The effect of ammonium suphate usage on the availability of soil phosphorus to citrus

Abstract

Glass-house experiments were carried out to study the effect of soil acidification, caused by the continuous use of ammonium sulphate, on the availability of soil phosphorus to citrus plants. The soils for the glass-house experiments were sampled in a tilled and a permanent sod treatment of a factorial field experiment, each at several levels of ammonium sulphate. Nitrogen levels in the soil were equalized by application of ammonium nitrate solutions. Growth and phosphorus uptake of lemon cuttings and orange seedlings were considerably better in soil from the no-ammonium sulphate plots than in soil from the high ammonium sulphate plots. Application of phosphate caused a marked increase in growth and phosphorus uptake only of plants grown in soil from the no-nitrogen plots. Increasing levels of superphosphate had very little effect if applied to the soil from the high ammonium sulphate plots. A considerably better response to superphosphate was obtained if applied to soil from limed plots, or applied together with a mixture of calcium and magnesium silicate. A good response was also obtained to crushed rock phosphate. Analysis of soils indicated that in the acid, high ammonium sulphate soil a greater proportion of soil phosphorus was present as aluminium phosphate than in the no-ammonium sulphate soil. The results are discussed in relation to citrus fertilizer practices and their effect on fruit quality aspects.