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 |
|
|
|
|
TOTAL SCORE |
22 |
*measure after good soaking rain
Note:
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.
DETERMINE SSP RATING:
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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