In recent years we have seen good growth in the New Zealand dairy industry associated with a rise in dairy cow numbers along with higher stocking rates and increased use of nitrogen fertilisers. Unfortunately, these trends have also had a number of negative impacts on our soils and have lead to increased nutrient losses to our ground waters and waterways. The severity of these impacts depends not just on farming practice but also on soil type and climate so that the same practices can cause less damage in some regions than in others. Concerns over these issues are being expressed throughout the country including in Canterbury and Southland and also in North Island regions such as the Waikato which has one of the largest concentrations of dairy cattle in the world.
Compaction and pugging
Trampling by stock, leads to compaction and pugging which increase the soil's density and reduce its porosity, aeration and drainage. As a consequence, trampling damage causes soils to stay wet and soft for longer which further increases their susceptibility to trampling damage. Trampling damage is usually most severe on wet soils, in winter, with heavy (i.e. older) animals and on heavy clay soils. Trampling affects both the topsoil and also the subsoil. Where trampling occurs over a number of years, the damage it causes tends to be cumulative.
Soils damaged by trampling suffer reduced water infiltration rates, reduced water-holding capacity and reduced root-zone depth. These combine to limit the amount of water available to the pasture in summer and so lead to reduced pasture growth. The pasture growth reduction can be offset to some extent by increased fertiliser usage but this tends to decrease soil pH and to increase nutrient loss so they are not remedial in their effects - more of a 'band-aid' approach. Research to quantify the impacts of trampling on pasture growth indicates production losses of 22 to 40% over the subsequent three months. Soil compaction can have greater negative effects on pasture production than any positive effects obtained from annual fertiliser (P, K, S) applications.
Reducing topsoil compaction
Trampling damage is greatest when the soil is wet, so good grazing management during wet conditions is critical. Compaction can also be minimised if key soil-quality indicators such as pH, calcium and organic matter content are maintained at optimum levels. A soil that has good physical structure and is high in organic matter has greater resistance to compaction and also recovers more quickly from trampling due to the activity of earthworms and other soil organisms. Associated reductions in nitrogen leaching will also have beneficial effects on the soil's physical quality. Reversal of soil damage, increases pasture growth which brings about corresponding increases in soil organic matter.
Reducing subsoil compaction
Normal practice is to apply lime to the topsoil to achieve appropriate pH and calcium levels based on soil tests. This is appropriate for the topsoil but alleviating subsoil compaction is more difficult. Deep ripping to 50 cm during spring has been shown to reduce deeper compaction and to stimulate root growth down below 30 cm but this is an expensive option that is not always economic because it does not deliver a long term gain if the underlying causes of the subsoil compaction are not addressed.
A soil test (basic soil test profile + exchangeable aluminium) at 30-50 cm will show if subsoil acidity is an issue. Low pH and calcium levels and high exchangeable aluminium may combine to restrict deeper root growth. A visual inspection of the soil profile looking for signs of compaction may also be useful. Where subsoil acidity is a problem there are good reasons to increase the target pH and calcium levels in the topsoil so as to increase calcium movement down through the soil profile.
Being more soluble than lime, gypsum is more mobile so lifts soil calcium levels deeper down and improves soil structure without raising its pH. Applications of gypsum at rates between 2000 and 4000 kg/ha are more effective than lime in delivering calcium deeper into the soil. Where pH adjustment is also required, a 60:40 lime:gypsum mix at about 5000 kg/ha is an option. Combined lime + gypsum mixes also facilitate the movement of calcium to the subsoil. Incorporation of lime and/or gypsum by cultivation or deep ripping will speed the effects of these treatments.