Managing inputs in a variable environment

Figure 1: The effect of soil PAWC on modelled "total gross margin" averaged over 30-year periods of farming in central Queensland.

Howard Cox examines how growers can use the new national Whopper Cropper project to better manage climage and climate-influenced variables such as soil"s plant available water content

In northern regions, the value of stored soil water on crop yield is well known. However, there are other related factors that also play important roles. These include the water-holding capacity of the soil (also called plant available water content - PAWC), soil nitrogen status and in-crop rainfall.

Also, contrasting SOI (Southern Oscillation Index) phases can result in different in-crop rainfall ranges and can be used to influence other inputs.

The PAWC on its own can have a big impact on yield and returns (Figure 1). This PAWC (also called the "bucket size") can be a physical effect (shallow soil) or a chemical effect (salinity).

The effect of the PAWC is also influenced by location. For example, a larger PAWC will be of less benefit in a region that does not get sufficient rain to take advantage of the extra soil capacity.

The cropping season also affects the response to soil moisture content. In northern regions, winter crops are often very dependent on stored soil water. Therefore, a larger soil PAWC (when filled) can lead to higher returns. Summer cropping has a wider sowing window that allows time for refill of the soil profile. Depending on location and soil PAWC, this can lead to much higher summer crop yields, and with less variation.

The effects of interacting inputs can be quite complex. Whopper Cropper was designed to enable quick and easy comparisons of the effects of variable resource inputs, such as nitrogen fertiliser and soil moisture at sowing.

Single-level responses or interactions between factors can be easily graphed and compared side to side. Figure 2 was generated with only a few clicks of the computer mouse by choosing options from crop type, crop maturity, sowing date, plant population, soil PAWC, soil water at sowing, nitrogen fertiliser rate, soil nitrogen and SOI phase. Gross margins can also be generated after entering costs and grain prices.

Figure 2: Effect of crop type and soil PAWC on wheat and sorghum yield at Emerald

Figure 2 shows that at Emerald, higher soil PAWC increases potential yields to a greater extent in sorghum than wheat. The higher PAWC also reduces the yield variation in sorghum but not in wheat (wheat is more responsive to PAWC and higher yielding the more southerly the location).

For the national Whopper Cropper project, 1.8 million combinations of the resource and management inputs and the SOI phase forecast system will be generated. This is a large number, but in reality most people will be interested in a small set relevant to their own location.

This new national project is part of the "Managing Climate Variability" program in which GRDC is a major partner.

Soil PAWC and variability of in-crop rainfall also affects nitrogen fertiliser decisions.

Shallow soils with low nitrogen fertility are a risky proposition to fertilise with bagged nitrogen in an environment with widely varying in-crop rainfall.

Important questions could be: what are the critical values of soil PAWC for economic response? What are the economic responses to nitrogen fertiliser? How does the response vary with location? What is the interaction with SOI phase? These questions can be investigated with Whopper Cropper.

A new version of Whopper Cropper will soon be available to growers, advisers and consultants through workshops conducted by Nutrient Management Systems, starting in April 2005.

GRDC Research Code DAQ00006.
For more information: Howard Cox, 07 4688 1381, fax 07 4688 1193,; Chris Dowling, 0407 692 252