The management of rice straw, fertilisers and leaf litters in rice cropping systems in Northeast Thailand. 2. Rice yields and nutrient balances

The maintenance of soil organic matter (SOM) and the balancing of nutrient flows into and out of the rainfed rice cropping systems in Northeast Thaila nd is of paramount importance to arresting the decline in soil fertility an d crop yields. A system where small applications of leaf litters from local ly grown trees are applied annually to rice paddy soils prior to transplant ing is described. The annual application of 1500 kg/ha of Cajanus cajan, Ac acia auriculiformis, Phyllanthus taxodifolius and Samanea saman for five se asons resulted in increases in rice grain yield of 48, 35, 32 and 23% above the no-leaf litter control, respectively. Average annual nutrient inputs f rom the leaf litters, in kg/ha, ranged from 62.7 N, 3.9 P, 17.9 K, and 3.5 S for Cajanus cajan to 24.5 N, 1.5 P, 8.1 K and 2.0 S for Acacia auriculifo rmis. Nutrient balances, determined by the difference between the inputs (f ertiliser and added leaf litters) and outputs (grain and straw) indicated n et positive N and P balances of up to 457 and 60 kg/ha. respectively, after five seasons of leaf litter applications. Sulfur and potassium balances re sulted in net deficits of up to -3 and -52 kg S and K/ha, respectively, whe re no leaf litter was applied and rice straw was removed following harvest. Calculated apparent nutrient recoveries reflected the decomposition rate o f the added residues and were highest for P and K, reflecting their higher soil residual value than mobile nutrients such as N and S. Sustainable farm ing systems will require that crop yields are stable through the maintenanc e of soil fertility and the balance of nutrients in the system.