Residual Contribution of Green Manure to Humic Fractions and Soil Fertility

Abstract

Green manure (GM) of jack bean can improve the quality of soil organic matter and increase soil fertility. The objective of this study was to evaluate the residual contributions of fertilisation with jack bean (Canavalia ensiformis) to (i) organic carbon (OC) and total nitrogen (TN) contents in the soil and in the humic fractions, (ii) the percentages of nitrogen (N) derived from GM (NdGM) applied at different seasons and rates and (iii) the chemical attributes of the soil at different depths. The treatments consisted of two rates of GM required by the coffee tree (146 and 584 g pot−1 of the dry mass of jack bean), two application seasons and one control. Seasons refer to fertilisations with enrichment of 15N for each rate, applied in 2009 and 2010. Soil samples were collected at four depths for analysis of humic substances (HSs), NdGM and chemical attributes. In general, the organic carbon and total nitrogen contents in the soil and in the humic fractions increased with green manure fertilisation. We found higher percentages of residual nitrogen for the highest rate applied in 2010, 1 year after the first season in 2009. The humic substances, organic carbon and total nitrogen contents in the soil and organic carbon and total nitrogen contents in the humic fractions increased to a depth of 0.40 m in soil fertilised with jack bean. The residual nitrogen from jack bean in the soil is dependent on the rate and seasons of application. The residual contributions of the fertilisation with jack bean in the chemical attributes of the soil are dependent on the applied rate and the soil depth.