In search of the ideal soil
GroundCover™ Issue: 48
The promise of a farming system that is both economically and environmentally sustainable drew about 120 growers to the first conference of the Victorian No-Till Farmers Association, held at the Longerenong campus of the University of Melbourne near Horsham in September. Speakers from Canada and Western Australia shared their scientific research and hands-on experience.
Kay Ansell reports.
Richard MacEwan reduces the complexities of soil science to deceptively simple principles. The gritty stuff on which all agriculture relies is “a three-phase system of solids, gases, and liquids”, he told the first Victorian No-Till Farmers Association conference.
But in demonstrating that the proportions of all three are crucial to “ideal soil”, he generally argues that maintaining that balance can be anything but simple, especially under traditional cultivation systems.
A balance of organic and mineral matter and the quality of the organic matter determine the fertility of the soil, and the proportions of gas to liquid will change as the soil wets or dries. Tillage increases the ratio of space to solid but traffic has the opposite effect and causes compaction.
Mr MacEwan, a senior research scientist with the Department of Primary Industries, PIRVic, Bendigo, Victoria, displayed a cross-section of pasture during his presentation which cuts to the heart of the no-till debate.
A ‘before’ picture shows particles that are well spaced, porous, and well connected, containing earthworms and scats.
Three years after it has been ‘improved productively’, that is, cultivated, rolled and fertilised, the pasture is more compacted and resistant to water. “Erosion is a natural consequence,” he says. “No-till would have prevented that. We have to ask: is tillage necessary?”
He distinguishes between ‘soil quality’ (the material you start with) and ‘soil health’ (the condition of the soil) and says that the functions that soil must provide are varied and sometimes conflict.
Mr MacEwan says: “We need soils that will: support plant growth, receive, transmit and store water, support animals and wheeled traffic, resist erosion, store and recycle nutrients, absorb wastes and act as environmental buffers.”
It is up to farmers, as the managers of the land, he says, to balance these soil functions to maximise production and to protect the soil resource and environment.
“We are farming carbon from the atmosphere in wheat and other crops – and there is a hidden harvest in all of this,” he says. “In natural ecosystems this carbon would accumulate locally and provide benefits to soil structure, topsoil depth and life in the soil.
By harvesting the carbon in crop and pasture-based products, we deprive the soil of much of this carbon input and we need to understand and manage the consequences of this.”
Mr MacEwan says the average rate of soil formation is 0.04 tonnes per hectare per year but topsoil losses from crop production can be considerable. Average soil losses from winter crops are 1.5 t/ha/y and from summer crops 8.0 t/ha/y.
Quoting CSIRO soil scientists Beckmann and Coventry, he sums it up thus: “We are mining a non-renewable resource and the mine has a limited life span.”
So much for the bad news – the good news is that farmers can have a huge influence on soil structure, even if they cannot change their soil type, he says.
“What is soil structure? It’s the arrangement of the solids and the spaces. The spaces make soil useful to hold water and air to let roots grow.
“The spaces between aggregates are filled with either gas or water and the number, proportion, size and shape and connectivity of all those spaces can create an ideal structure for plant growth,” he says.
At its most basic, ideal soil connectivity has particles that offer an “open surface” and provide low enough physical resistance to let roots in with sufficient “anchorage” to resist erosion.
Ideal soil also balances the “three-phase system of solids, gases and liquids”, providing large plant-available water storage and good gas diffusion – because roots and animals in soil need to breathe underground. It resists compaction from machinery or animals and it is resilient – it will bounce back, he says.
“What is ideal soil? It should be open and soft but strong and resilient, with high water content but lots of air.”
This is why ideal soil is a paradox and why we need to think about separating the soil we traffic from the soil that we use to support plant growth. Zero and minimum tillage farming improves the conditions for soil protection and plant growth.
We need to take advantage of this improvement by controlling traffic in cropped paddocks.
Mr MacEwan advises growers to check the condition of their crop’s roots to monitor what their soil is doing and to seek advice in interpreting what they discover.
For more information:
Richard MacEwan, senior research scientist with the Department of Primary Industries, PIRVic, Bendigo, 03 5430 4326, firstname.lastname@example.org
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