Over the past 10 to 15 years, scientists have realised that the top 5mm or so of the soil profile (unofficially dubbed the ‘soil skin’) have a profound effect on the soil’s health and production potential.
“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 entire soil profile beneath it is negatively affected,” says soil scientist, Dr Neil Miles.
Soil skin health and quality is particularly compromised when there is no organic cover.
A bare soil surface bakes under the sun to form a hard crust, which limits permeability to air and water. Fragile germinating seedlings struggle to emerge successfully from beneath a crusted soil skin, and an uncovered soil is open to water and wind erosion.
Organic soil cover
Miles points out that there is a clearly visible difference between a soil surface that is always covered in organic residue, and one left bare.
The covered soil skin will be considerably cooler, tend to stay moist, be rich in organic matter on which beneficial soil Healthlife such as earthworms feed, and have a friable structure.
“Bare soil is not farming with nature. Soils must be covered with organic matter, such as crop residues or cover crops, all the time,” he adds.
Lubricating the soil
Scientists regard bacteria, fungi, mycorrhiza, arthropods, earthworms and moles as the key factors in biological soil health. However, Miles adds that soil organic matter (SOM) is no less important.
“Soil organic matter lubricates the whole soil ‘gearbox’. It influences the physical structure, the biology, the chemical aspects, and the nutrients required for soil health. Building organic matter in soils year after year is vitally important. It’s 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,” he says.
He highlights a seminal 2011 UK barley field trial that provides important evidence of the value of soil organic matter. The study revealed that barley yield was far greater in a land where the SOM was significantly higher, at 6,2%, than in an adjacent land where it was at only 1,7% (see Graph 1).
What was notable about this trial, Miles explains, was that the production potential of the lower SOM land was greatly influenced by the quantity of nitrogen (N) fertiliser applied.
With no N applied, a yield of less than 2t/ha was achieved. A 50kg/ ha N application achieved 3,5t/ha, 100kg/ha N achieved just over 4t/ha, and 150kg/ha N resulted in just over 4,5t/ha yield.
In contrast, N fertiliser applications to the higher SOM land had negligible effects on final barley yield.
Even with no N fertiliser applied to this particular land, the average barley yield was just under 7t/ ha. When 50kg/ha, 100kg/ha, and 150kg/ha of N were applied to the higher SOM land, the yields from these applications were all just over 7t/ha.
“This shows that the lower the soil organic matter in a land, the higher the land’s applied nutrient requirements. This adds significant input cost burdens to crop production, and also has the potential to contribute to undesirable run-off of nutrients into the environment,” Miles points out.
Increasing soil organic matter
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, such as sunn hemp and black oats, between primary crop production periods.
Another is to apply animal waste such as chicken manure, or pig and dairy slurry. These are rich in organic matter and nutrients, and will promote SOM build-up in soils.
Allowing livestock, such as beef cattle, to graze on crop residues and cover crops also helps promote soil health and build SOM levels, as the dung contains high quantities of nutrients.
The grazing should be well managed, however, to ensure that infield livestock do not deplete soil cover excessively, and that soil compaction through hoof action is addressed effectively.
Miles stresses that farmers and their consultants should aim to follow management practices that build organic matter in the soil.
He cautions them, however, to be wary of ‘snake oil’ products that promise miraculous improvements to SOM and overall soil health.
Farmers should understand that ideal SOM levels vary among soil types under different management systems, so there is no one-size-fits-all approach to optimising each soil’s minimum SOM.
Miles explains that as the clay content of a soil increases, its average organic matter content also increases.
Sandy soils do not have high organic matter levels as they cannot retain organic matter in the same way as loam and clay soils can.
“So don’t fall for products and talk that promise to dramatically increase soil organic matter in sandy soils,” he advises.
Keeping this in mind, it is crucial to implement management practices that maximise all soils’ organic matter content.
Measuring soil health
Miles stresses the need to continually measure biological soil health in order to develop appropriate management plans. Earthworms are an indicator of biological soil health; the more earthworms in a given area, the healthier the soil.
Counting earthworms is fairly easy, but this number should not be used as the sole indicator of biological soil health.
Measuring the biomass of soil microbes provides more meaningful data.
Laboratories measure soil health from the flush of carbon dioxide (CO2) released after wetting a previously air-dried and sieved soil sample.
Additional 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 useful indication of the soil’s microbial Healthbiomass and of its N reserves,” Miles says. Recently, this test was performed on soil samples from a tillage trial in KZN.
The trial area had been tilled the same way for 16 years. The CO2 test showed that microbial life in no-till soils was three times greater than that in conventionally tilled soils.
Microbial life in conventionally tilled soils was similar at 0cm to 5cm and 0cm to 15cm depths, which makes sense given that tilled soils are regularly inverted. In contrast, microbial life in no-till soils was much greater at the 0cm to 5cm depth than the 0cm to 15cm depth.
Unfortunately, Miles says, it is difficult to develop understandable result formats for farmers.
“This is because tests are sensitive to differences in soil management practices, and there are no universally applicable threshold values as the results depend on total organic matter levels for each land. The soil sampling depth is also critical.”
This article is based on a presentation delivered at the 2017 Conservation Agriculture Conference held in KwaZulu-Natal from 5 to 7 September 2017.