APSIM (Agricultural Production Systemsof both the total amounts in the whole projle and their distribution with depth. Since neither of these datasets included measurements of the runof component of the water balance, this aspect of model performance was evaluated, and shown to be generally good, using data from a third source where runoff had been measured from contour bay catchments. 0 1997

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Two models that differ markedly in how they represent the crop-soil system have been used to simulate soil processes and crop production in the long-term experiment at Hermitage Research Station, Warwick, Queensland. The experiment was designed to examine the effects of tillage, stubble management, and nitrogen (N) fertiliser on the productivity of a winter cereal-summer fallow cropping system. it commenced in 1968 and the treatments have been maintained until the present. CENTURY operates on a monthly time step, considers all soil N transformations to occur in a single soil layer, and has a very simple crop growth routine that does not deal with crop phenology. APSIM provides a framework whereby a model of a cropping system is configured from component modules, which operate on a daily time step. For simulating the Hermitage experiment, modules to represent the dynamics of soil-water, N, surface residues, and growth of a wheat crop were used. The water and N modules deal with a multi-layered soil, whilst the wheat module develops leaf area, intercepts light, and accumulates and partitions dry matter in response to weather, soil-water, and N. Both models were specified to simulate the whole experimental period (1969-92) as a continuous run. The ability of these models to simulate grain yields, soil-water and drainage, nitrate-N, and soil organic matter were examined. Both models predict, in agreement with the observed data, that for this continuous cereal cropping system there has been a decline in soil organic matter for all the treatments and a reduction through time in the capacity of the soil to mineralise and accumulate nitrate during the fallows. CENTURY performed better than APSIM in predicting the relative yields of the N treatments but was less satisfactory than APSIM for absolute grain yield, soil-water, and drainage. Yield predictions with APSIM were sensitive to carry-over errors in the water balance from one season to the next, so that in some seasons large errors occurred in the predicted relative yields. Both models reproduced the observations well enough to indicate their suitability for providing useful insights into the behaviour of cropping systems where the focus is on depletion of soil fertility.

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The tolerances of a range of tropical grain legumes to salinity were compared during early vegetative growth of plants grown in pots with NaCl added to a sandy loam soil to achieve electrical conductivities (sat. extract, ECe) over the range, 1.3-13.8 dS m-1. Tolerance, based on the ECe at 50% of maximum growth (in parenthesis) was of the order: Sesbania cannabina (13.2 dS m-1) > guar cv. CP 177 (10.1 dS m-1) > guar cv. Brooks (9.8 dS m-1) > cowpea cv. Caloona (9.0 dS m-1) > soybean CPI 26671 (6.7 dS m-1) > pigeon pea cv. Hunt (5.4 dS m-1) > black gram cv. Regur (5.0 dS m-1) > pigeon pea cv. Royes (4.9 dS m-1) > green gram cv. Celera (3.5 dS m-1). Genotypes exhibited differences in Na+ accumulation, with black gram, green gram and pigeon pea accumulating large quantities in shoot tissues, compared with effective exclusion of Na+ by Sesbania, guar and soybean. Smaller relative differences existed between species in terms of Cl- uptake, and the relative yield reduction was closely related to the amount of cl- in shoots. These results are discussed in terms of current understanding of the nature of salt tolerance in nonhalophytes.

The applicability of models in addressing resource management issues in agriculture has been widely promoted by the research community, yet examples of real impacts of such modelling efforts on current farming practices are rare. Nevertheless, simulation models can compliment traditional field experimentation in researching alternative management options. The first objective of this paper is, therefore, to provide four case study examples of where models were used to help research issues relating to improved nutrient efficiency in low-input cropping systems. The first two cases addressed strategies of augmenting traditional farming practices with small applications of chemical fertilizer (N and P). The latter two cases explicitly addressed the question of what plant genetic traits can be beneficial in low-nutrient farming systems. In each of these case studies, the APSIM (Agricultural Production Systems Simulator) systems model was used to simulate the impacts of alternative crop management systems.The question of whether simulation models can assist the research community in contributing to purposeful change in farming practice is also addressed. Recent experiences in Australia are reported where simulation models have contributed to practice change by farmers. Finally, current initiatives aimed at testing whether models can also contribute to improving the nutrient efficiency of smallholder farmers in the SAT are discussed.