Quality parameters of organic amendments from Umbumbulu and Msinga farms and their effects on nitrogen and phosphorus mineralization.

Abstract

Conventional agricultural management practices that farmers in Africa and South Africa have practiced have led to a decline in soil fertility. Organic amendments have shown to improve soil quality and fertility status when incorporated into the soil. Smallholder farmers manage their fields differently according to resource endowment, distance of fields from the homestead (i.e homefield and outfield), and labour. The use of organic inputs as fertilizers to remediate the soils from which the loss of nutrients occurred depends on their decomposition rates and nutrient release patterns. Factors such as soil type, climate and application rates of the amendments affect the decomposition and mineralization of these amendments in soils. The objective of this study was to determine (i) carbon and phosphorus pools from different fields from uMbumbulu and Msinga as affected by farmer typology and (ii) the characteristics of organic amendments and their decomposition and mineralization of nitrogen and phosphorus in soil. Three typologies (i.e. resource constrained, moderately resourced and resource endowed) were selected for both Msinga and Mbumbulu. Two fields per typology were used, namely homefield (<100m from homestead) and outfield (>150m from home) for Msinga while for uMbumbulu it was fields with mixed cropping and monocropping system. Three farms were selected per typology and field type with three replications. Soil samples were collected from the farms of different typologies at 0 – 20cm and analyzed for soil organic carbon (SOC) and phosphorus pools. Organic amendments including cattle manure, goat manure, accelerator and maize residues were sampled from different farms in Msinga and uMbumbulu and characterized. Composite samples of these amendments, separately and in combination, were then incorporated in soils and incubated for 84 days during which soil pH, P and mineral-N (ammonium-N and nitrate-N) were analyzed. Farmer typologies did not affect carbon and phosphorus pools of the soils on farms at Msinga and uMbumbulu. Carbon pools under different cropping systems and typologies for uMbumbulu showed significant difference with total carbon concentrations being the highest under monocropping system (40.3 g/kg) followed by c- POMC, f-POMC, MAOC and DOC and also under resource constrained typology, total carbon was the highest (44.6 g/kg). Carbon pools under Msinga did not follow the same trend both under cropping system and typology since there was no significant difference. More P was in a reductant P form in uMbumbulu soil both under different cropping systems and typology with concentration of 224-310 mg/kg under cropping system and 145-447 mg/kg within typology. Available P had lower concentrations in both cropping system (8.9-11.8 mg/kg) and typology (9.6-11.7 mg/kg) with Al-P and Fe-P showing no significant difference in uMbumbulu soil. Msinga soils followed the same trend of P pools showing no significant different in Al-P and Fe-P as uMbumbulu. Msinga soils showed more positive correlation between carbon and phosphorus pools than uMbumbulu soils. Msinga amendments appeared more beneficial than uMbumbulu with high pH levels and cattle manure having low C/N ratio content which allows rapid decomposition. More nutrients were available for plant uptake as Msinga amendments had higher concentrations of bases. The Accelerator had higher ammonium-N concentration (128 g/kg N on day 84) than other treatments showing higher decomposition rate in the uMbumbulu soil. When the manures were combined with maize residues, they had lower ammonium-N concentration due to C:N ratio of the maize residues. After day 7 of incubation, nitrate-N and mineral-N concentration increased in all treatment in both mg/kg soil and g/kg of N present. Like in the first incubation experiment (uMbumbulu soil), the control in the Msinga soil had higher nitrate-N than all treatment combinations containing maize residues between 14 to 56 days of incubation except for the accelerator+maize residues. Maize residues in both experiments (uMbumbulu and Msinga soils) showed lower mineralization of N and P and Msinga amendments had higher nutrient mineralisation than those from uMbumbulu. The findings of the study imply that carbon and phosphorus pools in the two study sites could be affected by environment factors more than management practices and that maize residues will require a longer period of time to allow maximum decomposition and mineralize nutrients compared to the accelerator, cattle, and goat manure. More studies need to be done on environmental factors such as climate, parent material, and topography, as they might be the primary drivers of carbon and phosphorus pools.