Science - August 23, 2010

Good yields with less phosphate

Strict norms for phosphate applications do not seem to affect crop yields at livestock farms on sandy soils. This has been the finding after long-term use of balanced fertilization on De Marke experimental farm, part of Wageningen UR.

De Marke experimental farm
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For many years, agricultural extensionists advised farmers to apply more phosphate on their fields than the vegetation could absorb. Lower applications could reduce yields, was the thinking. Overdoses were necessary to compensate for ‘unavoidable loss of phosphate’. This policy led to many Dutch soils becoming saturated with phosphate, which ended up in ground water.
In order to meet European water quality requirements, norms for agricultural fertilizer applications are being tightened up. Policymakers are aiming at balanced fertilization, which means not applying more phosphates than the vegetation can absorb. Many farmers are afraid that will reduce their harvests. And among agronomists there are different opinions on whether balanced fertilization will reduce soil fertility.
Layers of soil
De Marke, an experimental dairy farm in Hengelo in Gelderland, started with balanced fertilization in 1989. De Marke mainly has grasslands, but also grows maize and other grains as fodder crops. Researchers measured every year how much phosphate had been absorbed by the crops, how much had leached into ground water, and now much was stored in various layers of soil.
When the testing began, there was a lot of phosphate stored in the soils at De Marke. Through balanced fertilization, this went down by 16 percent. And the amount of readily available phosphate in the soil went down by as much as one quarter. Yet crop yields did not go down. ‘Using balanced fertilization you can get perfectly good crop yields, even in the long term’, says researcher Koos Verloop. The reason for this, he explains, is that the availability of phosphate in the soil first goes down with balanced fertilization, but then stabilized at a level at which plants can go on growing fine.
Sandy soils
It does help, Verloop adds, if farmers practise crop rotation, alternating between grass, grain and maize. When the soil is ploughed over, the phosphate becomes more available, the research has shown. But he remarks that these conclusions are valid for agriculture on sandy soils. ‘Research is also being done on clay and on peaty soils, and the picture there is not straightforward. It also makes a difference what you are growing. My colleague Bert Smit has established that balanced fertilization for fast-growing outdoor horticultural crops is harder to do successfully than it is with less fertile soils.’ Verloop thinks the De Marke findings are significant. ‘Maize is known to need a lot of phosphate. It is remarkable that even with this crop you can get away with long-term balanced fertilization.’
The research is part of the ‘Cows and Opportunities’ programme, and is financed by the ministries of VROM and LNV, the dairy board and the European project Dairyman. Phosphate concentrations in the soil have never before been measured over a long period at low phosphate applications. It was high time this was done, says Verloop. ‘Soils react slowly to changes in phosphate applications. Much of the phosphate is stored in soil particles, and the soil goes on supplying phosphate after you have lowered applications. So research on the effects of phosphate fertilization is a long-term business.’
The research was published in July in the professional journal Nutrient Cycling in Agroecosystems.