Relationship between the distribution of soil macropores and root elongation

We studied the role of soil macropores in the promotion of root elongation. The roots of maize plants were grown in undisturbed soil columns 30 cm in diameter and 10 cm in height. The number of visible macropores with or without roots was counted in horizontal sections cut every 2–3 cm. The soils used were taken from the BC and C horizons of a Brown Lowland Soil, the BCg and Cg horizons of a Pseudogley and the B horizon of an Ordinary Andosol. The distribution patterns of the soil macropores varied with the soil types. Brown Lowland Soil was characterized by a large number of tubular macropores with a clustered distribution pattern. Ordinary Andosol was characterized by a small number of tubular macropores with a scattered distribution. Pseudogley was characterized by a linked distribution of interstitial macropores. The degree of clustering or linking of the macropores was more pronounced in the B horizons of both the Brown Lowland Soil and Pseudogley than in the C horizons. χ2 test revealed that the root elongation depended on the presence of macropores in each type of soil. Moreover, the distribution of the roots corresponded fairly well with the distribution of the macropores. The proportion of roots that elongated into macropores was higher in the soils with abundant clusters or linkages of macropores. To analyze this relationship, we introduced the concept of entropy as an index to characterize the distribution of macropores. The more pronounced the degree of clustering or linking of the macropores, the lower the value of entropy. The value of entropy, however, was also affected by the mean density of the macropores, and the larger the number of the macropores the higher the value of entropy. To eliminate the influence of the mean density, the ratio of the actual entropy (S′) to the entropy (S) calculated on the assumption that all the macropores were distributed independently. The values of S′/S were low in the B horizons of the Brown Lowland Soil and Pseudogley while high in the Ordinary Andosol. The values of S′/S showed a strong negative correlation with the proportion of the roots penetrating the clustered or linked macropores. This fact suggests that the distribution patterns of the macropores may reflect the mechanical strength, bulk density, and other factors in the soil which hamper root elongation.