Having and maintaining healthy soils is vital if sugarcane growers want to achieve consistent and sustainable yields. Dr Neil Miles, a senior soil scientist with the South African Sugarcane Research Institute, elaborates on the interrelationships between soil chemistry, physics and biology.

Over the past 10 – 15 years, scientists around the world have discovered that the top 5 millimetres or so of a soil – unofficially dubbed the ‘soil skin’ – have a profound impact on the health and production potential of the soil as a whole.

“The soil skin affects aspects such as water infiltration, aeration of the soil profile, nutrient cycling, and soil biology. So, if this top layer of soil is damaged or lost for any reason, the functioning of the whole soil profile beneath it is negatively affected,” says Miles.

And the health and quality of the soil skin is compromised especially when it has no organic cover. A bare soil surface bakes in the hot sun to often form a hard crust that is largely impermeable to air and water. An uncovered soil is also prone to water and wind erosion.

ORGANIC MATTER BENEFITS SOIL LIFE

Miles points out that there is an easily visible difference between a soil surface that is always covered in organic residues and one that is bare. The covered soil skin will be far cooler, is likely to be moist, and will contain the organic matter necessary for a good and friable soil structure. Soil organic matter (SOM) is also food for beneficial soil life such as microbes and earthworms.

“Bare soil is not farming with nature. Sugarcane soils should ideally be covered with organic matter, such as crop residues between harvests or by cover crops between plough-out and replant. Sugarcane growers should do everything in their power to keep their soil skin covered all the time,” he adds.

When scientists talk about biological soil health, they are interested in things like bacteria, fungi, mycorrhiza, arthropods, earthworms, and moles. According to Miles, favourable levels of SOM are crucial for the proliferation of these organisms.

He explains, “Soil organic matter lubricates the whole soil ‘gearbox’. It positively influences the physical structure, the biology and the chemical properties of soils. Building organic matter in soils year after year is vitally important. It is food for microbes, it’s a slow release form of nutrients for plants, it improves water infiltration and retention, and it creates good soil tilth. It also enhances a soil’s cation exchange capacity, an aspect of particular importance in sandy soils.”

Field trials in the United Kingdom have shown that the lower the SOM in a crop field, the higher are this field’s applied nutrient requirements, such as nitrogen (N). In the case of sugarcane, low organic matter levels add significant input cost burdens to crop production, and also have the potential to contribute to undesirable runoff of chemical fertilisers into the environment, Miles points out.

One option to build SOM, in addition to leaving crop residues as cover on a soil surface, is to plant cover crops or green manure crops between periods of plough-out and replant. Examples of these cover crops/green manure crops include sun hemp and black oats.

Another option is to surface apply, and then possibly incorporate into the soil through discing or irrigation, the likes of sugar mill by-products and animal wastes such as chicken manure, pig slurry or dairy slurry. These products will contribute to the build-up of SOM levels, contribute nutrients for crop growth and also stimulate soil life.

“Sugarcane growers and their consultants must always be targeting management practices that build organic matter in the soil,” Miles urges.

NEW SOIL TESTS PROPOSED

Growers should, however, understand that optimum SOM levels vary between soil types, and that there is no one-size-fits-all answer to what each soil’s minimum SOM level should be.

“As the clay content of a soil increases, so its average organic matter content also increases. Sandy soils will, therefore, not have high organic matter levels because they can’t retain organic matter like clay and loam soils can. But implementing management practices to maximise the relative soil organic matter content of sandy soils, and all other soils, is critical,” says Miles.

In recent times a number of new soil health tests have been proposed by researchers. Miles says that perhaps the most promising of these is the measurement of the carbon dioxide released when a dried soil is rewetted. Other tests determine a soil’s water extractable carbon (C) and N content, its C:N ratio, and its biological N release.

“Measuring the flush of CO2 from a soil sample gives a rapid and reliable indication of the soil’s microbial biomass and of its organic N reserves,” Miles explains.  “However, scientists are grappling with the interpretation of soil health indicators, since optimum levels vary not only with soil types but also with the field management regimes imposed on the soil.”

Contact Dr Neil Miles at: Tel – 031 508 7437 email – neil.miles@sugar.org.za website

This article is based on a presentation given at the 2017 Conservation Agriculture Conference held in KwaZulu-Natal from 5 – 7 September 2017 website – www.notillclub.com

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