Beware damage to soil structure

The winter months are when farm soils can come under particular pressure from things like pugging and compaction.

So farmers are advised pay particular attention at this time of year to ensuring, as much as possible, their soils remain healthy, as this is essential to any profitable farming operation and its long-term sustainability.

If the soil is physically healthy and fertile, crop and pasture production will be high.

The physical structure of soil controls the movement of air and water through the soil, and the ability of roots to penetrate into the soil. It also provides habitat for beneficial organisms, including earthworms. Soil with good structure has a significant number of pores that provide aerobic conditions, good drainage and high water-holding capacity.

Plants require about 10 elements in large quantities (macronutrients), and about eight in small quantities (micronutrients). Of the major elements, carbon, hydrogen and oxygen are obtained from oxygen and carbon dioxide in the air. Others include nitrogen, phosphorus, potassium, sulphur, calcium and magnesium. Minor elements include iron, manganese, copper, zinc, molybdenum, boron, chlorine, silicon and cobalt. In natural ecosystems when plants and animals die many of these nutrients are cycled back into the soil. However, in farming ecosystems, plant or animal biomass is removed with harvesting. To supply essential elements for plant growth farmers add nutrients to the soil in a number of ways such as using fertiliser and animal effluent application to land. So, during the winter months, it’s important to safeguard this investment in soil health and fertility and the time and money involved.

Compaction and pugging of wet soils in winter can particularly damage the soil structure.

Pugging is caused by animals’ hooves sinking into the soil surface (sometimes as deep as 15 centimetres) when they tread in very wet soils. This leaves a ‘puddle’ effect and can lead to compacted layer of soil.

Such compaction occurs when the soil is compressed or squeezed. In addition to being caused by animal treading, vehicles or farm machinery also contribute to this problem.

Compaction on dairy and drystock sites is a particular concern as it reduces the number and size of pores available for water and gas movement in soil. It reduces aeration, nutrient uptake, root growth and distribution, and potentially decreases infiltration and increases runoff. The most sensitive indicator of compaction is macroporosity. Previous research reveals that macroporosity below 10 per cent will inhibit pasture growth.

Soil scientists have found that compacted soil can reduce the amount of dry matter in pasture by 200 kilograms per hectare per month. (Aerating the compacted soil at the correct depth and time can increase the amount of dry matter by about 30 per cent within six months.)

Other problems caused by pugging and compaction include more frequent and persistent surface ponding, as well as increased sediment, nutrient and effluent losses to waterways through surface run-off. It also takes longer for pugged or compacted pasture to recover after grazing and weed invasion often occurs in the bare sites pugging and compaction create.

Problems like these can be minimised by:

reducing stock density, especially on sensitive paddocks like those that are wet

not feeding out on sensitive paddocks

constantly monitoring pugging and compaction during at-risk periods and moving stock off before damage occurs

grazing the back of the paddock first.

Another important issue for soil health management and water quality is how animals are wintered.

Losses appear to be exacerbated by high density urine patches deposited at times of the year when plant growth rates are low and drainage is high (the types of conditions typical in winter). Consequently, on a per hectare basis, nitrogen leaching losses from grazed winter forage crops are high relative to losses measured under pasture.