Sticky soils with erosion-fighting qualities are under investigation for use in macadamia orchard floor management.

NSW Department of Primary Industries soil scientists in conjunction with local soils consultants and growers are conducting a four-year project funded by HAL to investigate several methods of rapidly stabilising soils.

These include fast growing grasses, polyacrylamide (PAM), poly vinyl acetate (PVA) and molasses.

“Saving soils in northern NSW macadamia orchards has been a difficult challenge,” said NSW DPI researcher Justine Cox.

“The combination of dense shade under mature trees, heavy rainfall in the growing area and the conventional system of mechanically harvesting macadamia nuts off the ground makes soil management difficult,” she said.

“Previous NSW DPI studies have identified the permanent groundcover smothergrass as the most suitable long-term solution for stopping erosion in the low-light, orchard-floor environment.

“But how to remediate the impact of erosion in some older orchards without groundcover is another issue this project covers.”

Ms Cox said some growers are creating inter-row v-drains or dish-drains using a grader or a rotary hoe fitted with a belt to transfer inter-row soil to the eroded tree-line.

“But immediately after these works the whole orchard floor is exposed to the risk of erosion,” she said.

“The rehabilitation approach of this project is to transfer inter-row soils, rich in organic matter, to the eroded tree line and then to quickly stabilise the soil until a permanent ground cover can be established.”

“Quick germinating and growing sub-tropical summer species will probably be most effective at rapid soil stabilisation of a large area but it will be difficult to get them to persist in the shaded tree row. This is where the other treatments will be tested.”

PAM is widely used for dust control and PVA, which is biodegradable, to reduce soil movement.

Readily degradable organic material such as molasses, cellulose or starch may provide a binding agent and stimulate stabilisation by increased soil microbial activity.

Ms Cox said the project would also look at saving soils in younger orchards where the tree row is often maintained bare by herbicide but there is usually a full inter-row ground cover.

“We will trial soil stabilisers in this bare strip; we will also evaluate whether using herbicide only before harvest time or maintaining a ground cover in the tree line will be effective.”

A demonstration plot will be set up to show growers the options for remediation.

Field trials will assess the effectiveness and cost-benefit of the most promising stabilisation methods.

Sites with and without groundcovers and soil treatments will be monitored over three years for harvest efficiency, soil movement and structural properties, water infiltration, root growth and soil biology.

The effects of blowers and sweepers on soil movement will be evaluated; trunk collars and collection bottles will measure stem flow of water down trunks.

The information will be used to compare the effectiveness of the techniques to hold down soil, allow water penetration, and to aid the recovery of the topsoil and promote root growth.

Contact Justine Cox, NSW DPI, Alstonville, 6626 2400.

This story appears Agriculture Today.