Management of clay soils for rainfed lowland rice-based cropping systems: an overview

The problem of concern in this project is that in the dry season following a lowland rice (Oryza sativa L.) crop, yields of post-rice crops are generally low, despite adequate water commonly being available in the soil pro®le to grow a potentially high yielding dry season (DS) crop without irrigation. Maize (Zea mays L.) yields are as low as 1 Mg haÿ1 or less, soybean (Glycine max L. Merr.) and cowpea (Vigna unguiculata L.) at 0.3 to 0.8 Mg haÿ1 in Indonesia and mungbean (Vigna radiata (L.) Wilzek) around 0.5 Mg haÿ1 in the Philippines. These are all very much below the yield potential of these soils. For example, mungbean yields of 2.2 Mg haÿ1 have been achieved by IRRI in the Philippines on these soils without irrigation or additional fertilisers. The causes of low yields of DS crops after rice are mainly poor crop establishment and poor root growth due to soil physical constraints. These result from the breakdown of soil structure during wet cultivation (puddling) for rice. Yields are also limited by biological and chemical constraints. As a result of these low yields, farmers are reluctant to invest in post-rice crops. Therefore, land after lowland rice (at least 51 million ha in Asia according to Huke [Huke, R.E., 1982. Rice Area by Type of Culture: South, Southeast, and East Asia. International Rice Research Institute, Los BanÄos, Philippines, 32 pp.]) represents an underutilised resource that can be used to meet the food requirement of the ever increasing population of the developing world. To increase the utilisation of these soils, improved management practices are required to enable dry season crops to use the stored water in the soil pro®le after the rice crop. This paper outlines a project which was established with the general objective of developing soundly based soil management technologies that can overcome soil physical limitations to DS crop production after lowland rice. The speci®c objectives of the program were 1. to test a range of soil management and agronomic practices that have the potential to overcome adverse soil physical conditions for DS crops after rice, across a range of soil and climates; 2. to evaluate these practices by 2.1. measuring the changes in soil physical conditions throughout the complete cropping cycle from rice to DS crops; 2.2. determining the performance of the DS crop (establishment and growth) and its ability to extract soil water. 3. to determine the mechanisms involved in dispersion due to puddling and in ̄occulation and structural development as the soil dries after draining surface water from rice ®elds. Relevant outcomes from this project are described in the following papers in this issue. # 2000 Elsevier Science B.V. All rights reserved.