Mighty microbes

15/Aug/2014

A simple set-up, but with major results: agricultural crops rely heavily on soil biodiversity. © Franz Bender

Intensive agriculture is its own worst enemy. If it were to broaden its perspective and pay more attention to what lives underground, it could cut costs and increase yields. By Ori Schipper

​It’s crazy. There’s no other word for it when you consider the excessive use of fertilisers in the agricultural sector. To be sure, it makes for high yields in crop cultivation, but the plants only utilise half of the available nitrogen from the fertiliser. The other half escapes into the atmosphere or is washed away and pollutes our rivers and lakes.

By focusing only on the highest possible yield, intensive farming is going down the wrong track. Apart from anything else, this wasteful use of plant nutrients obscures the fact that the world’s total reserves of phosphorus, for example, will only last for some 50 to 100 years. Now Franz Bender and Marcel van der Heijden of the Agroscope research institute in Zurich-Reckenholz have produced results that prove how expanding our current perspective could prove doubly rewarding.

These two researchers have conducted an experiment that shows how unseen subterranean biodiversity has a decisive, positive impact on the nutrient efficiency of maize and wheat. They took earth samples from nearby grazing land, sterilised it and then added to their samples either a greater or a lesser variety of soil organisms. Then they put the earth into lysimeters, large containers with a drain hole at the bottom through which rainwater seepage escapes.

A quicker nutrient cycle

The chemical analysis of the water seepage allowed them to draw conclusions about the amount of nutrients washed out, which differed greatly according to whether the earth in the lysimeter had been given a larger or a smaller number of soil organisms. The soil teeming with bacteria, fungi and worms had a quicker nutrient cycle. The nitrogen was chemically converted by the subsurface organisms and subsequently retained in the soil. In fact the organism-rich soil lost only half of the nitrogen to the rain when compared with the organism-poor soil.

Microbes in the soil also unlock phosphorus from the predominant chemical compounds that often make it inaccessible to plants. Above all, the symbiosis with mycorrhizal fungi means maize and wheat absorb roughly a fifth more nitrogen and almost twice as much phosphorus when they grow in soil that is rich in organisms.

All this has an impact on the productivity of the plants. The more life there is in the soil, the greater the growth of maize and wheat – something that Bender and van der Heijden have now been able to prove for the first time. "Organisms in the soil and mycorrhizal fungi have the potential to improve agricultural yields both qualitatively and quantitatively", they write in their article.

When arable land is farmed intensively, the living networks in the soil are often disrupted. As a result, the nutrients fed into the soil with the fertiliser remain largely unused and seep away, write Bender and van der Heijden. If there was wide adoption of practices such as crop rotation, reduced ploughing and direct sowing – all of which are beneficial to the invisible creatures in the soil, and all of which are already common in Switzerland – then agriculture could profit greatly. If we were to abandon our pointless extravagance with nutrients, then farmers would not only save on the cost of fertiliser – and not even at the expense of high yields – but also help to solve environmental problems such as polluted lakes.

(From "Horizons" No. 101, June 2014)

 

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