Poor soils require new thinking

Ryegrass is an established part of soil improvement strategies for high yields in several Goulburn Valley orchards which began testing a theory presented by a local soil scientist in the 1990s.

Bruce Cockroft began soil studies more than 50 years ago and from his own research and reviews of work by others concluded that such practices as mulching and ripping to improve soil structure and root penetration were not enough to make a significant difference.

The part he suggested ryegrass could play was to add organic matter and increase porosity through the soil profile via its vigorous and fibrous root system. Sheaths of soil particles develop around these roots (called rhizosheaths) consisting of fine particles glued by root exudates, fine fungi and root hairs. These break up when ryegrass roots die but can leave soil in a more penetrable state.

It was an idea the Turnbull Brothers at Ardmona thought was worth trying and since first using ryegrass as a tool nine years ago they have been refining management and applying the principle to old and new blocks.

Chris and Al Turnbull grow peaches, nectarines, pears, apples, plums and cherries and their irrigation methods range from flooding and mini-jet sprinklers to conventional and sub-surface drip. Water is monitored using gypsum block moisture sensors at depths of 30cm, 60cm and 90cm, and measurements show water infiltrates much better now and is taken up by the tees more efficiently. This improvement is particularly welcome with the advent of cuts to water allocations.

They say there has been a steady building of organic matter over the years and soil structure has improved deeply under the tree lines and across the width of the large banks on which trees are planted.

“Before the ryegrass treatment you could try to push your hand into soil on the bank on which trees were planted and it was like concrete,” Al Turnbull said. “Now a hand will go in easily and pull out a clump of crumbly soil.”He believes the capacity of fruit trees to grow and produce well has been assisted by deeper taproots and healthy, active lateral roots able to explore a greater volume of soil.

The Turnbulls, like others who have taken up the idea in the district in the last 10 years, say soil management is evolving. Challenges have included finding ways of getting strong establishment of ryegrass each year and maximising growth across the banks where it is most needed before it has to be sprayed off in spring to prevent competition with the developing fruit.

As part of its charter to improve technologies for water use, whether these are physical or biological, the National Program for Sustainable Irrigation has supported the development of Bruce Cockroft’s work and extension of improved soil management to irrigators.

An important message from the prominent soil scientist is to recognise that while some parts of the world have what he calls “super soils” with good levels of organic matter and other properties which sustain production, Australian soils are older and lack the attributes of the very productive soils. Our soils are characterised by structurally fragile shallow topsoils, and subsoils that are difficult for the roots of agricultural crops to penetrate.

“Such is the nature of our soil that most organic matter applied to the surface is gone in a matter of months and while earthworms churn it up little is done to increase organic carbon levels and improve structure at a reasonable depth,” Bruce Cockroft said.

“However, within rhizosheaths organic matter is produced and protected and growers are seeing the results of sowing ryegrass as part of their soil management.”

Caption: For growers like Al Turnbull ryegrass has become a regular tool for soil improvement.

Citation

Land & Water Australia. 2010. Poor soils require new thinking. [Online] (Updated March 29th, 2010)
Available at: http://lwa.gov.au/node/3739 [Accessed Wednesday 29th of February 2012 07:55:06 PM ].