Environmental performance of the aerobic rice system in three soil types in the Philippines

The aerobic rice system is a new approach to grow rice in water-short environments. It entails the cultivation of high yielding rice in unpuddled, nonflooded aerobic soil, with (in dry season, DS) or without (in wet season, WS) supplementary irrigation. Since the aerobic rice system is still being developed, there is a need to gain experience on the systems_ potential on different soil types. The authors tested aerobic rice in the Philippines in three soil types: heavy clay at IRRI, light clay at IRRI, and sandy loam at Dapdap, Tarlac. In heavy clay, different rice varieties were grown under aerobic and under flooded conditions in DS and WS (2001-03). In sandy loam, different rice varieties were grown under aerobic conditions with three irrigations regimes in the 2003 DS. In light clay, variety Apo was grown in aerobic soil under the irrigation regimes at five fertilizer N levels in the 2004 WS. The authors monitored crop growth parameters, yields, water inputs and leaf nutrients. Aerobic rice yields were 4-6 t/ha in heavy clay. Compared with flooded conditions, the aerobic rice yields was 28% lower in the DS but saved almost 50% irrigation water during the crop growth period and increased water productivity by 11%. Aerobic rice yields in sandy loam "collapsed" in the irrigation treatments receiving irrigation once a week and once I two weeks only. Yields were 3-5 t/ha with irrigation twice a week. Measured nematode populations were very high averaging 1,200 juveniles/gram root (range: less than 100-7,057 juveniles/gram root). Yields in light clay were also low, and ranged from less than 1 to more than 4 t/ha. Two replicates performed very poorly, but two other replicates gave good results. Leaf iron (Fe+) content was high, which may be toxic to the plants, and zinc (Zn) and manganese (Mn) contents were very low, which were close to concentrations indicating deficiency. Reasons for the poor crop performance on the light and medium soils may include (1) inadequate water availability since percolation rates were relatively high, 2) the presence of nematodes, (3) possible other soil-borne diseases, (4) possible micronutrient imbalances. In various aerobic rice experiments in the WS (IIRI, unreported data), yields were relatively high and stable, and none of the problems encountered in DS experiments were observed. Priority should be given to understanding the causes for low yields in the DS to successfully develop an aerobic rice system for this season.