Three experiments on soybean water relations were carried out for three years at Mlalir Farm of Sindh Agriculture University Tandojam. The crop was irrigated in response to 3, 5 and 8 bar soil moisture tensions and irrigation schedules were also tested at different stages of vegetative and reproductive growth. The fertility levels were 50-60-30 and 50-90-30 kgs/ha. The Sultanpur silt loam contained 23.5% and 8.65% moisture at field capacity and wilting point. The infiltration rate of the soil was 7.06 mm/hr. Averagely, the cumulative ET was 767, 643 and 535 mm, when the crop was irrigated at 3, 5 and 8 bar tensions. The ET increased with the crop age with maximum at 105 to 115 days after sowing, which corresponded to the pod fill stage. The increase in moisture stress forced plants to draw higher percentage of water from the lower layers than the plants under less stress. The KC increased with crop age and soil moisture availability. The highest KC occurred at 105 to 115 days after sowing, following the same pattern as ET. The grain yield reduced by 28% and 20% at 8 and 5 bar as compared to 3 bar moisture stress. It was about 10% more in 90 than 60 kg P2O5 /ha treatment. Similarly positive effects of higher P2O5 were observed for yield components. The water stress also reduced; height, nodule weight, pods and grains per plant, and seed weight. The increase in phosphorus augmented number of pods and grains per plant as well as seed weight. The plants under stress and higher fertility were more efficient in producing grain/mm of water consumed up to 5 bar tension. The oil and protein content of seed was not influenced by moisture stress. The moisture stress at any growth stage of crop reduced grain yield. However blooming, pod formation, and pod fill were the most critical stages for water stress, which caused high reductions in number of pods and grains per plant, seed weight and yield. The soil moisture stress at early vegetative growth also caused reduced height and plant population.

INTRODUCTION