Direct and Residual Effects of Phosphate-Enhanced Organic Fertilizers on Soil Fertility and Crop Production

Abstract

Full recycling of phosphate with presently available farmyard manure can satisfy only one fifth of Moldova’s annual phosphate requirement. Crop nutrients from other sources are needed. Data from long-term experiments with different doses of sewage sludge and sugar-factory waste in crop rotation have been studied to establish their direct and residual effects on soil phosphate status. In the first 8 years after applying 40 t/ha of sludge, the P2O5 content was increased from 6.0 to 70 mg/100 g soil – a very high level of assurance; after 23 years, P2O5 decreased to 2.4–2.8 mg/100 g soil – corresponding to a moderate level of assurance. Doubling the dose of sludge gave higher P2O5 levels but not in proportion to the amount of sludge applied; treatment with 80–160 t/ha of sludge produced background levels of 4.9–6.2 mg P2O5/100 g soil over 23 years. Fertilization increased crop yields by 16–42 %, depending on the dose.