Giving up on organic matter

20 Nov 2013

Last week I visited Rothamsted Research with a few arable farmers.  As always, it was very good value and we were updated on many aspects of the organisation’s research. Pesticide resistance and availability and soil issues were the main themes.

One area that caused particular interest was the evidence behind a new HGCA and Defra funded project on how to get the best out of organic manures. The whole industry is not only aware of the desirability to increase organic matter levels in long-term arable soils but also the futility of trying to achieve such an objective. 

Changing to a system of interspersing short periods of arable crops between medium-term grass leys could help but would also have a negative impact on both production and profits. The alternative is to use a lot of organic manures or amendments at regular intervals. However, there are simply not enough of these organic sources to have a significant impact on organic matter levels on more than a very small percentage of our arable land.

So how best to use these limited organic sources to maximise their benefits? Some research at Rothamsted has provided a clue and is the basis of the new project. Researchers found that the benefits from annual farmyard manure application, in terms of spring barley yields, were quickly available when judged against the same amount of manure being applied annually since 1852 (see figure). This yield benefit, recorded in 2002 after just two successive autumn applications of farmyard manure, could not be explained either by the nutrients in the manure or by an increase in soil organic matter. So what is the possible explanation?

Rothamsted spring barley FYM project

Figure: spring barley yields (t/ha) at the optimum doses of mineral (bag) nitrogen.
The blue line is where mineral nitrogen only has been used annually since 1852, the green line is where both farmyard manure and mineral nitrogen have been used annually since 1852 and the red line is where organic manure and mineral nitrogen have been used annually since 2000 (i.e. first manure application in autumn 2000).  Graph kindly provided by Andy Whitmore of Rothamsted Research    


Regular readers of my blogs (if there are any out there!) will have guessed the possible explanation because I’ve banged on about this issue before. The application of crop residues and organic manures and amendments increases the level of the biomass of fungal, bacterial and fauna in the soil. These can act as a surrogate for organic matter.  However, to maintain these benefits they need annual applications.

It is for this reason that the value of straw to the cereal farmer is more than just the nutrients it contains. Many farmers reported that their land worked more easily after a couple of years of incorporating rather than burning straw. However, despite positive effects on the soil, Rothamsted has not found a benefit in terms of winter wheat yields from the annual incorporation of up to four times the average straw yield.

Why were increased yields recorded from the regular incorporation of organic materials recorded in spring barley and not in winter wheat? It’s my view, which cannot be verified, that spring barley yields are more likely to benefit because the crop is established in more hostile growing conditions and has a shorter growing period. We all recognise that winter wheat yields are more likely than spring barley to compensate for set-backs in the first couple of months of growth. The opposite may well also be true, that spring barley yields more than winter wheat yields are likely to benefit from improved soil conditions in the early stages of establishment.

To continue my theorising, how best should we use the limited UK supplies of organic manures and amendments? Based on very limited information, it would seem that using smaller amounts annually rather than large amounts intermittently in largely spring crop orientated rotations may be the best way forward to maximising yield benefits. However, applying smaller amounts annually will result in extra costs. I’m sure that the Rothamsted project will provide guidance as to the way forward.