Pictured above: Dr Gordon Rajendram and Mark Robertson (holding Humates) on a pumice soil farm in Taupo. Leading Hamilton-based soil scientist Dr Gordon Rajendram says nitrate leaching has become one of the most critical current challenges for pastoral farms across New Zealand threatening both water quality and environmental sustainability. And he has the backing of a growing number of farmers who are putting into practice his advice. “In grazed pastures, urine patches from livestock are the primary culprit: they supply nitrogen far in excess of plant demand, making the surplus vulnerable to leaching as nitrate,” says Dr Rajendram, who is at the forefront of promoting practical, research-based solutions to alraming issue. In Gore, Southland, rising nitrate concentrations in the town’s water supply have raised concern among residents, while further north, Environment Canterbury (ECan) is facing growing public pressure to address nitrate contamination across parts of Canterbury’s groundwater network. These are not isolated issues; they’re unfolding in different regions across New Zealand, affecting both farmers and communities alike. Why Carbon Makes a Difference Adding carbon to soils, through humates, biochar or other organic sources, helps retain nitrogen in forms that plants and microbes can use, rather than allowing it to convert into mobile nitrate. Carbon inputs stimulate microbial immobilisation of nitrogen, slow nitrification (the conversion of ammonium to nitrate), and support denitrification processes that convert nitrate into benign dinitrogen (N₂). Biochar, sometimes nicknamed “black gold”, has attracted considerable scientific attention. Made from plant material under low-oxygen pyrolysis, it acts like a sponge in the soil, holding water and nutrients and altering microbial activity. In a recent study (Clough et al., 2020) published in Soil Biology and Biochemistry, Professor Tim Clough and colleagues demonstrated that when biochar or carbon-rich amendments comprised a high fraction of soil volume, nitrate losses were dramatically curtailed. That research supports the notion that around 10% by volume biochar (relative to soil) can very significantly reduce nitrate leaching, reinforcing the promise of high-carbon strategies. Another meta-analysis of 30+ studies (Cayuela et al., 2014) showed that biochar reduces emissions of nitrous oxide (N₂O) by an average of 54%, with stronger effects for higher-quality biochars and optimal application methods. Humates are yet another valuable tool. These naturally occurring substances enhance soil structure, stimulate microbial life, and boost the soil’s capacity to retain nutrients and moisture. Trials reported in Farmers Weekly found that humates applied with urea increased pasture yield by up to 30% while reducing nitrogen losses. Long-term trials led by Dr Peter Espie showed nitrogen leaching reductions of around 61% and pasture gains up to 11% over ten years. Supporting Research Findings Together, these findings show that adding sufficient carbon, through humates, biochar or other organic sources, can dramatically cut nitrate leaching, even from intense urine patches. Expert Insight Dr Rajendram emphasises: “By pairing carbon-rich amendments with strategic farm management, we can trap more nitrogen in the soil where plants can use it, or let microbes convert it harmlessly to N₂, rather than losing it to waterways. This is about giving farmers workable solutions, not criticism.” What This Means for Farmers Farmers are suppporting Dr Rajendram’s ground-breaking scientific approach in growing numbers and noticing marked differences from adopting his advice. Dr Rajendram has been consulting to Taupo dairy farmer Mark Robertson for about six years. “Dr Rajendram’s recommendation was to apply humates every time I used my tow n fert machine at approx solid fine ground humates 10kg per hecate with a cost of around $7 per hectare and I have applied approximately 150kg per hectare of humates to date,” Robertson said. “The aim is to build carbon levels with humates or biochar to bind up as much urinary Nitrogen as possible and the results are heading in the right direction,” Robertson said. “Dr Rajendram is a leader soil health and his input is very important to the growth of pasture. It is the right time for a changed approach in terms of what we are looking at for the future,” Robertson said. Northland farmer Michael Mackinven is feeding biochar to cows and has confidence it is the right approach to take. “Dr Rajendram is an absolute expert in his field of soil health and provides very high level advice. We are very fortunate to have him and the wealth of knowledge he brings to our industry,” Mackinven says. “Dr Rajendram has been on the farm taking soil tests and giving his expertise to help us work out the best products such as humates and fish and the evidence we have seen with the topsoil development is positive,” Mackinven said. “I have total confidence in his judgement and his years of experience stand him in good stead,” MacKinven said. “For farmers, this is a practical and science-backed path forward. By adopting carbon-based amendments such as humates and biochar, they can reduce nitrate leaching, improve nutrient efficiency, and safeguard waterways-all while maintaining profitable production. As Dr Gordon Rajendram, leading New Zealand soil scientist, highlights, these strategies combine productivity with sustainability, protecting both farms and the environment and we are taking his guidance with his years of experience standing him in good stead,” MacKinven said. References Contact Dr Gordon Rajendram 021 466 077 | rajendram@xtra.co.nz www.gordonrajendramsoilscientist.co.nz Contact MediaPA Phillip Quay MediaPA 027 458 7724 phillip@mediapa.co.nz
