ROOT DEVELOPMENT OF 2 COTTON CULTIVARS IN RELATION TO POTASSIUM UPTAKE AND PLANT-GROWTH IN A VERMICULITIC SOIL

Cotton (Gossypium hirsutum L.) cultivars exhibit marked differences in yield on vermiculitic soils where late-season K deficiency causes yield reductions. A two-year field study was conducted to determine whether differences in root growth contribute to the differing sensitivity to late-season K deficiency of two cultivars. Cultivar 'Acala GC510' accumulated 41% more dry-matter, 37% more K, and proeuced 22% (1986) to 28% (1987) more seed cotton than the sensitive cultivar 'Acala SJ-2'. Levels of extractable K were greatest in the surface 0-0.30 m, decreasing sharply below this depth. Root-length and surface-area density of both cultivars were greatest in the 0.10-0.20-m soil layer but least in the surface 0-0.10 m. Fluctuations in soil matric potential were greatest in the 0.10-m topsoil layer, which limited root extension in this zone. The K-efficient cultivar had a larger mean root diameter and an increased rate of root extension after peak bloom that resulted in 58% more total root surface area by mid-August. Differences in root surface area at 0.10-0.30-m depth were positively correlated with cultivar differences in K uptake, and root development in this subsurface layer appears to be critical for adequate K acquisition by cotton in soil with low subsoil K supply. Root surface area of both cultivars at 0.15-0.30 m was negatively correlated with soil bulk density, but root surface area of the more K-efficient cultivar was consistently greater across the range of measured bulk density. The results demonstrate significant genotypic differences in K uptake and sensitivity to late-season season K deficiency that were associated with differences in the determinacy of root growth after peak bloom and/or in root-growth response to soil bulk density. © 1990.