Uso consuntivo da água pela soja sob condições extremamente secas

Abstract

Em um campo piloto de drenagem com, aproximadamente, 1,1 hectare, estabelecido num solo da unidade de mapeamento Vacacaí (Planosol), foi avaliado o uso da água do solo pela cultura da soja (Glycine max (L.) Merrill) variedade davis, em condições extremamente secas. O conteúdo de umidade do solo foi medido semanalmente com uma sonda de nêutrons da Didcot Instruments, em três tratamentos de drenagem, divididos em parcelas subsoladas e não subsoladas, resultando em dezoito perfis com quatro profundidades de medida: 250; 450; 750 e 950mm. Nenhum dos tratamentos mostrou diferença significativa, pelo fato de o lençol freático ter permanecido abaixo dos drenos durante todo o período de cultivo. As diferenças entre os blocos foi a causa da maior variação no rendimento. Os rendimentos dos blocos 1, 2 e 3 foram, respectivamente, 1.700, 1.850 e 2.000kg/ha. As diferenças constatadas devem-se, principalmente, às diferentes lâminas de àgua consumida pelas plantas, respectivamente 145, 178 e 203 mm para os blocos 1, 2 e 3. As lâminas de água consumida pela cultura nos diferentes blocos foram inferiores à evapotranspiração potencial (516mm) para o mesmo período, indicando a época excepcionalmente seca no decorrer do ano agrícola 1978-79, e influenciando diretamente na desuniformidade e baixo rendimento da cultura.

The soil water used by soybeans (Glycine max (L.) Merrill), variety Davis, was measured in a field drainage study during conditions of extreme drought. The experimental field of approximately 1.1ha was located in a soil of mapping unit Vacacaí (Planosol). There were three levels of drainage treatments replicated as main plots in three randomised complete blocks. The subsoiling treatment was applied to half of each main plot randomly selected to form a split-plot design with a total of 18 sub-plots. The soil moisture content was measured weekly using the neutron scattering technique, at depths of 250, 450, 750 and 950mm at one location in each sub-plot. The analysis of variance of the crop yields indicated that none of the treatment ejects were significant which is explained by the fact that the water table stayed below the level of the drains for the entire crop cycle. The block ejects though also not signficant, were the largest contributor to yield variation. The block-average yields were 1700, 1850 and 2000kg/ha in blocks 1, 2 and 3, respectively. The block yield diferences were explained by the differences in the total quantities of water extracted from the soil by the plants in each block. The cumulative depths of water extracted were respectively 145, 178 and 203mm for blocks 1, 2 and 3, being much lower than the potential evapotranspiration (516mm) for the same period. This was attributed to the exceptional dry growing season of 1978/1979 and was the cause of relatively low yield and variability of the crop development.