Nitrogen, phosphorus and potassium availability as influenced by humate and fulvate soil amendment

Abstract

Citrus fruit in South Africa is produced mainly for the export market where it competes with other countries such as Spain, Turkey, USA and Egypt. South Africa is the third largest exporter of citrus after Spain and Turkey. Therefore, quality and shelf life play an important role in maintaining the competiveness of South African produced citrus. Plant nutrients and especially the macro nutrients such as nitrogen (N), phosphorus (P) and potassium (K) play an important role in ensuring yield, quality, and shelf life. However, the efficiency of applied fertiliser is less than 50% for N, less than 10% for P and about 40% for K due to the leaching. Thus, by using humate and fulvate amendments the N leaching from soils can be reduced. The objectives of this study were to determine the effects of humate on: (1) The culturable soil community and microbial activity in a sandy clay and a sandy clay loam soil; (2) the reduction in N, P and K losses; (3) the uptake of N, P and K in potted citrus and (4) the cation exchange capacity of soils. Four experiments were conducted: Experiments on the viable microbial population and dehydrogenase activity were done in a microbiology laboratory, leaching column studies were done in a soil physics laboratory and pot trials were conducted in a glass house at the experimental farm of University of Pretoria. Sandy clay and sandy clay loam soils were supplemented with 220-50-80 kg ha-1 which represent 100% of the recommended N, P and K application rate and 165-37.5-60 kg ha-1, which represents 75% of the recommended N, P and K application rate. The soils were further amended with humate low ash and humate high ash or with fulvate at a rate of 200 kg ha-1. Controls included soils without any amendments and with 100% and 75% of the N, P and K recommendation. Experiments on microbial population and dehydrogenase activity were done in triplicate and leaching column and pot trials had four replications. Quantification of heterotrophic bacteria and fungi in both soils indicated, after four weeks, an increase in bacterial and fungal counts for soils treated with humates and a fulvate compared to soils with no humic acids. Results from leaching column experiments indicated a decrease in N leaching when humates and fulvate were added to the soils, while inconsistent results were found for P and K leaching in both soils. Pot trials indicated that humates and fulvate reduced N and P leaching, while N, P and K uptake were higher for the soils with humate or fulvate. The study indicates that humates and a fulvate increased the cation exchange capacity of both soils.