With the recent increase in stock levels of dairy cows comes an increase in the use of nitrogen fertilisers. Unfortunately these trends have had a number of negative impacts on our soils and have lead to increased nutrient losses through ground waters and waterways.
"It is useful in the transition period in dairy cows 2 – 4 weeks pre & post calving, and can be used as an anionic salt to counteract the effects that high potassium & sodium concentrations have on increasing hypocalcemia. Gypsum, a readily available form of calcium, is 100 times more soluble than lime and is more suitable for the digestive system during this period"
The severity of these impacts is influenced by farming practice, soil type and local climate as farmers with the same practice will experience different levels of success determined by their region. These issues are being expressed throughout the country from Canterbury to 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 increases the soil’s density and reduces its porosity, aeration and drainage. As a consequence soil stays soft and wet for longer which further increases its susceptibility to trampling damage. Heavy animals traffic damages topsoil and subsoil and is more severe on wet, clay soils.
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. Pasture growth reduction can be offset by increased fertiliser usage but this decreases soil pH and increased nutrient loss.
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
Good grazing management is critical when the soil is wet and more susceptible to damage. Maintaining key soil-quality indicators (such as pH, calcium and organic matter content) will also minimize compaction.
Activity from earthworms and other organisms improve soil structure and allow greater resistance to, and better recovery from, compaction.
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 gain in soil organic matter.
REDUCING SUBSOIL COMPACTION
Applying lime to the topsoil will deliver appropriate pH and calcium levels on 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 and only a short 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.