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Red soils cover 21% of China's surface: some 2.10 6 km 2 . Much of the area is hilly. Rice grows mainly in the lowlands. Only a fraction of the uplands in the red soil area is cropped, but yields are low due to inadequate topsoil as a result of erosion and nutrient mining. The remainder is wasteland that is not used for cropping and a little for grazing. But these areas have a high agricultural production potential if some constraints are removed. The World Bank has set up a major project named “China Red Soil II”. It comprises a total area of over 9.10 4 ha for development or rehabilitation, with 215 small watersheds including one named Qingshishan. Techniques of system analysis and optimization are applied to this watershed, and a watershed development planning model (QINGS) has been created. QINGS includes seven objectives, 32 activities and some constraints, two scenarios; three land use forms and two irrigation systems are available to the model for selection. This complexity provides a web of possibilities and there can be many solutions for suitable development. By running QINGS a number of times for successively more specific goals, a window of opportunities is established where land use practice is the key factor to development and where all goals are met to a certain degree. Feasible solutions can then be identified in which reduction of soil erosion, increased income and sustainability in land use are combined. Local farmers and technical officers can identify and accept the best compromise. QINGS is a tool for watershed developers that allows them to quantify opportunities and constraints, and set specific targets for developments with the local population. Results of QINGS can empower local farmers in negotiations with watershed developers. After 5 years of red soil development supported by the World Bank Project, various activities were initiated, including new crops (e.g. bamboo), trees (e.g. mulberry), livestock, fish and pearl production, and an ecological and social benefit forest. Infrastructure was improved (such as irrigation system, traffic conduction, biogas ponds and so on). The year 2000 total net income is 870,000 yuan, compared with 265,000 in 1994 before the project. Before the project, the forest cover rate was 9%, now it is up to 50%; soil erosion is at low level, and water supply has increased the length of the secure growing season by more than 50 days. The social employment conduction is also improved, especially the employment of rural women. The whole structure of agro-ecology system is optimized.

Percolation ponds are constructed in many parts of the tropics to augment fast-depleting groundwater resources. Increased agricultural productivity due to additional well irrigation water is the direct benefit from the pond schemes. Usually, location-specific empirical relationships are adopted for hydrologic and economic design of ponds. In this paper, an approach is presented to evaluate hydrologic pond design, by combining watershed, aquifer and crop simulation models of the pond system. A watershed model (CAPSIM) was developed based on the ‘state variable approach’ describing the dynamic catchment-pond hydrologic processes such as runoff, runoff due to interflow, evapotranspiration, direct percolation, pumping and irrigation. CAPSIM operates on a daily basis, using rainfall and pan evaporation data. Based on the Alternating Direction Implicit Algorithm (ADIA), a finite difference Aquifer Simulation model (ASIM) was developed to simulate two-dimensional recharge mounds beneath the pond. Predictability of CAPSIM and ASIM was verified with observed field data from one pond site, from which the models were improved. The models were further validated separately for three pond locations and for different years of weather data, by comparing observed pond and well water levels. The crop model, SAHEL, was adapted to the rice-growing environments of pond command areas, by representing water stress, runoff and well irrigation. Total Additional well Irrigation made possible Due to recharge (AID-recharge) in two space dimensions during a crop season was predicted by linking CAPSIM and ASIM. AID-recharge thus computed was input to the crop model for predicting the spatial distribution of crop response to different well irrigation scenarios. A series of ‘distance-capacity-yield’ curves were developed, which quantify the benefits associated with a modification in the pond structure. The batch of models thus linked could serve as a decision support tool for pond designers for evaluating desired hydrologic economic strategy.

There is potential in cultivating cowpeas (Vigna unguiculata (L.) Walp.) in rice-based cropping systems in the humid tropics as a source of proteins for humans, forage for ruminants, and for improvement of soil fertility. Seasonal and yearly variability is, however, a major constraint. A statistical stability model and a mechanistic simulation model were used to quantify yield variability of early and medium maturity groups of cowpea cultivars in pre- and post-rice environments at Los Banos, Philippines. Four field experiments, with 24 cultivars in each experiment, were conducted in pre- and post-rice environments, to determine the response of the cultivars to varying moisture and to water table depth regimes, and a stability analysis was performed across the regimes using a statistical model. In the post-rice environment, irrespective of moisture and water table depth regimes, the medium maturing cultivars tended to outyield early maturing cultivars, whereas in the pre-rice environment, early maturity demonstrated a distinct yield advantage. The statistical stability model indicated that medium maturing cultivars performed well in high-yielding environments, whereas in low-yielding environments their yields were comparable with those of early maturing types. However, results of the statistical stability model are limited because of the short time series of basic data. To broaden the analysis, a simulation model of the cowpea crop was calibrated and validated for its performance. Measured and simulated dry weights of crop parts (leaves, stems and pods) and soil moisture contents at 10-cm soil depths throughout the growing season indicated satisfactory performance of the simulation model. Long-term simulation results using the validated model indicate that TVX1948-012F (medium maturity) performs betters (800–2250 kg ha−1) than IT82D-889 (early maturity) (430–1620 kg ha−1) with a fluctuating shallow water table in the post-rice environment, whereas IT82D-889 is superior (80–1150 kg ha−1) to TV1948-012F (30–1150 kg ha−1) in the pre-rice environment. Yields are highest and least variable for the 15 January planting in the post-rice environment, and for the 15 April planting in the pre-rice environment.

The question of how many grams of an organism can grow heterotrophically from only 1·0 g of glucose and adequate minerals has been put forward many times. Only a few attempts have been made to answer this question theoretically and these attempts were rather rough. In this paper, it is demonstrated that the yield of a growth process may be accurately computed by considering the relevant biochemistry of conversion reactions and the cytological implications of biosynthesis and growth. Oxygen consumption and carbon dioxide production by these processes are also computed. The weight of the biomass synthesized from 1·0 g of substrate and the quantities of gases exchanged are independent of temperature. These results are obtained by adding the individual equations describing the formation of each compound synthesized by the organism from the substrate supplied. The sum represents an equation which accounts for all substrate molecules required for biosynthesis of the carbon skeletons of an end-product, whose chemical composition is given. It is then calculated how much energy is required for the non-synthetic processes which form a part of biosynthesis, such as intra- and intercellular transport of molecules and maintenance of RNA and enzymes. The additional amount of substrate required to provide this energy by combustion is easily calculated. Adding this substrate to the amount used for skeleton synthesis gives an overall equation which quantifies the substrate and oxygen demand as well as carbon dioxide evolution during biosynthesis of 1·0 g biomass. For example, it requires 1·34 g of glucose with adequate ammonia and minerals to synthesize 1·0 g maize plant biomass in darkness; during this process 0·14 g oxygen are consumed and 0·24 g carbon dioxide are produced. It has been described elsewhere that similar results were obtained experimentally with growing plants. Such results depend considerably upon the chemical composition of the biomass being synthesized and upon the state (oxidized or reduced) of the nitrogen source. Other parameters, such as the number of ATP molecules required for protein synthesis, the possibility for utilization of alternative pathways for synthesis or energy production, the presence or absence of compartmentation of synthetic processes and variations in the P/O ratio between two and three, under many conditions affect results of the computation less than 10%. Since maintenance of cellular structures is not considered, the approach concerns the gross yield of biosynthesis. It predicts therefore the dry matter yield of heterotrophic cells from a given quantity of substrate at high relative growth rates.

There is a growing interest in cultivating wheat in Southeast Asia for reduction of the drain of foreign exchange and for crop diversification. A crop growth model was used to establish potential and rainfed yields of wheat following rice in different parts of this region. A comparison of measured and simulated yields indicated satisfactory performance of the model. The results indicate that the yield potential of irrigated wheat is between 3·0 and 5·0 t ha−1 between 10 S and 23 N latitude at sea level, but it is higher at altitudes between 500–2000 m. Such yields are stable and relatively insensitive to sowing date. Rainfed yields are lower, and have a high year-to-year variability. The gap between potential and water-limited grain yields is small if wheat is sown towards the end of the rainy season on a deep and well-drained soil.

A strictly linear relation was established between the carbon and ash contents of plant biomass and the relative amount of substrate required to produce this biomass. Data concerned storage organs of crops of widely different biochemical composition. Amounts of substrate required were calculated by an analysis of the metabolic pathways for the synthesis of the principal components of biomass. Carbon contents were derived from the biomass composition. A linear relation was also established between biomass carbon and ash contents and the CO 2 loss during biosynthesis. The linearity of both relations and the ease with which the carbon and ash contents of new biomass samples can be determined accurately imply that the previous procedure for determining the conversion efficiency for plant biomass and growth respiration can be simplified substantially with a gain in accuracy.

Summary Maize plants hydroponically grown on nutrient solution were used to study the effect of alterations in N supply on N concentrations in the organs as related to the ability of biomass production. Omission of nitrate from the nutrient solution induced in the vegetative organs a breakdown of nitrogenous compounds which were partly redistributed towards the ears. These processes took place more at the expense of membrane proteins than of the buffer-soluble proteins and resulted in N concentrations lower than the theoretical minimum; then the biomass production stopped. Re-supply of nitrate to young N-starved plants induced an increase of N concentrations in all plant parts and the resumption of biomass production. The newly produced amount of membrane proteins was larger than that of buffer-soluble proteins. However, the ratio of buffer-soluble to membrane protein concentrations in mature leaves did not return to the original value. It was concluded that re-supply of nitrate resulted in a recuperation of N-starved plants to a restricted extent.