Field studies were conducted to determine the effect of interrow management on root growth and rooting pattern in corn (Zea mays L.) and to relate these with changes in soil environment. Four cultural treatments, 1) control 2) 5‐cm deep postplanting cultivation, 3) 2‐cm thick straw mulch, and 4) interrow compaction, were established on sandy loam and loamy sand soils during 1970 and 1971. Straw mulch and cultivation enhanced root growth in the upper 15 cm of soil and increased the lateral spread of roots both years. The mulch treatment had less roots than the control below 15 cm. The cultivated plants had more symmetric root distribution. Interrow compaction inhibited lateral spread of roots in the surface layers and caused downward growth of roots. At the 5‐cm depth straw mulch reduced the maximum soil temperature by 2.6 C in 1970 and 2.3 to 6.3 C in 1971. Greater lateral spread and enhanced root growth in the surface layers of mulched plots were associated with reduction of soil temperature and reduction of moisture losses and resulted in better plant growth and higher yield.

Abstract. Usual residue‐management options are to remove the residue, use it as mulch with or without undercutting or to incorporate it into the soil. While the role of surface mulch in evaporation has been widely studied, the information on the effect on evaporation of mulch with undercutting or residue incorporated into soil, particularly in relation to soil type and evaporativity (Eo) is lacking. We studied the effect of wheat straw used in various ways on the course of evaporation loss from soil columns with three soils at Ludhiana, India and one soil at Bushland, Texas, USA, under two Eo's Energy‐limited evaporation rates under mulch (Eom) followed the soil‐specific relation Eom/Eo= a e(bRes+cEo), where Res is residue rate t/ha and a, b and c are constants; Eo, is expressed in mm/d. In an effort to model the total evaporation (CE) during the energy‐limited stage ‘U’ was obtained from appropriate CE versus time curves and (CE‐U) was regressed over (t ‐ ti)0.5 to obtain the slope ‘α’ (Ritchie 1972) for the soil‐limited evaporation stage. The observed ‘U’ was independent of mulch rate and Eo but was strongly affected by soil type, Values of ‘α’ decreased with increase in mulch rate and decrease in Eo and coarseness of soil. The otherwise short lived benefit of evaporation reduction with mulch per se, which peaked after a few days was maintained when residue was mixed with soil at the stage when evaporation reduction reached a maximum; this benefit continued for several weeks. Cumulative evaporation values computed from ‘U’ and ‘α’ agreed closely with the observed values under straw mulch for loamy sand and clay loam soils and for ‘undercut’ and ‘residue mixed’ treatments on all soils regardless of Eo, and for all situations under small Eo. However, for sandy loam and silt loam soils under Eo of 10 mm/d, the modified square root of the time function of Jalota et al. (1988) gave a better fit.

Harvest index (HI), the ratio of grain to aboveground dry matter, is reported to be a species-related parameter and is recommended for screening cultivars. But the fact that it is affected by environmental stress limits its use for intercrop or intercultivar comparisons. Fair comparisons should be based on estimated genetic HI for n given environment, but a procedure to determine the same is lacking. We propose that the slope of an upper-bound in the grain yield I'S. dry matter plot passing through the origin approximates the uenetic HI because the highest grain yields against given dry matter represent the least-stressed and/or stress-adapted plants and passage of the line through the origin is necessary to satisfy the definition of HI. This HI also provides a useful reference for interpreting agronomic data with respect to stress effects associated with management practices. The His of sorghum (Sorghum bicolor (L.) Moench), corn (Zea mays L.), and wheat (Triticum aestivum L.) were estimated by the upper-bounds of grain yield (corrected to dry weight) vs. dry matter yield taken from existing reports in the literature. Harvest index of sorghum and irrigated corn ranged between narrow limits of 0.48 to 0.53 and 0.58 to 0.60, respectively. Harvest index of irrigated wheat ranged from 0.38 to 0.47. Stress e,ffects on HI are illustrated in plots of published and unpublished (dry) grain yields vs. dry matter yields and reasons for the same are discussed.

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