The Effects of Fertillizers, Manure and Trace Elements in Continous Cropping Rotations in Southern and Western Uganda

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Stephens, D.
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Eighteen factorial fertilizer trials, sited in pairs at nine stations, have shown that the largest responses were of maize and beans to nitrogen and of sweet potatoes and cotton to potassum on frequently cropped, strongly acid ferrallitic soils. Responses to phosphorus were smaller and to magnesium or soil applications of micronutrients usually negligible. The effects of farmyard manure were partly explained by its nutrient content, particularly potassium. After four years ammonium sulphate had lowered the pH of the 0-8 inch soil layer by 0.3 unit per ton of fertilizer per acre and removed approximately 1.2 X its chemical equivalent of exchangeable cations from the top 16 inches, while about one-third of the applied phosphorus, one-half of the potassium and three-quarters of the magnesium remained extractable by the Truog method and ammonium acetate respectively. The potassium fertilizer also appeared to depress the level of extractable phosphorus. Manure raised the level of exchangeable potassium slightly more effectively than potassium fertilizer and increased soil nitrogen, but it had no significant effect on the level of Truog phosphorus. Four years of continuous cropping depressed the pH and exchangeable potassium but had little effect on other exchangeable bases or Truog phosphorus: it also probably reduced the level of organic matter at the rate of 3 per cent of its initial value each year. The pH of the soil was the best indicator of site responses to phosphorus or potassium and the pH or exchangeable potassium the best indicator of crop yield. Leaf analysis confirmed the importance of potassium deficiency but levels of nitrogen or phosphorus in leaf samples were only occasionally correlated with yields.
East African Agricultural And Forestry Journal, 34 (4), pp. 401-417