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Soybean Response to Phosphorus and Potassium Fertilizers as Affected by Soil Drying

USDA-ARS, National Soil Tilth Lab, Ames, IA 50011

J. B. Zahler

Comité Central Mennonite, B.P. 1307, Ouagadougou, Burkina Faso

*Corresponding author.

ABSTRACT

Soybean, Glycine max (L.) Merr., response to P and K fertilzers can vary with the amount of rainfall received during a growing season, but it is often difficult to determine whether growth is limited by unavailability of nutrients and/or water. A 2-yr study was conducted in a rhizotron at Ames, IA, to determine whether drying of the fertilized, surface soil layer reduces plant growth and utilization of P and K fertilizers when subirrigation replenishes water below the fertilized surface layer. Low, medium, and high fertility levels were established on soil obtained from the upper 0.15 m of a Sparta loamy, fine sand (sandy, mixed, mesic Entic Hapludoll). Two surface-drying treatments, one beginning during late flowering (dR2) and the other after the start of seed fill (dR5), and a control were imposed. Shoot tissue concentrations and dry weights at the fullseed stage (R6) and mature seed yield were measured. Surface drying reduced total shoot accumulation of P and K. Leaf and pod P concentrations were lower for the dR5 treatment than for the dR2 treatment. Shoot K concentrations of the dR2 treatment increased less than those of the other treatments as soil K levels increased. Shoot dry weights at R6 and mature seed yield were reduced at the low fertility level. Surface-drying treatments reduced total shoot dry weight (42.8 and 40.9 g plant ^–1) compared with the control (49.5 g plant^–1) in both years, but yield was reduced only in 1984. This study shows that drying of the fertilized soil layer reduced shoot growth and utilization of applied P and K fertilizers even though water was available at greater depths.

Joint contribution: USDA-ARS and Journal Paper no. J-13016 of the Iowa Agric. and Home Econ. Exp. Stn., Ames; Project 2659.

Received for publication June 15, 1988.

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