Good groundcover keeps our soil where it should be – on our paddock

By: David Walker, Executive Officer
March 21, 2010

Our soils have taken hundreds of thousands (if not millions) of years to develop. What a pity, then, to have such a valuable asset wash away when we get heavy rainfall. 

Some of the heavier falls of rain this summer have produced quite a lot of erosion in some areas, as evidenced by the amount of sediment (mud and silt) in the runoff.

Research that was done right here on the Liverpool Plains at the Gunnedah Research Centre gives us a pretty good pointer as to how to keep our soil. As shown by the following graph, this research shows that soil loss is negligible if groundcover is maintained at 70% or better. 

At 10% groundcover, an average of 70 tonnes per hectare of topsoil is lost each year. This equates to the top 300 mm (one foot) of the most important part of the soil disappearing over a 30 year period. Hard to believe, isn’t it? But when you think of the topsoil that has disappeared from what used to be “the best wheat growing country in Australia”, the area around Keepit Dam and Manilla, then the 300 mm loss is quite believable. At 70% groundcover it would take 600 years to lose that amount of soil. 

But just what is groundcover? It is usually expressed as a percentage of the total ground surface area, and is all plant material that covers the ground including: crops, stubble, pasture plants and their residues, leaf litter, bark and twigs. Of course, the amount of ground cover is influenced by external factors such as climate – it’s a lot harder to maintain groundcover in a drought than in a good season. But as far as practicable, land managers should work within these constraints to maintain at least 70% ground cover. Even through the worst of the drought, it has been very apparent that some farmers have been able to maintain good cover, while others have let their land become completely bare and desolate.

And what are the other benefits of good cover? As well as holding on to your soil, it makes a huge difference to the amount of rainfall that you keep on your farm for your use. The same research has shown just how much rainfall is lost along with the soil.

Going back to the 10% groundcover example, an average of 175 mm rainfall will run off each year – that’s 7 inches of rain that you’re not keeping. I’m sure most of us would have just loved an extra 7” of annual rainfall over the last few years!

Holding onto your topsoil means that you are holding onto your plant nutrients, both in the fertiliser that has been applied and in ‘natural’ fertility. These nutrients are concentrated in the top few centimetres of soil. 

Soil organic matter is also concentrated in the topsoil. It helps improve soil structure and the ability of the soil to hold moisture, and then give it up as growing plants need it. It is the top soils that best hold soil moisture, and when it is lost the poorer soil horizons below mean that plants strike moisture stress much sooner.

So how is groundcover maintained? Cropping management involving stubble retention and zero or minimum tillage makes a huge difference, whereas multiple cultivations sees all groundcover quickly disappear. Stubble burning, of course, means zero groundcover - instantly!

Grazing management also needs to aim at maintaining the magic 70% groundcover. As well as not keeping stock in paddocks until all groundcover is removed, this is much easier if pastures are managed to encourage perennials, rather than annuals. This means that the paddocks need a period of REST so that perennials can recover from grazing. Set stocking is the enemy of groundcover.

LPLM 02 6741 8375

www.lplmc.com.au

A healthy catchment for a productive future