Farmers fearing that manufactured nitrogen (N) and phosphorus (P) fertiliser supplies might dry up should be rest assured. But economic and environmental pressures mean their use of them needs to improve.

Those were the underlying messages in presentations from two Nuffield Farming Scholars following worldwide studies.

Lincolnshire farmer and fertiliser supplier Nik Johnson noted that 75% of global reserves of rock phosphate lay in just three countries. But the price “spike” of 2008 had encouraged development of resources elsewhere, and a meeting with agriculture minister Jim Paice showed that DEFRA was more concerned about the Water Framework Directive’s impact on farmers’ ability to use phosphorus fertiliser.

“We won’t run out of phosphorus,” said Mr Johnson. Supplies and prices were more likely to be driven by geo-political decisions and unexpected events such as extreme weather, he believed.

However, there was plenty of scope for growers to use phosphorus fertilisers more efficiently through a better understanding of the way plants obtain the nutrient from the soil, he explained.

“How is it that we can get 4t/acre of wheat off land with a P index of only 1 while on another with an index of 2 or even 3 we get nowhere near that?”

He urged growers to regard their soils’ P reserves as they would their bank account. “It’s your working capital. You can add to it and withdraw from it, but it must be monitored.”

The first step was to determine the critical amount of P needed to allow their crops to perform best. Once that was found all future applications simply needed to balance inputs with outputs.

“The problem is that that critical level is different for every farm, crop and soil.” That was because of highly complex processes determining how much P from the overall pool of reserves became available to the crop, he explained.

Although DEFRA’s Fertiliser Manual (RB209) offered sound advice, Mr Johnson believed it could be improved to allow growers make better application decisions without the exercise becoming too complicated.

To help kick-start an upgrading project, involving Phil White of the James Hutton Institute in Dundee, he called for 100 arable farmer volunteers to take part in a three-year study. Each would need to dedicate about 0.4ha of land to soil testing and yield monitoring experiments.*

GM TO REDUCE ARTIFICIAL N DEPENDENCE

As non-renewable energy sources dwindled, nitrogen fertiliser was inevitably becoming more expensive to manufacture. So growers had to strive to make better use of cheaper alternative sources, urged Yara’s Mark Tucker.

Using renewable energy to make N fertiliser, perhaps even on-farm using wind turbines, was a possibility. “The technology is out there to do it if the economics eventually become right.” But that would not address the problem of N overloading, he pointed out.

However, the main goal was to improve the efficiency with which crops used N to help overcome the environmental problems linked with it, he stressed.

“Since we started using artificial fertilisers we’ve doubled the loading of nitrogen into the global nitrogen cycle,” said Mr Tucker.

Crops’ use of N was only 30-50% efficient, and nitrogen fertiliser was a key driver of “dead zones” in the world’s oceans; halving nitrogen-based agricultural emissions would offer the same environmental benefit as taking all 321m cars in the US, UK and France off the road, he calculated.

The immediate answer to the problem was a return to more “circular” farming, he suggested. That meant better rotations including pulses. “The UK pea and bean area is only 56% of what it was in 2001. That means £2.42m worth of N hasn’t gone back into the soil.”

NIAB TAG work had highlighted the benefits of green manures and bi-cropping with clover, he added. “Conventional agriculture can really learn lessons here from the organic sector.”

Biodigestates and composts also had a role to play, and all-arable farms might need to consider re-introducing livestock – “N-factories on legs.”

There was also a need to improve N recommendations, one promising line of research being DNA fingerprinting of soils to help understand why some were better at supplying nitrogen than others.

Eventually, however, GM breeding technology had most potential to reduce farming’s reliance on manufactured nitrogen fertiliser, he believed.

Already GM work in the US by Arcadia had boosted N use efficiency by up to 30%, and the “holy grail” of non-leguminous plants being able to fix their own nitrogen was not too far off, he suggested.

“Proof of concept of such techniques could be as close as 7-10 years away.”

* Growers interested in taking part should email Nik@jseco.co.uk