Abstract:
Globally agricultural production is facing serious challenges to provide adequate food
supply to meet a growing population. However, the reduced capacity of soil to support and
sustain agricultural production as a result of soil fertility decline is impacting negatively on
agricultural growth. Increase in the price of inorganic fertilisers and limited availability of
nutrients from organic amendments has reduced progress in improving soil fertility. This
research therefore aims at contributing knowledge towards evaluating the
maximisation/optimisation of nutrients in compost and secondary treated sewage effluent
(STSE) amended soils to meet the nutritional requirements of crops for sustainable crop
production and environmental protection. STSE was irrigated on soils (sandy loam and clay
loam) amended with greenwaste compost in soil incubation, glasshouse/pot and lysimeter
studies. Perennial ryegrass (Lolium perenne) was grown in the pots and lysimeter studies.
The incubation experiment showed that for a clay loam, N mineralisation in treatments with
STSE alone and combinations of compost and STSE was higher than the applied N.
Increasing compost quantity in compost and STSE nutrient integration resulted in reduced
net N mineralisation in the clay loam soil. In the sandy loam, increasing compost
contribution in compost and STSE nutrient integration resulted in an increase in net N
mineralisation. Cation exchange capacity, microbial diversity, quality of available carbon
and drying and rewetting cycles influenced the net nitrogen mineralisation dynamics in both
soil types. Increasing the contribution of STSE while reducing compost quantity resulted in
increased nitrogen use efficiency and ryegrass dry matter yield. The environmental threat to
ground and surface water pollution through NO3
--N leaching may be enhanced by the
inclusion of STSE in integrated compost and STSE nutrient supply to plants. Similarly, the
threat to eutrophication due to phosphorous leaching is likely to be higher with integration
of compost and STSE. Ryegrass dry matter yield reduced with increasing compost
contribution while the concentration of N in ryegrass herbage for the combinations of
compost and STSE was above the minimum requirement for N in herbage for productive
grazing and dairy cattle in the pot experiment. Using compost and STSE of similar
characteristics, the ideal approach to maximise nutrient potential from compost through
irrigation with STSE is when 25% compost is integrated with 75% STSE with respect to
nitrogen supply.