roots

19 June 1999




Going right

back to your

roots

Many UK farmers could be locking up their soils inherent fertility, reports Tom Allen-Stevens.

HOW is it that some of the best crop managers in the country are not achieving the sort of yield they expect from their soil? They should be tailoring their inputs to the soil – not the crop.

So says independent agronomist and soil expert Neil Fuller. He believes many farmers put on fertiliser without knowing for sure whether it is being taken up by the crop or locked up by their soil.

"Many soils carry enough nutrients to feed up to 300 years worth of continuous wheat crops, yet modern farming practices tend to lock up rather than release this potential. Nutrition is the key to growth and health, and is a real limit to realising genetic potential," he maintains.

Mr Fuller has worked with farmers all over the world to help them unlock their soils potential, and the results are impressive: South African growers have achieved potato crops of up to 80t/ha, against previous yields of 65t/ha, without having to significantly step-up inputs.

Now he is back in the UK and spoke recently to a select gathering at the Royal Agricultural College (RAC), Cirencester. His audience ranged from small, organic gardeners to larger, conventional Cotswold farmers.

Mr Fullers approach is based on a comprehensive analysis of soil fertility and nutrient availability. Then he concentrates on the soils biological activity and how that interacts with the growing crop. He believes this approach could ultimately help growers to:

&#8226 improve fertility

&#8226 reduce inputs

&#8226 increase yields.

Simple science

Heres how those locked up nutrients can be released. There is an entire interdependent food web that exists beneath the soil surface, from earthworms and beneficial beetles to beneficial fungi, bacteria and other microbes which are responsible for feeding and protecting the plant.

Many microbes gradually break down organic matter (crop residues etc) to form humus and build soil structure. Others release nutrients from inherent soil minerals, in forms that can be readily absorbed by plant roots, and can transfer these nutrients directly into the root. They behave rather like the bacteria animals have in their digestive system. The key difference, of course, is that plants do not have a stomach but rely entirely on the ability of the soil to support beneficial micro-organisms. Farming practices – anything from ploughing to pesticides – can play an enormous part in changing this.

As the plant roots work their way down through the soil, they release energy-rich organic acids, using up to two-thirds of their energy reserve. These substances attract soil microbes that feed and multiply. As they do this, they release and convert soil nutrients which the plant can consume.

One of the most important soil microbes is mycorrhizae. These beneficial fungi invade plant roots, growing between the root cells and extending hyphae (long, microscopic hairs) out into the soil. Using enzymes, the hyphae release minerals from the soil and transport them back into the root, for the mutual benefit of both plant and fungus. This symbiotic relationship can increase the root surface by up to 100,000 times, significantly improving water and nutrient uptake from soil reserves, which can translate into yield and quality benefits.

The presence of the fungi also triggers the plants own immune system, providing a sort of vaccination against attack from pathogenic fungi (like mildew, septoria and take-all).

Fungicide manufacturers have long been aware of the ability of plants to produce their own defences, known as Systemic Acquired Resistance (SAR), and products such as Novartis Bion are now available (though not yet in the UK) that enhance this activity.

Mr Fuller is very keen on this biological approach to crop protection: "Imagine being able to make your own blight sprays on farm from a cupful of native microbes and some simple, organic materials. This is already happening overseas and is here in trials this year."

Another naturally-occurring group of microbes are the rhizobia that work inside legume roots to fix atmospheric nitrogen. Rhizobia can generate up to 250kgN/ha/year, and are the foundation to productivity in organic farming systems.

Other types of rhizobacteria have been found to work without a host plant, fixing nitrogen directly from the air into the soil. These microbes have the potential to replace applied nitrogen in row crops and reduce the risk of nitrate leaching.

Mr Fuller believes that generations of farmers have been coerced by manufacturers into using fertilisers and expensive chemicals that they simply do not need, but which provide an easy, short-term benefit.

Moreover these chemicals can unbalance and destroy the delicate ecosystem that thrives in a healthy soil. As levels of soluble phosphate increase, the benefit obtained from mycorrhizal activity decreases, and will effectively cease altogether when high levels of nitrogen fertiliser are used. Farmers will then have to carry on using large amounts of these soluble fertilisers to maintain the level of availability of these basic nutrients through artificial means.

"This simply replaces inherent soil fertility with purchased inputs. The problem is that fertilisers cannot replace the ability microbes have to build soil structure and protect a crop," maintains Mr Fuller.

Applied nutrients can have a devastating effect on microbes: "An application of 60kg/ha of muriate of potash will put a band of chloride in the soil that may be 20 times more concentrated than the chloride level in a swimming pool," states Mr Fuller.

This will have a far greater effect on the beneficial microbes than it will on the pathogens such as take-all. Without competition, the pathogens can dominate the soil and challenge the crop. By maintaining a diverse range of active microbes, the effect of take-all can be reduced and the length of time before take-all decline occurs can be diminished.

Damaging

Selecting the most appropriate fertilisers and cultivation techniques is just part of the story. The application of some pesticides can also have a detrimental effect on microbial activity, Mr Fuller believes. "Residual herbicides will temporarily knock out many of the beneficial algae and bacteria, accelerating the loss of nitrogen and phosphorus from the soil. These microbes will regenerate, but repeated use of residuals can be very damaging. Measuring microbial activity indicates that some soils are biologically dead."

He also questions some of the new technology being introduced into agriculture: "The strobilurins are highly effective at resisting fungal attack, but late applications can remain active even after crop residues are incorporated. This will reduce the ability of beneficial microbes to break down straw and release nutrients. We are also seeing some GM crop residues being toxic to beneficial organisms."

Although talking to a largely organic audience, Mr Fuller did not believe that improving the biology of the soil was a practice restricted to organic farming: "If you can harness the soils inherent fertility, anyone can feed a bigger crop for less, and this goes as much for conventional farmers as it does for organic."

"Managing a living soil isnt the answer to solve all of farmings current problems, but its certainly part of it. It can have real benefits for the farmer, the consumer and the environment," he points out.

The problem:

Just a quick study of the RAC field reveals tell-tale signs of nutrient lock-up:

A) high calcium content of the typically thin, clay-rich Cotswold brash

B) white, relatively unadventurous roots showing little or no microbial activity taking place in the soil

C) residues of two previous crops.

Mr Fullers verdict?

The calcium in the soil will lock up any triple super phosphate applied. Potash is difficult to hold in this soil and should only be applied when the crop can take it up in large amounts. This crop will run short of magnesium long before it uses up applied nitrogen.

Mr Fullers prescription?

Sulphur and sulphur-based products will improve nutrient uptake. Apply phosphate at drilling, use ammonium sulphate and foliar-applying magnesium and micro-nutrients to improve crop growth.


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