Dr Gordon Rajendram: Canterbury's Leaching Season and the Hidden Losses Below Ground
As Canterbury moves into autumn and winter, farms are entering the main leaching season. The period between April and October is when nutrient losses below the root zone are at their highest risk.
During winter, soil temperatures drop, and pasture growth slows significantly. Once soil temperatures fall below around 6 °C, ryegrass growth becomes limited, meaning plants are not taking up nutrients efficiently. At the same time, rainfall continues moving water through the soil profile. When soils become saturated, drainage increases, carrying soluble nutrients deeper into the ground.
Canterbury presents a unique challenge compared to many other farming regions in New Zealand. Naturally, much of the region only receives around 600–900mm of annual rainfall. However, once irrigation is added to farming systems, soil moisture levels become similar to much wetter regions of the country.
When heavy winter rainfall occurs, nutrients can be flushed below the pasture root zone before plants can utilise them.
Most farmers associate leaching with nitrate nitrogen, and rightly so. Nitrate is highly soluble and easily moves with water through the soil. However, nitrogen is not the only nutrient being lost during winter drainage.
Important nutrients, including calcium, magnesium, potassium, sodium and sulphur, can also be leached from the soil profile. These nutrients are essential for both pasture growth and animal health.
Research I was involved in showed that intensive grazing systems can lose significant quantities of magnesium and calcium annually through leaching. In some situations, approximately 30kg of magnesium per hectare per year was being lost below the root zone. If these nutrients are not adequately replaced, soil fertility gradually declines over time, which is what the Ministry of Agriculture found was occurring in the 1980’s and 1990’s.
These losses equate to a large sum in terms of dollar value.
One of the key lessons for farmers during the leaching season is to be careful with highly soluble fertilisers during periods when soils are cold, wet, and pasture growth is limited.
If soluble nitrogen and sulphur fertilisers are applied when there is little plant uptake occurring, there is a much greater chance that nutrients will move beyond the root zone during drainage events. Timing fertiliser applications closer to active pasture growth periods can improve nutrient efficiency and reduce unnecessary losses.
Soil texture, structure, and drainage pathways also influence leaching risk. Lighter soils and free-draining profiles allow faster movement of water and nutrients compared to heavier, more retentive soils.
Healthy soils with good structure, balanced fertility and efficient irrigation management are better at holding nutrients and supporting plant uptake.
Contact Dr Gordon Rajendram
021 466077
rajendram@xtra.co.nz
www.gordonrajendramsoilscientist.co.nz
Contact Media PA
phillip@mediapa.co.nz
027 458 7724