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Residual straw affects the physical, chemical, and biological attributes of soil and can influence plant transpiration and the evaporation of water from the soil. Therefore, in this study, we evaluated the effect of straw on evaporation and transpiration of sugarcane. The experiment was conducted in a 2.5 ha area irrigated via a central pivot. The experiment consisted of two treatments, namely, with and without straw removal of soil. Evaporation was determined by means of equations and transpiration with sap flow sensors using the heat balance method. Evapotranspiration of the crop was measured using the Bowen ratio method and compared with the sum of the collected soil evaporation and transpiration data. On the basis of relationship analysis between the evapotranspiration of the crop measured using the Bowen ratio method and the sum of the sap flow combined with estimates of the evaporation of soil water, we obtained coefficient of determination values of 0.65 and 0.69, and angular coefficients of 1.01 and 0.96 for treatments with and without straw, respectively. We accordingly found that the use of straw on the soil reduces the evaporation of soil water and increases the transpiration and evapotranspiration of the crop.

Reactive natural phosphate is a slow and gradual solubilizing fertilizer, which makes it difficult to use in neutral to alkaline soils. Nitrogen fertilizers which acidify the soil may increase the possibility of using this phosphate fertilizer commercially. Two greenhouse experiments were conducted to compare responses of Xaraés palisadegrass (Urochloa brizantha syn. Brachiaria brizantha cv. Xaraés) and Mombasa guineagrass (Megathyrsus maximus syn. Panicum maximum cv. Mombasa), when different combinations of P and N fertilizers were applied during the establishment phase in non-acidic soils or with corrected acidity. The experiments were carried out in a completely randomized design with 3 fertilizer combinations (simple superphosphate plus urea, SSU; natural reactive phosphate plus urea, RPU; and natural reactive phosphate plus ammonium sulfate, RPAS). There was no difference in tiller density, leaf numbers, forage mass, leaf mass and stem mass for either forage on SSU and RPAS treatments but they exceeded those on RPU. Soil pH was lower in soil fertilized with ammonium sulfate than in soil fertilized with urea. Applying natural reactive phosphate plus ammonium sulfate seems as effective as simple superphosphate plus urea in promoting increased growth in tropical grasses on low-P soils. Longer-term and more extensive field studies are needed to determine if these results can be reproduced in the long term, and the level of soil acidification over time.