Cotton Root Growth and Soil Water Extraction

Abstract

AbstractThe inherent productivity of many fine‐textured soils depends on timely extraction of water stored deep in the profile. Growth of crop roots and their ability to extract water are of central importance to efficient utilization of these soil resources. We observed the rate at which the soil volume was explored and water was extracted by cotton (Gossypium hirsutum L. ‘GP3774’) roots in a fine‐textured soil in a subhumid environment. Measurements of root growth and water extraction were made for 2 yr in a 2.3 m deep weighing lysimeter, 3.0 m2 in area and containing an intact core of a Frio taxadjunct silty clay loam (fine, montmorillonitic, thermic Cumulic Haplustoll). Water extraction was measured with neutron probes at 10 or 11 soil depths in the lysimeter at approximately 5‐d intervals. Root length density [Lv, cm root (cm3 soil)−1 = cm−2] was estimated at 7‐d intervals by counting roots visible against clear observation tubes installed horizontally in the lysimeter every 0.2 m between 0.5 and 2.1 m. Depth of rooting increased up to 35 mm d−1 for part of the season. Maximum Lv at 0.5 m was 1.5 and 0.8 cm−2 in 1984 and 1985, respectively; at 2.1 m comparable values were 0.24 and 0.04 cm−2. When the lysimeter surface area was divided into six 0.5‐m2 regions of soil, the coefficient of variation in Lv among the regions reached 100% both early and late in the season and both at 0.5 and 2.1 m. Crop rows did not affect the spatial distribution of Lv at depths measured. Growth of the root system required about 90 kg ha−1 d−1 for part of the season. Maximal water uptake rate per length of root was approximately 5 mm3 water (mm root)−1 d−1 and occurred at 1.7 to 1.9 m. Redistribution of dry matter invested in roots near the soil surface to deeper in the profile would probably increase total water extraction by the crop.