4 things scientists know about how soil affects fertility

Improving organic carbon in soils can improve nutrient uptake and reduce waste.

To understand the biology and physics of soil, scientists at Rothamsted Research have been exploring aggregate at microscopic levels of 100 microns (one thousandth of a millimetre) or less using x-ray computer tomography.

Such close observation is needed because carbon is sequestered at this size of pores, and the water-holding capacity of soil can be seen at this level, explains Andrew Neal from the department of sustainable agriculture sciences at Rothamsted.

See also: How to achieve healthy grassland soils

At a recent Maize Growers Association conference, he set out what they have learned so far.

1. Soil structure is crucial to soil fertility and efficiency

There is a strong relationship between the total amount of organic carbon (OC) in soil and the connected porosity – the degree to which pores are connected and have the capacity to hold water.

Degraded soil, free of plants for more than 50 years, had very low OC and very little connected porosity. These soils were then converted to arable and grassland and benefited from increased carbon and connected porosity over 10 years. But permanent grassland stored the most carbon and had the best porosity.

You can only generate so much connected porosity, and all soil will have its limit.

Soil texture is very important – clay soils perform very well, and sandy soils are less effective – but by managing carbon in soils, you can increase system performance.

2. Improved connectivity improves hydraulic conductivity

Hydraulic conductivity is the ability of a soil system to move nutrients around for plants or to move oxygen for microbial processes.

Improved connected porosity results in improved hydraulic conductivity, which means nutrients and water move through soil very effectively.

A soil that is low in connected porosity and hydraulic conductivity has a very high anoxic volume (lack of oxygen), even in dry conditions.

“Highly connected pore space not only stores water for resilience, but also controls the amount of oxygen that governs microbial activity. We can trace all of this  back to the amount of organic carbon in soils,” says Prof Neal.

3. Genes drive important processes

The sequencing revolution that has taken place in the past 20 years has enabled scientists to get a much better understanding of soil microbiology.

The soil structure is influenced by the genes and organisms present in the soil.

“If you feed that soil carbon, you generate a good, open structure that stores water, stores nutrients and moves them around effectively, and is very aerated. If you don’t put a lot of carbon in there, you end up with a soil that is very poorly connected, doesn’t hold much water, doesn’t store a lot of carbon and the organisms have to revert to anaerobic processes, which are negative for efficiency and climate change.

“We can see how the abundance of genes that drive important processes – particularly for agricultural production – evolve in response to these physical changes that are coming about by our management of carbon,” adds Prof Neal.

One example is dissimilatory nitrogen metabolism ­– when microbial processes switch from using oxygen to nitrogen – which drives nitrous oxide release from soil into the atmosphere. This means nitrogen doesn’t get to the crop and causes a warming effect. The number of genes present in a system that drives this process is much higher in poorly connected, bare fallow and arable systems than in grassland.

4. Not all N is bad

Broadbalk – one of the oldest continuing experiments in the world, which tests the effects of inorganic and organic fertilisers – shows using farmyard manure as nitrogen changes the way nitrogen is partitioned in the soil.

Rather than losing a lot of nitrogen, nitrous oxide emissions are lower and increase the accumulation of nitrogen in soils where farmyard manure is spread.

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