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Phosphorus Uptake by Soybeans as Influenced by Moisture Stress in the Fertilized Zone¹

Reports on the effect of soil moisture stress on plant uptake of P have been conflicting. The effect of the dynamic variation in soil moisture on P uptake by soybeans (Glyfine max L. Merr.) was studied in vivo. A split-root technique with plants grown in 3 liter containers in the greenhouse was used whereby moisture stress was localized in the zone having the only source of P for the roots, while water and P-free nutrients were supplied through the other portion of the root system. The procedure minimized the effect of plant-water deficit.

Differences between moisture treatments accounted for 81.5% of plant P content when the effects of fertilizer treatments were removed, while variation in soil moisture content within each moisture treatment accounted for only 8%. The extent P uptake was impaired by high moisture stress was conditioned by the P status of the plant which in turn was determined by the previous soil moisture history of the zone supplying the P. After watering, plants exposed to a high moisture stress resumed P uptake at a rate exceeding that of plants absorbing P from the adequately watered compartment.

The range of response to which increasing levels of fertilizer P was obtained increased as the period of moisture stress in the fertilized zone was prolonged. Temporary luxury consumption of P, induced by high levels of soil P and foliar application, benefited soybeans whose P supply was curtailed by high moisture stress.

Moisture stress appeared to affect P uptake largely through its influence on diffusion rather than mass-flow. It is suggested that the inconsistencies of field results reported in the literature studying the relationship of P uptake and moisture stress could be caused by the peculiar P uptake behavior found in these experiments.

Key Words: Plant-water deficit • Translocation

² Senior lecturer, Dep. of Agron., Univ. of Fort Hare, South Africa (former graduate student) and professor of agronomy, Purdue Univ., respectively.

Received for publication February 8, 1975.