CRITICAL SOIL BULK-DENSITY AND STRENGTH FOR PEA SEEDLING ROOT-GROWTH AS RELATED TO OTHER SOIL FACTORS

Bulk density and soil strength are two major soil physical factors aff ecting root growth of pea seedlings. This study was conducted to deter mine the influence of soil texture, organic carbon content and water c ontent on critical bulk density and strength. Soil from the plough lay er (PL) and beneath the sub-soil (SUB) was used. By soil packing and a djusting the water content between 30% and 100% of field water capacit y (FWC) a wide range of bulk density (1.3-1.7 Mg m(-3)) and strength ( 0.24-6.66 MPa) were obtained. Pea (Pisum sativum L.) was grown in the packed cores of 100 cm(3) for 72 h at 20 degrees C. Regression models were developed to explain root growth in terms of bulk density, soil s trength, silt and clay (<60 mu m) content, organic carbon, and water c ontent. The regression curve of root growth as a function of soil stre ngth showed that 40% of maximum root length can be regarded as an indi cator of very poor root growth. By substituting this value into the ro ot growth equations we calculated a critical bulk density and strength in terms of fraction<60 mu m, organic carbon percentage and water con tent. The values of critical bulk density in both layers and of critic al soil strength in the sub-soil increased with a decreasing content o f fraction<60 mu m. Irrespective of fraction<60 mu m content, the crit ical bulk density and strength decreased as soil water content decreas ed. Critical soil strength was more sensitive than critical bulk densi ty to changes in fraction<60 mu m content and water content. This stud y provides data and a method for predicting critical bulk density and soil strength in relation to other soil properties for pea seedling ro ot growth. (C) 1998 Elsevier Science B.V. All rights reserved.