Effects of compression of some subtropical soils on the soil properties and upon root development

Abstract

Representative soils of four Great Soil Groups from Hawaii were compressed artificially to develop a study on the effects of compaction in subtropical soils. Although this study was primarily concerned with the Low Humic Latosols which are of great importance agriculturally in Hawaii, representatives of the Hydrol Humic Latosol, Gray Hydromorphic Clay and Alluvial Soils were also included. The dominant criterion investigated was the effect of increasing soil bulk density on sugarcane roots. A method was devised to produce the conditions under which roots could be grown in soils which were compressed to various bulk densities. The relationship of the effects of variations in bulk density and aeration porosity on root morphology and proliferation was examined and is discussed. With increasing soil bulk density, roots of sugarcane develop quite normally until a density is reached at which proliferation is reduced and then roots and rootlets gradually become more distorted. Roots are incapable of penetrating a soil compressed above a critical bulk density. Seven stages of degradation to root proliferation and development are described for this continuous trend between these extremes. Each of the se stages has been correlated with bulk density . and aeration porosity values for each soil studied. Even though the particle density values. determined by standard means, for soil material are similar (2.86 to 2.87 gm/cc), a Grey Hydromorphic Clay allowed cane roots to enter soil at 1. 89 gm/cc while a Hydrol Humic Latosol did not allow roots to enter soil compressed to 0.96 gm/cc. Bulk density, per se, plays an even less significant role when variations in particle density are considered: a soil horizon of a Humic Ferruginous Latosol with a particle density of 4.01 gm/cc allowed good root distribution and proliferation at bulk densities as high as 2.71 gm/cc. Despite this great variation in the soil bulk density associated with a particular stage of degradation to root proliferation and development for widely different soils, when one particular horizon of a particular soil series is compared, the seven stages of degradation described occur at remarkably similar bulk densities. Aeration porosity values correlate much more closely with each of the seven stages of root degradation than do bulk density values. However, variations in aeration porosity are quite wide for a particular soil material which has been compressed at different moisture contents. Some preliminary investigations, not developed further in this study, indicate that there is a still closer correlation of each stage of root degradation with air permeability. Detailed investigations of other factors establish the actual weight and volume of roots within representative soils which had been compressed to various bulk densities. Distortion to the cells of roots was not established, but morphological distortions are related to increases in soil bulk density. Rates of root elongation decrease with increasing soil bulk density. Despite reduced proliferation and distortions, radio rubidium investigations indicate that roots may function when they are able to penetrate compressed soil. ' A system for estimating root development in an unknown soil is proposed from particle density and moisture retention characteristics.