Otimização do uso de água e nitrogênio no cultivo do algodoeiro herbáceo na região oeste da Bahia.

Abstract

Cotton cultivation is expanding in the western part of Bahia and needs technical-scientific research base to improve the orientation to the producers. Seeking to contribute for the development in the area, this work was accomplished at Santa Cruz Farm, located in Barreiras-BA (coordinates 12º03'09” of latitude South, 44º57'46,6” of longitude West of the meridian of Greenwich, altitude of 660 m) during March to September of 2004. The research consisted of two experiments where the growth, the financial returns and technological characteristics of fiber of the herbaceous cotton (Gossypium hirsutum L. r. latifolium Hutch.) cultivar Deltapine Acala 90, as function of water depth (Experiment I) and of interaction between water depths and doses of nitrogen (Experiment II) were evaluated, under central pivot, in a Yellow Latosol, loamy sand. The treatments of depths of water studied were 0.75, 0.90, 1.05 and 1.20 of the evapotranspiration of the crop (ETc) (L1 to L4) and the levels of N were 108, 168, 238 and 308 kg.ha-1 (N1 to N4). The growth variables consisted of: reproductive phytomass (FR), vegetative phytomass of the aerial parts (FVPA) and its partitions, phytomass of stem (FC) and leaves (FF), number of leaves (NF), leaf area (AF) and mean length of leaves (CMF) at 120 days after emergence (DAE) and height (AL), number of internodes (NE) and stem diameter (DC) at 160 DAE. Besides this the effects of the treatments on the technological characteristics of cotton fiber and physical and economical return of cotton were also evaluated, considering: cost of nitrogen of R$ 2.3.kg-1 (CN), of the water of R$ 0.36.mm-1.ha-1 (CL) and cotton price of R$ 1.2.kg-1 (PZ). The growth at 120 DAE optimized at water depth (up to 120 DAE) between 441 and 477 mm, and with N levels superior to 270 kg.ha-1; for the growth in height at 160 DAE the respective values were between 622 and 655 mm and the dose of 298 kg.ha-1 of N; the effect of the factors on DC was interactive; the production function obtained for the cotton, under the fertilizer management of the farm (212, 117 and 236 kg.ha-1 of N, P2O5 and K2O, respectively), had the maximum point at water depth of 671 mm and production of 5,522.8 kg.ha-1 of cotton, but the economical depth (L*) was found to be 656.5 mm for the relationship: CL/PZ=0.3; Z obtained with L* was 352.34 kg.ha-1 superior to that obtained under the water management of the farm, resulting in an additional net return of 25.82%; the optimal economical combination of water and nitrogen was 631 mm of the water and 287 kg.ha-1 of N, with Z of 5,581.82 kg.ha-1, for the relationship CL/Pz=0.3 and CN/PZ=1.92 and fertilizer levels of P and K used in the Experiment II (177 and 209 kg.ha-1 of P2O5 and K2O). The technological characteristics of fiber varied little with the treatments and independent of the depth of water or N doses studied attend the requirements of the national textile industry.