Grains Research and Development

Date: 01.04.2002

Opportunity cropping and subsoil constraint by Gary Alcorn

Soil chemist John Standley prepares the Toowoomba DPI soil rig for deep probe sampling as part of the PAWC project.

QUEENSLAND-BASED researchers are finding conditions matching a batch of 's' words — soil moisture, subsoil, salinity and sodicity — are closely linked when it comes to assessing plant-available water capacity (PAWC) for cropping.

Toowoomba research agronomist Brett Robinson and Nic Christodoulou, an extension agronomist based at St George, are cooperating under the GRDC-supported Western Farming Systems Project. This long-running program is combining computer and laboratory research with grower-led field trials to develop and test a whole-farm systems approach to management and decision making.

PAWC is the maximum amount of soil water available to plants and is the difference between the drained upper limit and the wilting point when the plants' roots cannot extract further moisture. In wheat crops this value is mainly a function of the soil type, Dr Robinson said. Sands have low PAWC around 50 mm, while well-structured clays can have PAWC values around 300 mm.

For example, a coolibah grey clay near Thallon in southwest Queensland had a PAWC of 160 mm. But, conditions

in the subsoil often prevent roots growing into this zone and extracting soil water, so the PAWC is reduced.

"Two common conditions are salinity and sodicity, which are very different, but sometimes occur together," Dr Robinson said.

  • Salinity is due to excess salts, including common salt (sodium chloride) in the soil water. Common salt is toxic to plant roots. Salinity can be measured by either the electrical conductivity (EC) or chloride (CI) concentration — the lower the numbers the better. Salinity often shows up in clay soils below about 60 cm depth.
  • Sodicity is due to excess sodium in soil minerals. The sodium turns the soil into a plasticine-like mass when wet and into baked brick when dry — both of these extremes can kill plant roots. Sodicity is measured by a test that extracts the positive ions from the soil. A high percentage of sodium (exchangeable sodium percentage, ESP) indicates a problem, i.e. an ESP greater than 15-20 per cent in subsoil.

Dr Robinson said where salinity or sodicity is severe, pastures usually become more desirable than crops, which demand stored soil moisture for growth and a reasonable yield.

Computer program helps

By using the HOW WET computer program, available at www.apsru.gov.au/products, growers can use their rainfall records to estimate the amount of soil moisture available to planned crops and pastures. "Once the profile is full, to continue fallowing is a waste of further rainfall." HOWWET calculates soil moisture from rainfall records for various soils throughout Queensland.

The table 'Opportunity cropping: what's on offer' (see below) compares the many advantages and disadvantages of opportunity cropping. The strategy of understanding and measuring plant-available soil water makes the benefits of opportunity cropping more achievable, and reduces the risks for growers, Mr Robinson said.

Opportunity cropping: what's on offer
WhatWhen it worksWhen it doesn't
Paddock healthBetter soil structure
More available nitrogen
Better infiltration of rain
Less run-off and deep drainage
[No downside]
Mix of enterprisesA wider range of crops
Better cash-flow
Less reliance on wheat price
Lower price risk
More management
More labour
Higher expenses
Higher production risk
Farming efficiencyBetter use of rain
High water use
Good water-use efficiency
Better use of machinery
Failed crops — wasted rain
Low water use
Low water-use efficiency
Water-use efficiencyHighLow
YieldsAverage to highLow to average
Personal outlookYou're laughingYou wonder why you bothered

Program 4 Contact: Dr Brett Robinson 07 4688 1343 email brett.robinson@nrm.qld.gov.au; Mr Nic Christodoulou 07 4620 8125

Region North