Leading New Zealand Soil Scientist Dr Gordon Rajendram – An Expert On Humates, Biochar and Carbon - Calls For A More Scientific Strategy To Slash Nitrate Leaching

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

• Landcare Research (Manaaki Whenua) lysimeter trials showed that carbon applications can significantly reduce nitrate leaching, with results indicating reductions of around 75 percent under ryegrass and white clover, 51 percent under lucerne, and up to 91 percent at higher rates. While large amounts achieved the greatest effect, the key is applying smaller quantities more frequently for sustainable results. Carbon helps tie up nitrogen in the soil, preventing it from leaching away while supporting healthy plant growth.
  

• Dr Gordon Rajendram has carried out extensive work on nitrate leaching and knows what is required to achieve meaningful reductions. Pure carbon is one of several proven solutions, and the right approach can substantially cut leaching without reducing stock numbers. With increasing regulatory pressure in areas where nitrate levels are rising, Gordon can help farmers identify practical and effective ways to restore soil balance and maintain productivity.

• A ScienceDirect study (Qin et al.) found that with ample dissolved organic carbon (DOC), nitrate is preferentially denitrified to N₂ rather than nitrous oxide.
• Research from Lincoln University and Landcare Research showed that increasing the available carbon supply reduces nitrification potential in soils, thereby lowering nitrate formation and leaching.
  

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

• Ministry for Primary Industries (MPI) (2018). N₂O and ammonia emissions from biochar-amended urea prills. Link
• Farmers Weekly (2022). Benefits of soil humates highlighted. Link
• Otago Daily Times (2023). Stimulant raised growth in 10-year trial. Link
• Clough, T. J., et al. (2020). Soil carbon fractions influence nitrous oxide emissions following urine deposition. Soil Biology and Biochemistry, Elsevier. Link
• Cayuela, M. L., et al. (2014). Biochar’s role in mitigating soil nitrous oxide emissions: A review and meta-analysis. Agriculture, Ecosystems & Environment, Elsevier. Link
• Manaaki Whenua – Landcare Research. Carbon inputs to reduce nitrogen losses from farms.
• Qin, S. et al. Denitrification and DOC effects. ScienceDirect.
• Lincoln University / Landcare Research Archive. Increasing the available C supply from plant roots reduces nitrification potential in soils.
  

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