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Effect of Exchangeable Sodium on the Rate and Pattern of Water Uptake by Raya (Brassica juncea L.) in the Field¹

Sodium can adversely affect crop growth, and the mechanism by which roots obtain water in sodic soils needs further study. Effect of exchangeable Na (ES) on the rate and pattern of water withdrawal by roots of irrigated raya (Brassica juncea L.) was studied in aquic Natrustalfs soils in a field experiment. Root water extraction was measured by monitoring the change in water content and the flux divergence with time in the soil profile. Flux divergence was calculated from the knowledge of the hydraulic conductivity vs. water content relations established in the field for different percent of ES soils and the hydraulic gradients obtained in situ during crop growth. Both the rate (R,) and pattern of water withdrawal from the soil profile by the roots were significantly affected by the amount of Na in the surface and subsurface layers. The R, measured in soil having ES of 38% in the 0 to 15 an layer was 0.141 cm ¹⁴C-sucrose compared to 0.233 an day^–1 in soil with ES of 4y0. In high ES soil 78y0 of R, came from the 0 to 15 an layer and the layers below 30 cm did not show any change in water content. In low ES soil 0 to 15 and 15 to 30 cm layers contributed about 77% of the total R, and water extraction by the roots was apparent to the 60 an depth.

Key Words: Exchangeable Na • Hydraulic conductivity • Hydraulic gradients • Root water extraction • Sodic soil

² Scientist S-1, S and Head, Division of Soils and Agronomy, respectively.

Received for publication October 16, 1978.