Dairy Farming has an effect on the health and structure of soils.
Addition of lime and fertilisers resulting in increased plant growth rates with a corresponding increase in soil organic matter can improve soil physical structure and nutrient status. However, high stocking rates, increasing numbers of cattle and trampling of wet soils is a major cause of soil compaction. Increasing rates of nutrient leaching down the soil profile and decreasing pH in either the topsoil or the subsoil is a consequence of the increase in nitrogen fertiliser application and the increase in dairy cow numbers over the last 10 years.
This is an issue in most dairy farming regions but particularly Canterbury and Southland. The result is a reduction in soil quality through compaction of the soil and acidification of the subsoil. Where this damage has occurred over several years, the effects may be cumulative.
Water infiltration rates and the water holding capacity of soils are affected, reducing summer growth rates and increasing the risk of more pugging damage.
A simulation of dairy cow treading by AgResearch showed that the effects of treading common with normal grazing management decreased pasture yield by up to 14% with an average reduction of 9% due to effects on soil density and porosity. Further research in the Manawatu quantified treading damage reduction in pasture production at between 22% and 40% in the subsequent 3 months. On this basis, compaction of the soil can have a greater negative effect on pasture production than the positive effect expected from a base fertiliser (P, K, S) annual application and can have long term effects.
Reducing Compaction
The worst effect to pastures occurred after trampling wet soil, so grazing management during wet conditions is very important.
Compaction effects can also be minimised where key soil quality indicators pH (acidity), Calcium and Organic Matter are at optimum levels. Soil with good physical structure has some resistance to compaction and also recovers from physical damage due to the activity of earthworms and other soil organisms.
Reduction in leaching loss of nitrogen down the soil profile will have a beneficial effect on soil physical quality.
Management of the subsoil
The normal practice is to apply lime to the topsoil to achieve appropriate pH and calcium
levels froma soil test. This is fine for the topsoil but alleviating compaction in the subsoil is
more difficult. Deep ripping to 47cm duringthe first 3months of spring has been shown to
reverse the effects of compaction and stimulate root growth below 30 cmdepth however
this is an expensive option that is not always practical.
Conduct a soil test (Basic Soil test profile + exchangeable Aluminium) from the soil profile
at 30 – 50 cm. If subsoil acidity is an issue, the pH and calcium levels will be low and
exchangeable aluminium will be high whichmay combine torestrict root growth. Conduct
avisual inspectionof thesoil profile, looking for anyindicationsof compaction.
Deep ripping is a good option where the pan can be fractured and this process may
facilitate the physical movement of calcium down the soil profile however if the underlying
causeof the compaction problem is not addressed, thisis an expensive short term remedy.
Where subsoil acidity is a concern there is reason to increase the target pH and calcium
level in the topsoil to increase calcium movement down the soil profile.
Gypsum is a good option to lift calcium levels below the normal cultivation depth due to
different chemical properties in comparison with lime. Gypsum will improve soil structure
without raising pH.
Application of gypsum at rates of between 3 and 10 t/ha is known to bemore effective than
lime in moving calcium down the soil profile. Combining lime + gypsum as a treatment
facilitates leaching of calcium into the subsoil. Where pH adjustment is also required, a
60/40 blend of lime and gypsum at about 5t/ha is anoption.
Incorporation of lime + gypsum by cultivation or deep ripping will improve the effect of this
treatment. Effective treatment will require around 5t/ha of a 60/40 mix of lime + gypsum.
The objective is to minimise the adverse effects of compaction of pasture soils through
increased soil water holding capacity, water infiltration rate and root growth.
