oped and developing countries (Algozin et al., 1988; Bowen and Wilkens, 1998; Jagtap et al., 1993;Lal et al., In low-input systems, where most nutrients become available from Singh et al., 1993;Thornton and Wilkens, 1998). soil organic matter (SOM) and residue turnover, the applicability of DSSAT (Decision Support System for Agrotechnology Transfer) crop DSSAT version 3.5 incorporates 16 crops {maize (Zea simulation models is limited because (i) it recognizes only one type mays L.), wheat (Triticum aestivum L.), rice (Oryza of SOM (i.e., humus) and recently added, but not yet humified, resisativa L.), sorghum [Sorghum bicolor (L.) Moench], mildue; (ii) it does not recognize a residue layer on top of the soil; (iii) let [Pennisetum typhoides (Burm.) Stapf & Hubb.], barnewly formed humus is given a fixed C/N ratio of 10; (iv) only one ley (Hordeum vulgare L.), bean (Phaseolus vulgaris L.), litter pool is recognized for N although three are recognized for C; soybean [Glycine max (L.) Merr.], peanut (Arachis hy-(v) for residues with C/N ratios Ͻ25, the three litter pools for C pogaea L.), chickpea (Cicer arietinum L.), cassava (Mandecompose at a rate that is independent of the residue's N concentraihot esculenta Crantz), potato (Solanum tuberosum L.), tion; and (vi) SOM and residue flows are independent of soil texture. sugarcane (Saccharum officinarum L.), tomato [Lyco-A SOM-residue module from the CENTURY model was incorpoersicon lycopersicum (L.) Karsten], bahiagrass (Pasrated in the DSSAT crop simulation models, and a residue layer was added on top of the soil. Modifications were also made in the palum notatum Fluegge), and sunflower (Helianthus ansenescence module of CROPGRO, a model within DSSAT, so that nuus L.)}, with several more under development. The senesced material is now added daily to the soil. Evaluation of the model handles management strategies that involve crop model, using a data set of 40 yr of bare fallow, showed an excellent rotations, irrigation, fertilization, and organic applicafit [product moment correlation coefficient (r ) of 0.983] between tions. Although crops (or cultivars) and crop managesimulated and measured values for SOM-C. Soil N from decomposing ment (e.g., mechanization) may differ from country to SOM and residues was evaluated with data from a Brazilian expericountry or even from village to village, the effect of ment with seven leguminous residue types. By incorporating the CENfertilizer or irrigation on crop production is likely to TURY SOM-residue module, DSSAT crop simulation models have follow similar biophysical and biochemical pathways. become more suitable for simulating low-input systems and conduct-However, there is an important difference between ing long-term sustainability analyses.

oped and developing countries (Algozin et al., 1988; Bowen and Wilkens, 1998; Jagtap et al., 1993;Lal et al.,

No abstract

A preliminary study was made involving the nature of inheritance of Al toxicity in wheat. The difference in tolerance between the susceptible and moderately resistant varieties tested appears to be controlled by a single dominant gene.

Yield of irrigated cotton (Gossypium hirsutum L.) increases as the interval between water applications is decreased, even if the total amount of water applied is unchanged. Experiments were undertaken to compare season‐long water relations of high‐frequency dripirrigated cotton (I‐ to 2‐d intervals) to cotton irrigated at approximately 2‐wk intervals. The crop was grown at two locations in central Arizona on a Mohall sandy loam (fine‐loamy, mixed, hyperthermic Typic Haplargid) and an Avondale clay loam (fine‐loamy, mixed, hyperthermic Typic Torrifluvent) soils. In 2‐yr trials at each location, irrigation frequency had inconsistent effects on midday leaf water potential (ψw) during vegetative growth. When the crop developed a heavy fruit load, however, ψw of plants on the longer cycles was much lower than that of drip‐irrigated plants, even after irrigation when ample soil moisture should have been available. Plant hydraulic conductances, estimated from regressions of single‐leaf transpiration rate against ψw, were high in both treatments early in the season. Hydraulic conductance decreased greatly during fruiting in plants on long irrigation cycles but less so in drip‐irrigated plants. Late in the season, after fruit maturation and daring plant regrowth, conductances were again high and similar in the two treatments. The results imply that during heavy fruiting, mild water stress associated with long irrigation cycles triggers deterioration of the root system that is very slow to be reversed. High‐frequency drip irrigation, by preventing cyclical stress, apparently minimized this deterioration during fruit filling.