Mechanical reinforcement which stabilizes soil on slopes has been attributed to plant roots. To measure such reinforcement, direct shear tests were made on 25‐cm diameter root‐permeated soil columns. Roots of alfalfa (Medicago sativa), barley (Hordeum vulgare), and yellow pine (Pinus ponderosa), each increased the shear resistance of homogeneous and compacted layers of silty clay loam at 30‐cm depth. One‐year‐old alfalfa had a much greater reinforcing effect than pine trees 16 months after transplanting or barley at its maximum growth. Barley had a greater effect in the clay loam than pine, but its effectiveness decreased as depth increased from 15 to 30 to 45 cm. Alfalfa roots were more effective than either pine or barley roots in increasing the resistance to shearing between a dense gravel‐sand layer (simulating weathered rock) and the overlying soil, increasing shearing resistance to as much as 5 times that of fallow soil. A model is presented of soil reinforced by nonrigid roots. Calculations are given of slope safety factor increases from root reinforcement.

Abstract. Light transmission through several sands, soils and size classes of glass beads was investigated. The glass beads served as a highly reflective model system for the sands. Light penetration was significantly better through sands than through either sandy or silty loams. For sandy soils, there was a shift in the spectrum leading to a change in the red‐to‐far‐red photon ratio with increase in depth: this ratio dropped by more than 30% in the first few millimetres, and in some cases increased again with further increases in depth. The depths at which these changes occurred in the soil profile were specific for a given soil type. Similar spectral changes were obtained when sample depth for a sandy soil was held constant, and particle (ped) size of that soil was decreased. No such changes, either with sample depth or ped size, were observed with loams. Per cent transmittance in the blue (442 nm) was more than three orders of magnitude lower than in the red (632.8 nm) for sandy soils. When dry sandy soils were water‐saturated, their transmittance increased by up to three orders of magnitude, and the red far‐red ratios in the transmitted light increased in all cases. By contrast, water‐saturated loams were essentially opaque. The possible significance of these spectral properties of soils in seed germination and seedling photomorphogenesis is discussed.

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Plant roots have been shown to increase soil shear resistance through direct mechanical reinforcement and thereby enhance soil stability on slopes. Because of their potential large diameter and length, the roots of trees may be especially significant in such soil reinforcement. To provide large rooting volumes simulating natural conditions and shear cross sections many times larger than potential tree root diameters, we constructed 12 1.22‐ by 1.22‐m cylindrical soil containers in which two artificial soil profiles were prepared. Replicates of each profile type were planted to alfalfa, yellow pine, or were kept clear of plants. A large pneumatic direct shear device sheared both root‐free and root‐permeated materials along a horizontal plane at the 0.6‐m depth at either constantly maintained shear stress levels (creep shear) or at constant shear displacement rates.Creep shear at the 0.6‐m depth showed that roots of 14‐month‐old alfalfa increased the shear resistance of homogeneous clay loam and a clay loam/gravel interface simulating a soil‐weathered rock boundary by 32 and 50%, respectively. Constant shear displacement rate experiments at the same depth on similar samples permeated by roots of yellow pine planted 54 months earlier showed shear resistance increasing steadily with displacement over the entire test displacement range. At 75 mm displacement, the shear resistance of the pine‐rooted soil was about two times that of the nonrooted in both profile types. The superiority of pine roots to alfalfa roots in increasing soil shear strength in these experiments is in accord with field observations that woody plants are more effective than herbaceous plants in stabilizing soil against slips and slides. Pine root‐size distributions determined 11 months after the last shear test were combined with root tensile strengths and Young's moduli previously measured and used in model simulations of root reinforcement with good results.

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