Hamilton-based soil scientist Dr Gordon Rajendram on the effect of rate of nitrogen fertiliser on cation and anion leaching
The leaching of nutrients in soils generally occurs between the months of April and October, and there’s a lot going on below ground – more than we imagine. This article explores the effect of the rate of nitrogen fertiliser on cation and anion leaching under intensively grazed dairy pasture, which can have far-reaching effects on the way we farm.
In New Zealand, there have been limited studies on the topic of leaching of anions and cations in intensive New Zealand dairy farming systems. However, it’s an important topic, as it’s vital to the soil health and long-term effects on New Zealand farms.
Leaching is the loss of water-soluble plant nutrients from the soil due to rain and irrigation. Plant-essential nutrients exist as anions and cations. Anions are elements or molecules that in their natural state have a negative charge. Cations are ones that have a positive charge in their natural state. Generally, soil is negatively charged and repels negative ions (anions), which are easily leached by rainwater and irrigation drainage. Negative ions that are leached take a positive ion with them; it’s part of nature.
The leaching of anions and cations is conditional on various factors including
the amounts and form of nutrients applied
in fertiliser, stocking rate, rainfall and, therefore, drainage, soil type, and extent of previous leaching.
The increased use of nitrogen fertiliser on dairy farms for the last few decades has led to the growing concern about the effect of nitrate leaching into groundwater.
A study I conducted in the mid-1990s along with Stewart Ledgard and other research at AgResearch set out to determine the amounts of Ca, Mg, Na, K, nitrate-N, sulphate-S, Cl, and inorganic P leached from intensively grazed dairy pasture at three sites (Waikato, Taranaki, and Southland) over a five-year period in the absence of N. In addition, nutrient leaching was determined in dairy farmlets at the three sites receiving 0, 200, or 400kg N/ha/year.
The quantities of cations and anions leached from the farmlets getting no nitrogen fertiliser represented a significant removal of these nutrients from the system, particularly under high drainage. Where no N was applied, approximately $240 of fertiliser was applied above ground, but $160 of nutrients was lost below the root zone in drainage. Once the N applied was increased to 400kg N/ha/year, the value of nutrients lost below the root zone jumped to $250/ha/year.
Another negative impact was clover N fixation decreased from 180 to 0kg/N/year when nitrogen application rates increased from zero N applied to 400kg N applied per hectare per year.
Leaching losses for cations and anions were similar between the different non-N-fertilised farmlets even though there were differences in soil drainage characteristics, region, and management practices.
This study showed that high-level leaching losses of Ca, Mg, Na, nitrate-N, sulphate-S, and Cl occurred from intensively grazed pasture and that heightened farming intensity, through the use of N fertiliser, increased the losses of Ca and nitrate-N.
Soil magnesium levels were decreasing on farms in the 1980s and ’90s. This study showed the reason why this was the case. Approximately 30kg per hectare of magnesium was being leached every year and not replaced in fertiliser or via rainfall.
These results have significant consequences on the sustainability of these systems, the potential for an increase in incidence of hypocalcaemia and hypomagnesaemia in animals, and the requirements for nutrient inputs in fertilisers. The research findings were included in the Overseer nutrient model. When farmers are required to determine their N leaching index, cations and anions leached are also generated by Overseer as part of the output.
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