Evaluating the use of subsoil water by wheat - Is it the pipes or the pump?

Wheat in September 2004 growing under automatic rainout shelters at the field site near Junee

By John Kirkegaard, CSIRO Plant Industry Project Leader

Photo: Wheat in September 2004 growing under automatic rainout shelters at the field site near Junee.

The use of subsoil water below one metre by wheat crops in southern NSW is under the microscope (literally) in a new GRDC-funded CSIRO project that seeks answers to some fundamental questions.

How well wheat crops use subsoil water, what factors influence subsoil water use, and what types of management or variety improvements could make better use of the water when it is needed are to be investigated by the project, which is funded as part of the GRDC"s SIP08 initiative.

The work is focused on the red earth and red-brown earths in southern NSW, which are generally free of the severe chemical constraints such as salt, sodicity and boron that severely restrict root growth on soils in many other areas.

However, these soils are very dense below the plough layer and very hard when dry, and roots are often found clustered into old root channels or soil cracks.

Data compiled from 15 years of agronomic experiments in the area has shown that wheat crops often leave water in the subsoil below 1m which should have been available to the crop and we want to know why.

The amount of water left can be as much as 50mm and is influenced by both agronomic management and seasonal conditions.

Water may be left behind in the subsoil if we do not have enough roots there or if the roots are not functioning well for some reason (ie a problem with the "pipes"). Roots are often "clumped" into old root channels and so may not have enough access to the soil water.

Even when there are enough roots, the amount of subsoil water used could depend on how long the shoots are able to stay green and active (ie a problem with the "pump"). On the first hot days in spring, the high biomass crops typical of southern NSW may "shut down" to conserve water, and may then be less able to pump water from depth.

This balance between the number of effective "pipes" and the duration of an effective "pump" can be influenced by management and by variety characteristics, and their interaction with seasonal conditions.

The project involves an integrated approach that combines:

In 2004, a rainout shelter with drip irrigation was constructed and used to establish wheat crops on two treatments with different amounts of subsoil water - one treatment was wet to 120 centimetres, and the other had an extra 30mm stored below 120cm to 180cm (Figure 1).

The crops were grown to produce similar biomass at anthesis and then rain was excluded to see if the additional water was used, and how much extra yield it produced. Local growers and agronomists from the FarmLink group are involved in a competition to estimate how efficiently the extra subsoil water will be converted into grain.

Water is generally converted at 20 kilograms grain/ha/mm (French and Shultz), but simulation studies by Dr Julianne Lilley suggest that the deep subsoil water could be converted at anything from 2 to 45 kg/ha/mm;

Once we know how valuable subsoil water is, we would like to know how best to use it. We are using management (sowing date, deep tillage, nitrogen management, sowing density) and different genotypes (winter vs spring, high vs low vigour; high vs low tillering) to manipulate the ratio of root and shoot growth to generate different patterns of subsoil water use and determine its overall importance to yield.

Dr Michelle Watt has found significant variation in the potential rates of root growth among genotypes and is investigating its impact on subsoil water use in the field;

We have found that 40 to 80 percent of wheat roots can be clumped into old root channels, and although this may be a good thing to allow penetration to depth, the roots that are within these pores may not be able to make adequate contact with the pore walls for efficient water uptake.

Root tips may be damaged as they penetrate dense subsoils. Professor Margaret McCully and Dr Rosemary White are using detailed field examination in soil pits, combined with microscopy to provide information on the morphology and function of deep wheat roots from the field.

We are uncertain about which types of roots and where along their length water and nutrient uptake is actually occurring; and

We have collected intact cores from different depths within the soil, and using an apparatus developed by Dr John Passioura, we are able to grow plants in the laboratory under controlled conditions where the transpiration demand on the plant can be slowly increased, and we can examine how well the plants are able to cope.

For plants growing in dense subsoils we would expect that reduced root growth and clumping in the harder soil would prevent root access to all of the available water. Using this apparatus we can determine where exactly the restriction to water uptake from subsoils may reside - with the roots, with the soil, or problems at the root/soil interface.

Together these different components of the project will generate significant new knowledge on how and when subsoil water is used by wheat, and pave the way for the development of improved management and varieties that can make best use of subsoil water for grain production.

Figure 1. The graph at left shows the two soil water profiles established under the rainout shelter. What will the extra water be worth?

Figure 1. The graph at left shows the two soil water profiles established under the rainout shelter. What will the extra water be worth?

For more information: John Kirkegaard, 02 6246 5095, john.kirkegaard@csiro.au
GRDC Research Code: CSP00049, program 4