O Ito and M Kondo Introduction

According to the agroecological zone system proposed by FAO,1 the terms arid and semi-arid, representing dry-land, cover areas with less than 75 days length of growing period (LGP) and 75 to 180 days of LGP, respectively. Dryland occupies 30% of global land area. Two-thirds of dry land exists in Asia and Africa. About one-third of dry-land in Asia and Africa belongs to the semi-arid region, where various management options are still available to the farmers to increase crop productivity and resource use efficiency, depending on environmental and socioeconomic situations. The typical soil types found in the dry land are Entisols, Aridisols, Mollisols, Alfisols and Vertisols with unfavorable chemical and physical characteristics.

The semi-arid tropics (SAT) hold one-sixth of the world population, half of which subsists on less than a dollar a day. The agricultural productivity in the region should be improved to meet an ever-growing population which will reach 8.5 billion by 2025 and exceed 10 billion by 2500, according to the prediction made by the United Nations. The major constraints to agricultural production in SAT are unpredictable weather and poor accessibility to agricultural resources, which forces the farmers to take an option of low input farming systems. Among several options available at the farm level, N fertilizer management would be most suitable to achieve an immediate increase in crop production in the area where most of the farmers have no access to irrigation facilities. The SAT soils are usually low in organic matter as compared with temperate soils. Since organic matter is a source of available N in the soil, many soils in the SAT can satisfy only a part of the crop N requirement, even at a low yield level, and therefore N fertilization is necessary to improve crop yields and land productivity.

The fertilizer application is a risky investment for the farmers in SAT because of the heavy rainfall at the onset of each growing season, which leads to a considerable leaching loss of the nutrient element applied, and unreliable rainfall, which periodically results in a fatal crop failure. Fertilizer application does not always bring more income by increasing crop yield; instead, it sometimes incurs a huge monetary debt and consequently puts the farmers in a more difficult financial situation. In contrast, the option to select a suitable cropping system does not require investment and needs only appropriate judgement based on agronomic knowledge obtained by long practical experience. Among various cropping systems, intercropping seems to be a very attractive system which may improve the stabilization of crop production in harsh conditions.

The crop combination in intercropping is usually determined so as to capture more solar energy, in other words, to achieve more light interception than monocropping by considering the canopy structure and growth duration of the crops. To improve the utilization of soil resources, the root system structure should be also considered. Combining crops with shallow and deep root systems may increase resource capture from extended soil layers where limited amounts of water and nutrients exist. In the case of a legume and cereal combination, it is reported that biological nitrogen fixation of the legume is enhanced by the presence of the cereal.2 The proper crop combination, and careful resource management in intercropping, will certainly improve the resource use efficiency and, ultimately, land productivity.

This paper focuses on crop and resource management in relation to cropping systems widely practiced in the SAT region. The management of crops and resources should be considered within the framework of farming operations as a whole, considering the flexible decision making adopted by farmers in harsh environments for their survival. This flexibility has created numerous cropping patterns at the farm level.

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