Fact Sheet 4: 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 OMand
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.
Improving soil structure leads to increased water capture
and holding by:
More water - more plant growth.
How to improve soil structure
Stubble helps to improve structure and retain soil water
in four ways:
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.
protection from raindrop energy by stubble
energy rainfall forming
a surface seal
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.
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
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
More organic matter - more soil organisms - more stable
aggregates - more water.
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.
Yield and WUE
performance (long term trials) with 60mm or 110mm of water loss
How to increase soil water storage
Soil texture is hard to change, but improvements can be
made by the addition of:
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:
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
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.
benefits from different levels of gypsum additions
Courtesy CRC for Soil
and Land Management
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