5.8 Increasing Plant Available Water

The soil profile is like a leaky bucket.  Only the rainfall that remains in the bucket can be used for plant growth.  This is Plant Available Water - PAW.

Rule of thumb

Every 50mm of PAW can be converted to one tonne of grain per hectare. Your soil profile bucket leaks through three holes.  Evaporation, run-off and drainage. Water is also lost by transpiration by plants. Crop transpiration is good, but if water is removed by weeds, this is just another hole in your bucket. The rate of water removal by plants and evaporation from a bare soil are the same (see PAN table on Fact Sheet 1 for rates).

How big is the bucket?

Three factors influence this:

Soil texture - amounts of sand, silt, organic matter and clay.  This can be changed by inclusion of OM and clay.

Soil structure - the size and number of aggregates and pore spaces.  This can be changed by retaining OM, adding gypsum, changing tillage practices.

Rootzone depth - how much soil the plant can access.  Difficult to change - the use of raised beds and deep ripping can change the depth of rootzone to a barrier.

Rootzone Capacity

Improving soil structure leads to increased water capture and holding by:

•  Increasing INFILTRATION

• Decreasing EVAPORATION

• Increasing STORAGE

• Increasing ACCESS

More water - more plant growth.

How to improve soil structure

Retain stubble

Stubble helps to improve structure and retain soil water in four ways:

v      Surface protection

v      Reducing evaporation

v      Increasing organic matter content

v      Feeding soil organisms

Increasing infiltration

Stubble acts like an umbrella, intercepting and shattering raindrops as they fall.

Without stubble cover, the soil exposed to the full force of the rain.  Raindrop impact can cause small soil particles (clays) to come to the surface.  Small particles have small pore spaces, which may block and form a surface crust, therefore reducing infiltration.

For an average winter rainfall on a red brown earth without stubble cover a surface seal will form in 10 minutes.

Soil protection from raindrop energy by stubble

High energy rainfall forming a surface seal

Reducing evaporation

Standing stubbles break up the wind, forming a layer of quiet air which draws less water from the soil.

This quiet layer of air becomes humid, further reducing water loss by evaporation.

Standing Stubbles can reduce water loss by Evaporation

Increasing organic matter content

Organic matter acts like a sponge holding more water in the soil.  It also helps to hold particles in the soil - both together as aggregates and apart to form pore spaces, ie improving soil structure.

Less water is wicked out of well structured soils - see Fact Sheet 2

Feeding soil organisms

Soil organisms produce 'gums' and 'strings' which help bind soil particles together to form aggregates.  Carbon from plant residues is an important feed source for soil organisms.

More organic matter - more soil organisms - more stable aggregates - more water.

Soil aggregation is essential for good water retention

Reduce tillage and trafficking

Tillage can cause the breakdown of soil aggregates degrading soil structure.  Conventional tillage will increase the soil surface area leading to more evaporation.  However, on soils with small soil pores a single cultivation will break the wick and conserve moisture.

In the Mallee Sustainable Farming Project, the first year of the tillage trials resulted in improved yield and WUE for the average of the direct drill treatments over district practice.

Trafficking of clay and heavier soils at field capacity can lead to soil compaction.

Ute mounted and trailed spray carts are prone to causing soil compaction due to the high axle weights on narrow tyres.

How to increase soil water storage

Soil texture is hard to change, but improvements can be made by the addition of:

•  organic matter (all soils),

•  clay (sandy soils)m,

•  gypsum (sodic soils).

Organic matter (all soil types)

Stubble retention over a period of 5-10 years changes the texture of the top 10cm of soil.  In the South Australian Long Term Trial, a loamy sand topsoil was modified to have the characteristics of a sandy loam due to the inclusion of stubble.

The soil can store an extra 4mm of water, supporting the crop for longer between rainfalls.

Clay spreading (sandy soils)

Incorporation of 100 tonnes/ha clay can increase water storage of the top 10cm leading to increased yield.  This effect has been observed to last 30 years and is estimated to still be going strong after 100 years.

Clay spreading also provides the extra benefits of:

•  increasing soil pH

• decreasing nutrient leaching

• improving soil wetting for germination and emergence.

Gypsum (sodic soils)

High levels of the sodium ions in the soil reduce the ability of clay particles to bind together into aggregates.  The addition of gypsum (high in calcium) displaces the sodium, allowing the clay particles to form aggregates.

Studies in the Wimmera have shown soil structure improvements leading to substantial increases in crop yield (see Table 2

Take Home Message

Yield and income are directly linked to the amount of water the soil can capture and hold for the crop.  Improve soil properties to improve PAW.

Courtesy CRC for Soil and Land Management

5.8 Increasing Plant Available Water

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