The conjunctive use of saline irrigation water on deficit-irrigated cotton

Cotton (Gossypium hirsutum) is able to survive relatively large levels of both water and salinity stress. The objective of this study was to evaluate cotton lint production and soil salinization under a conjunctive use strategy using saline water at deficit levels. A three-year experiment applying irrigation at deficit amounts on cotton was conducted in Pecos, Texas on a Hoban silty clay loam. Treatments were four irrigation water qualities, conjunctively applied. Initial irrigation was with water having an electrical conductivity (ECIW) of 4.5 dSm-1, representing about one-third of the total amount of water applied. Thereafter, treatments were applied using water of varying ECIW, e.g., 1.5, 4.5, 9.0, and 15.0 dSm-1 for all subsequent irrigations. Total irrigation plus rain was approximately two-thirds of full water requirements. Lint yields for the three years averaged 1050, 1008, 809, and 794 kg ha-1, respectively, and treatment levels did not decline over time. However, the soil salinity levels of the three more saline treatments increased throughout the test period. Yields declined due to salinity prior to reaching the published threshold value (Maas and Hoffman, 1977) of ECe = 7.7 dSm-1. Under the deficit conditions of two-thirds of the full water requirements, the threshold level was lowered to 4.5 dSm-1. The overall yield loss that resulted from limiting water by one-third was three times > than the yield loss from even the highest salinity treatment. Relative lint yield was reduced 3% for each dSm-1 of ECIW. The pre-dawn and solar-noon leaf water potential values decreased at a rate of 0.026 and 0.042 MPa per dS m-1 of the ECIW, respectively. Study conclusions were that yields within treatments remained stable for three years. However, the increase of salinity in the soil profile indicated that long-term viability of using highly saline water conjunctively is impractical under deficit irrigation conditions. In the short-term, however, saline water of up to 15.0 dS m-1 can be used at mid-season under deficit conditions on Hoban silty clay loam soil to secure 75% of the yield level obtained by using high quality water if a pre-plant irrigation of medium quality water is first applied.