Ways to Reduce Nitrate Leaching, Part 2, with Leading NZ Soil Scientist Dr Gordon Rajendram

In the previous article, I focused on management changes that can help reduce nitrate leaching. Alongside those strategies, soil and plant-based tools also play a critical role. Much of my work has centred on understanding how carbon, microbes and plant roots interact to keep nitrogen in the system for longer, rather than allowing it to be lost through leaching.

One option I often work with is the use of solid humates. Humates act primarily as a carbon source for soil microbes. When microbes have adequate carbon, they become more active and are able to temporarily hold nitrogen within their biomass. This slows down the conversion of nitrogen into nitrate and reduces the speed at which it moves through the soil profile. The key point is that nitrogen is still available to plants, but it is released more gradually.

Biochar works in a different but complementary way. Rather than acting mainly as a food source, biochar provides physical structure in the soil. It creates protected spaces where microbes can live and function more effectively, particularly under stress. At the same time, both biochar and humates have the ability to hold onto nutrients, including nitrogen, helping to keep them in the root zone instead of allowing them to leach. Used together, they support a more stable soil system where nitrogen is retained for longer periods.

Plant roots are another important part of the picture. Typical ryegrass roots extend to around 20 to 30 centimetres. There is a lot of nutrients lost during leaching, and in dollar terms, this could be as much as half or more of what you apply in a year as fertiliser. Deeper-rooting species can intercept nutrients that move further down the soil profile, capturing nitrogen before it is lost below the root zone. Encouraging deeper and more diverse root systems adds another layer of protection against nitrate leaching.

Foliar application is also a useful tool in this toolbox. By supplying nutrients directly to the plant through the leaf, foliar feeding can improve nutrient efficiency and reduce the reliance on soil-applied nitrogen. When plants are better balanced nutritionally, they use nitrogen more effectively, which means less excess nitrogen remains in the soil to be leached.

In Canterbury, I will soon be measuring the effectiveness of these systems more precisely using lysimeters to better understand where the nitrogen is going and how effectively it is being retained.

The key takeaway is that reducing nitrate leaching requires a toolbox, not a single solution. By combining management changes with carbon inputs, plant strategies and targeted foliar nutrition, farmers can significantly reduce losses while maintaining productive and economically viable systems.

Contact Dr Gordon Rajendram

021 466 077 | rajendram@xtra.co.nz

www.gordonrajendramsoilscientist.co.nz

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Phillip Quay

MediaPA

027 458 7724

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