Freelance writer Jane Carley gets down to earth to provide practical tips on how to look after your soil and make the most out of one of farming's most precious resources.
Soil is a precious resource and farming recognises that action must be taken to manage it, preserving structure and fertility for future generations.
No-one knows the value of good soil more than farmers, and it is farmers who hold the land management keys to restoring, and improving, fertility.
Weather conditions, landscape, land use and soil type, texture and soil structure can combine to increase the risk of erosion and runoff. A soil with poor soil structure has a high risk of generating runoff. This is greatest when poor soil structure is near the surface. Deterioration occurs when structural units are deformed as pressure is applied to a wet and soft soil; for example, by machinery or the movement of livestock.
When rainfall intensity exceeds infiltration rate and the soil becomes saturated at the surface, runoff occurs. Naturally well-drained soils rarely become saturated and readily absorb most rainfall. Where the surface loses its porosity, runoff can occur on well-drained soils, when rainfall is as low as 1mm/hr.
Soil texture influences the degree of percolation of water through the soil – those with large proportions of sand have relatively large pores through which water can drain freely and are at less risk of producing runoff. As the proportion of clay increases, the size of the pore space decreases, restricting the movement of water through the soil and increasing the risk of runoff.
Poor soil structure
The shape of wheel imprints from machinery in tramlines channels rain water. If tramlines become compacted, this can create pathways for runoff, which carries soil, nutrients and pollutants to the edge of the field, and potentially to rivers. Research has shown that 80% of runoff in arable fields on sloping land can come from these compacted tramlines.
Soils can restructure due to natural fracturing processes when clay shrinks and swells, or appropriate cultivation. Biological activity also restructures soil.
Avoiding bare soils or producing rough surfaces (e.g. ploughed land, coarse seedbeds, or pressed land with indentations) help to slow down runoff. Roughness provides storage of rainwater, allowing water to collect before it soaks into the soil.
For some fields, extra storage can be created if the ploughed land is worked across a slope and not up and down a slope. Rough surfaces also help to reduce wind speed at the immediate soil surface, preventing wind erosion.
Farming and Wildlife Advisory Group adviser Jo Oborn adds that good soil husbandry can increase the ability of soils to accept and store water.
“Sandy soils are fragile due to their low clay content – they don’t hold together well so are easily overworked, especially where rotations have focused on potatoes, maize and cereals,” she said. “As they don’t restructure they may need mechanical intervention to tackle plough pans, and while min-till is becoming increasingly popular, it is best used in conjunction with cover crops on this type of soil.”