Soil physical factors limiting establishment of sorghum and cowpea in two contrasting soil types in the semi-arid tropics

Abstract

Experiments were carried out to study which soil physical factors, if any, would restrict germination and emergence of sorghum ( Sorghum bicolor (L.) Moench.) and cowpea ( Vigna unguiculata (L.) Walp.), under field conditions in Tanzania. The study also aimed to consider the usefulness of mulching as a strategy to overcome such problems, and to contribute to the development of a strategy for modelling seedling emergence in seedbeds in the field. Sorghum and cowpea were planted at two sites in experimental plots which simulated local farming practices, with both bare soil and mulched treatments. The soil types were a sandy clay loam (Paleustaulf) and a sandy loam (Typic Untorthent), both of which were hardsetting. Sequential, daily harvests of the seedlings were related to measurements of soil matric potential and temperature at different depths. Penetration resistances (PRs) were also estimated from matric potentials, and strength characteristics obtained from the same soils. Germination of both crops was high (> 85%) in all of the trials. Emergence of sorghum seedlings was poor (0–30%) in all cases. In the sandy clay loam it was primarily the large mechanical impedance that prevented emergence, whereas a combination of high temperature, water stress and mechanical impedance all contributed to poor emergence in the sandy loam. Cowpea emergence was higher but still only 56% or less in the sandy clay loam due to the increase in mechanical impedance to shoot growth as the soil dried. In the sandy loam, emergence of cowpea was very good (> 85%). The greater percentage emergence of cowpea was related to its greater initial rates of root and shoot growth, which allowed it to avoid the greater stresses caused by temperature, drought and mechanical impedance, experienced by the sorghum. In comparison to sorghum, the larger cowpea shoots were also able to crack a greater thickness of overlying soil when they approached the soil surface and hence overcome mechanical impedance that might otherwise have prevented emergence. The mulched treatment reduced soil surface temperatures by up to 20°C, and delayed the onset of water stress and high mechanical impedance. However, in comparison to the bare soil treatment, the overall effect of these factors varied between a decrease in the percentage emergence of cowpea in the sandy clay loam and an increase in emergence of sorghum in the sandy loam. It is suggested that a modelling approach is required to understand fully these interactions.