The Living Network in Prairie Soils

Abstract

1. Interrelations of prairie soils and the living network of roots within them have been examined. Grassland soils contain and act upon a more extensive portion of prairie plants than does the atmosphere. In the large area of midcontinental grasslands in western Iowa, and eastern Nebraska and Kansas, the parent materials are wind-blown loess and glacial drift. Prairie vegetation, following or accompanying physical weathering, introduced the biological force which was largely responsible for constructional processes in the soil. The accumulation of organic matter, mostly from vegetation, in the A horizon was a major factor in soil formation in both Brunizem and Chernozem soils. 2. In Brunizems the A horizon of dark-brown loam or silty clay loam is 8-16 inches thick and the lighter colored clay loam or silty clay loam of the B horizon is 12-24 inches thick. Beneath these lies the pale-brown or light-yellow C horizon of parent materials from which these very productive soils have been formed. Roots of prairie plants penetrate entirely through the solum and deep into parent materials. The vegetation everywhere is associated with soils that are highly fertile because of the good supply of plant nutrient elements in the primary materials, an adequate amount of organic matter, and small amounts of leaching resulting from moderate (25-32 inches) precipitation. 3. The root networks of the most abundant bunch grasses of uplands have been thoroughly examined. Andropogon scoparius is dominant over half of an area of 60,000 square miles, Stipa spartea and Sporobolus heterolepis form much smaller communities. Their root systems are similar in size, depth, and degree of branching. The many hundreds of roots per square foot, with their innumerable interwoven branches of great tensile strength, form a continuous network throughout the soil mile after mile. In the surface foot alone the dry weight of the root network is 2.7-4.4 tons per acre. The network becomes finer and more open with depth but it is normally 5 feet deep and thus extends downward well into the parent materials. 4. The equally wide-spread network formed by taller, sod-forming, lowland dominants-Andropogon gerardi, Spartina pectinata, and Panicum virgatum-is coarser, much heavier, and not so intricately branched. It continues much deeper, however, often to 8-10 feet, in these better watered soils. 5. The root system of each species of grass has certain inherent characteristics such as length, depth of penetration, and degree of proliferation, but these may be modified by soil conditions. 6. The root network of forbs supplements that of grasses and is especially conspicuous in the parent materials. On loess hills the roots of several grasses may extend 5-8 feet into the parent materials, but roots of many forbs have been shown to attain depths of 17-20 or more feet. 7. Many changes in parent materials result from the presence of roots. Pores and channels are formed which accelerate the entry of water and air and thus increase chemical erosion. Roots absorb water and nutrients and transport them upward through the soil. Nodules with nitrogen-fixing bacteria occur at all depths on roots of legumes. Hordes of organisms that live in the soil find entry along old root channels and cause decay. They too finally die and add to the considerable, well-distributed organic matter from decayed roots. Thus, soil development and plant growth are intimately related even in the C horizon.