Exploitation of Soil Potassium in Layered Profiles by Root Systems of Cotton and Barley

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Abstract
On vermiculitic soils of the San Joaquin Valley, cotton (Gossypium hirsutum L.) often responds to K fertilization where other annual crops do not. These soils have markedly higher available K in surface soil than in subsoil. A pot study was conducted to compare rooting patterns, K uptake, and dry matter production of cotton and barley (Hordeum vulgare L.) in soil profiles with layered K availability. Topsoil (0.15 cmolc K kg−1) was layered to each of six depths above subsoil (0.09 cmolc K kg−1) in 17‐L pots with a total soil depth of 45 cm in all treatments. Two irrigation regimes were imposed and nutrients other than K were adequately supplied. For both crops, plant dry matter and K uptake increased linearly with increased topsoil depth but K uptake per unit increase in topsoil depth was 6.5‐ and 3.6‐fold greater by barley than for cotton with frequent and infrequent irrigation, respectively. Increased K uptake per unit increase in topsoil depth reflected coincident root and K distribution: barley root length density (RLD) was 2.7 times greater than RLD of cotton in topsoil layers but little different in subsoil layers. Poor exploitation of topsoil layers by the cotton root system was attributed to greater sensitivity to low soil water potential. A root system with little compensatory root development in the surface soil when the subsoil is low in nutrients may limit K uptake and crop productivity on layered soils in the San Joaquin Valley and may require management systems designed to promote more congruent root and nutrient distribution.