Forage Legume Response to Subsoil Compaction

Compaction strongly influences soil physical properties such as bulk density, pore size and continuity, aeration, permeability, penetration resistance and soil water and temperature regime (Panayiotopoulos et al., 1994). Adverse effects of compaction on plant root growth and concomitant poor plant growth and yields have been well recognized (Barraclough and Weir, 1988), especially in fine textured soils (Gomez et al., 2002). Soil layers compacted due to machine traffic which is highly resistant to penetrate plant roots are one of the most common problems in agriculture. (Camargo and Alleoni, 1997). In addition to preventing root growth in the soil, high bulk density interferes with the movement and distribution of water in the profile, increasing the risk of erosion and low availability of water and nutrients to the plant. Uptake of nutrients by crops is of great importance to the farmer as well as to society as a whole since it has a major impact on the economic outcome of crop production. Furthermore, nutrient uptake has implication for environmental health by way of nutrient leaching and run-off into water bodies. Compaction affects nutrient availability and uptake through a number of mechanisms. Aeration negatively affects the availability of nitrogen, manganese and sulphur which are involved in redox reactions, and the growth and function of roots (Lipiec and Stepniewski, 1995). Transport of nutrients in the soil is decreased as compaction normally increases mass flow transport (Kemper et al., 1971) and the diffusion coefficient at a given gravimetric water content. Compaction increases root-to-soil contact, which may facilitate nutrient uptake (Veen et al., 1992), but generally reduces root growth through its effect on aeration and mechanical resistance. Considering that the mechanical methods used to eliminate compacted soil layers are expensive and energy consuming, an attractive alternative could be to use plants with vigorous roots to modify the compacted subsoil (Dexter, 1991). The use of plants with vigorous roots as a strategy in compacted soil management provides more uniform rupture of compacted layers than the common mechanical methods (Camargo and Alleoni, 1997). Compaction of the soil below the depth of tillage is referred to as subsoil compaction (Jorajuria et al., 1997).