29 March 2002

Hunt is on for yield boosting X factor

Scientists are trying to

identify the mystery factor

that causes unexpectedly

high yield increases from

crops grown on soil

containing added compost,

as Mike Williams reports

TRIALS using green waste compost have shown substantial yield increases as well as cash savings through reduced fertiliser applications.

Preliminary results suggest that compost as a manure for farm crops may be seriously undervalued.

Composts used in the trials were made from garden and civic wastes, similar to those produced in large quantities by many local authorities, but a critical factor highlighted in the trials at the Harper Adams University College in Shropshire is that the compost must be fully matured or "stable".

The best results were achieved with material processed for 12 months, and there was no significant yield increase when using compost produced over a three-month period.

One of the aims of the research programme, which started more than five years ago and is continuing, is to assess the value of compost as a replacement for fertiliser nitrogen. For wheat, the results so far suggest the best yields were achieved with a 35 or 40t/ha dressing of compost together with 80% of the normal application rate for fertiliser nitrogen. The table shows that this can achieve a 2t/ha boost to the grain yield, valued last year at £70/t, and there was also a cost saving because of the cut in the N fertiliser application.

The extra yield, equivalent to a rise of about 25% on a typical 8t/ha wheat crop despite a 20% cut in fertiliser nitrogen, is better than expected, but the trials have also produced "substantial" yield improvements in some other crops and in grass.

The increased grain yield is not the only difference measured in crop plants during the trials. Crops grown in the presence of compost showed increased root development, with a 23% increase in maximum root length in wheat and a 14.9% increase in oilseed rape.

There was also a substantial gain in the dry weight of the root mass for the rape plants and a smaller gain for wheat. The above-ground weight of the plant material also increased sharply, with wheat averaging 52.1% and the rape recording a 47.3% gain.

Possible explanations for the yield increase include a theory that the presence of the compost triggers a rise in the uptake of nitrogen through the plant root system. This was tested by using "marked" nitrogen to allow rates of uptake to be measured and compared, and the results suggest the theory is correct.

More evidence comes from the fact that field beans do not show a significant response to the compost treatment, indicating that the different process for fixing nitrogen used by leguminous plants is not affected by the compost.

Alan Keeling, director in charge of the Harper Adams compost research programme, says the next stage is to try to find out how the compost actually triggers the increased N uptake.

"We already have some ideas about this and I think the factor influencing the rate of uptake is likely to be a complex chemical compound developed during the later stages of the composting process," he says.

"The compound does not appear to be available until the compost has been processed for 10 to 12 months. Composts processed for a much shorter time than this appear to have little or no beneficial effect on nitrogen uptake or crop yield, although they do have a value as a soil conditioner.

"We are trying to identify the compound, and if we are successful Harper Adams will be claiming the intellectual property rights to it," says Dr Keeling. "The fact that the compound can achieve significant increases in crop yield while allowing application rates for inorganic nitrogen to be reduced makes it very interesting for both economic and environmental reasons, and it could be valuable in the future." &#42

Cup hands for the real thing. Unexpected yield increases continue to baffle research scientists at Harper Adams.


£/ha

Extra grain yield –

2t/ha @ £70/t 140.00

20% saving in N fertiliser use

(20% of 175 kg N/ha

at £0.3/kg) 10.50

K from compost (3 kg/t

valued at £0.23/kg) 27.60

Total additional

income/saving 178.10

Less cost of compost

spreading and additional

cultivation 74.00

Net gain 104.10

*Figures are estimates based on results from a number of trials.