Soil macroporosity, soil health and plant production
Subsoils with sufficient macropores have little restriction to drainage and aeration. Poor aeration leads to the build up of carbon dioxide, methane and sulphide gases, and reduces the ability of plants to take up water and nutrients. The number, activity and biodiversity of micro-organisms and earthworms are also greatest in well aerated soils and are able to decompose and cycle organic matter and nutrients more efficiently.
Roots are unable to penetrate and grow through firm, tight, compacted soils, severely restricting the ability of the plant to utilise the available water and nutrients in the soil profile. A high penetration resistance not only limits plant uptake of water and nutrients, but greatly reduces fertiliser efficiency and increases the susceptibility of the plant to root diseases. Soils with good porosity will also tend to produce less greenhouse gases (carbon dioxide, methane and nitrous oxide) during wetter and warmer conditions.
Potential rooting depth is the depth of soil that plant roots can potentially exploit before reaching a barrier to root growth, and generally indicates the ability of the soil to provide a suitable rooting medium for plants. The greater the rooting depth, the greater the available water-holding capacity of the soil. During dry growing seasons deep roots can access larger water reserves to help alleviate water stress.
The exploration of a large volume of soil by deep roots can also access more nutrients. Conversely, soils with restricted rooting limit plant uptake of water and nutrients, reduce fertiliser efficiency, increase leaching and can decrease crop yield. A high resistance to root penetration can also increase plant stress and the susceptibility of the plant to root diseases.
Crops with a deep, vigorous root system help raise soil organic matter levels and soil life at depth. The physical action of the roots and soil fauna and the glues they produce, promote soil structure, porosity, water storage, soil aeration and drainage at depth. A healthy root system provides capacity for raising production and may provide significant environmental benefits. Crops are less reliant on frequent and high application rates of fertiliser and nitrogen to generate growth and available nutrients are more likely to be intercepted, thus reducing losses by leaching into the environment.
Subsoil compaction causes slower water movement through the soil in most circumstances. Depending on soil type, the severity of compaction and the depth at which it occurs, water may not drain readily through the root zone. The risk of perched water table and waterlogging is increased, particularly on loamy sands and heavier soils. There is little effect on sands. Waterlogging and low porosity increases the chance of low oxygen (anaerobic) conditions, which leads to a significant decrease in a plant’s root function and nutrient uptake.
Soil strength
An increase in the soil strength is the major problem of subsoil compaction. Subsoil compaction caused by farm vehicles and machinery results in a layer of higher strength soil commonly at 10-40cm depth although on deep sands along the south coast, compacted layers are often from 20-50cm depth. These traffic and plough hardpans may be a few millimetres to 25cm thick, depending on soil type, agricultural practices and other environmental conditions.
In compacted layers the soil strength can be too high for roots to grow normally. Roots may grow sideways along the top of the compacted layer or be restricted to cracks and old root channels. If they are able to penetrate the compacted layer, roots grow slowly, the root tips may become damaged and thickened, grow in a tortuous pattern and exhibit increased branching.
Soils with high nutrient exchange capacity clays gravels will not need as much depth or rooting volume as sands which have low nutrient exchange capacity; thus can grow on shallower soil and above a compaction layer, especially if rainfall is frequent enough.
Yields from deep sands are more sensitive to shallow root depth than loams Restriction of root depth to 60cm in deep sand can be a penalty of 1t/ha, loams and even more clays, can hold much more water and nutrient, so may require less depth than sand to supply a crop expected from the rainfall received.