Residual soil nitrogen under different sources of organic and inorganic fertilizers

Abstract

This study was carried out in Embu with the aim of establishing the quantity, form and depth of residual soil nitrogen after two cropping seasons, that is, the 1998 long rains (LR) and 1998 short rain (SR) seasons. The experimental design consisted of a complete randomized block design with 10 treatments replicated 3 times. The test crop (Zea mays L.) was grown alone or in alley cropping with or without fertilizer/prunings of calliandra or leucaena. For treatments that received tree prunings, the application rate was based on the biomass production before the season. Therefore the season. Therefore, 3089 kg/ha dry matter (DM) of Calliandra and 1033 kg/ha DM of leucaena were incorporated in the LR season. Similarly, 1954 kg/ha DM of Calliandra and 647 kg/ha DM of leacaena were incorporated in the SR season.

At the end of each season, soils were sampled at 0-20, 20-50, 50-100, 100-200 and 200-300cm depths. Organic and total soil nitrogen decreased from 2.4 at the topsoil and subsoil nitrogen though there were no significant differences across the treatments and seasons. Ammonium content was found to be below 5.9 mg N kg and was attributed to fast nitrification in these moderately acidic soils. However, significant differences were noted across depths and seasons. Nitrate formed the bulk of soil inorganic nitrogen and varied between treatments, depths and seasons. In all treatments, there was a general decrease in nitrate content from the 0-100cm depth, which is the maize rooting depth. A further nitrate content reduction in the other depths (200-3000 cm) was observed in alley cropping treatments (where trees were present) while there was an increase in the sole crop treatments.

Reduced subsoil nitrate in alley cropping treatments show the trees potential in recycling subsoil plant nutrients especially nitrogen. This is an environmentally positive aspect as it prevents further leaching of the nutrient which can otherwise become a water pollutant. However, treatments that received prunings and /or fertilizer in sole cropping had higher maize yields than those in alley cropping. These low yields in hedgerow inter-cropping can be attributed to competition for resources between trees and crops.

The present study therefore points out the need for further investigations that may lead to the establishment of a cropping system that would benefit from Calliandra and leucaena biomass so as to improve crop production and also reduce nitrogen losses by leaching.