10.0 lime rate were also somewhat associated with increased leaf content of Ca, B, Mo, P and Zn, and with slight increases in leaf Mn content. Yield decreases were generally associated with decreased N/K and K/(Ca+Mg)½ ratios in the soil and plant. Yield increases, however, were only significantly correlated with leaf contents on N, K and Zn in upland rice, and with increased Mo, P (at 10%), Fe (at 10%) and Ca (at 10%) in maize. Genetic differences in nutrient uptake were apparent as the rice leaves had more than double the Fe and Mn levels and about half the Al levels found in maize; increases in leaf Fe content at the low levels found in maize were generally beneficial, while increases at the high levels found in rice were associated with decreased yields; and increases in leaf Mn content at the low levels found in maize were yield limiting, while leaf Mn increases at the higher levels found in rice were generally not yield limiting. Exchangeable Al and leaf P content in maize were related, leaf Al content was not related to leaf P concentration. Because of confounding effects of soil fertility, soil pH was not strongly related to yield, the correlation being quadratic and significant only at the 10% level. Least squares mean grain yields were obtained at an ultimate pH of about 6.3-6.4 in upland rice and 5.8-6.4 in maize, but optimum mean rice and maize gain yields without applied N were obtained at pH 6.8. High yields at high N and P fertility were, however, generally obtained up to pH 7.0. The data suggest that liming criterion for Oxisols low in exchangeable Al should be based on a lime level required to bring the soil reaction to an eventual pH 6.5 for cropping cycles involving legumes and pH 6.0 for continuous maize cropping.