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Fact Sheet 1: What happens to rainfall on your paddocks?

Surface properties of the soil influence its ability to infiltrate water and resist loss by evaporation.  This table aids in the evaluation of surface soil properties, providing a means to estimate infiltration and evaporation rates.


Symptom Rating eg, Score
1 2 3 4
Water repellence No repellence problems Rarely repels Frequently repels Always repels 1
Clay content Sand Loamy sand Loam Clay 2
Slope Flat ground Slight slope Moderate slope Steep slope 2
Ground cover at break 50% of ground is covered 20% of ground is covered Some straw visible No grass or straw present 4
Aggregate stability (when dropped in still water) Soil aggregates do not collapse Aggregates break up a little Aggregates collapse into small pieces Aggregates collapse to a powder size 3
Run off/erosion Never Rare (1 year in 3) A few minor channels most years Most years large channels (> 50mm) 3
Surface crusting No surface crust Thin crust forms sometimes but not a problem for emergence There are often emergence problems - plant has to push up a 'plate' of soil Serious emergence problems in most 4
Pre-cultivation strength* Can push in a 3mm ros with the palm of your hand - no pain Can push in a 3mm rod with the palm of your hand - a bit painful Cannot push in a 3mm rod with the palm of your hand   3


*measure after good soaking rain


  A high score (low SPP) can indicate a poor soil, but high scores can also result if the soil is a clay or steeply sloped where even the best management will result in water wastage.


SSP rating = 1 - (Total / 85)

Example:        1 - (22/85) = 0.74

round down to SSP 0.7


The SSP rating indicates the fraction of rainfall water to enter the soil.  A soil with SSP 1.0 will allow all rainfall to infiltrate, whereas a soil with SSP 0.6 infiltrates 60% of rainfall water.


The SSP rating also helps us to determine soil water loss by evaporation and is used in water balance calculations.


This soil loses topsoil water at the PAN evaporation rate divided by 0.7.

Improving infiltration and evaporation

Stubble retention on all soil types helps to:

  •  protect soil from raindrop impact

  • reduce evaporation by cutting wind

  • improve structure by increasing organic matter content

After 5-10 years of direct drill and stubble retention at the SA Long Term trial Sites, the following were observed.  These would give a potential change in SSP (figures in brackets):

  •   Water use efficiency and yield increased over the life of the trial.

  • Runoff and erosion channels were reduced (3 to 2).

  • Surface sealing was reduced (4 to 2).

  • Aggregate stability improved (3 to 2).

  • Pre-cultivation strength of topsoil reduced (3 to 2).

  • Improved water holding capacity in top 10cm.

Stubble retention would change SSP for surface cover from 4 to 1.  The soil now has a total score of 14 which calculates to 0.84 (round down to 0.8).


At the start of the trial the soil rated a surface soil property (SSP) of 0.7.  The changes seen with direct drill and stubble retention would change the SSP rating to 0.8.  The soil now infiltrates 10% more water and so, in a season where 200mm used to infiltrate, the crop now has access to 220mm.  This extra 20mm potentially translates to an extra 0.4 tonnes grain per hectare ($60/ha at $150/tonne).


The increase in organic matter has also increased the water holding capacity of the soil.  The crop now has a bigger bucket to access, allowing it to last longer between rains without water stress.


Addition of gypsum to sodic soils and clay to non-wetting sands will improve soil structure, increasing the SSP rating of the soil in a similar fashion and leading to improvements in soil infiltration and evaporation. (see Fact Sheet 3- Calculating maximum plant available water of your rootzone)


Click here or on the the chart thumbnail on the right for an Excel chart of PAN evaporation rates for selected places in South Australia.

Evaporation from the paddock

  • Bare soil loses topsoil water by evaporation at PAN rate (see Fact Sheet 2- Does summer weed control save soil water? ).

  • Stubble covered soil loses topsoil water by evaporation at a slightly lower rate.

  • Plant covered soil loses little water by evaporation, but loses topsoil water and deep profile water by transpiration at the PAN rate.

Fact Sheet 1: What happens to rainfall on your paddocks?

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