Needle scandal’s toll
Stubble improves water infiltration
IN THE last Rural Weekly I speculated about water infiltration rates into our very dry soil profiles.
I espoused some positive experiences I have observed over many years, with fully and deeply cracked soils down to 1m and lots of wheat or barley stubble remaining in the upright position.
I suggested this is a good prospect of absorbing water into the soil profile, no matter how heavy the rain falls from the sky. Definitely the more difficult issue to be faced with is for rewetting a soil profile, on soil types with little to no surface stubble, developing a shallow hard pan and no structure in the top 20cm of soil area.
So let us assume last December you have a near full profile of soil moisture from the 2017 zero tilled winter cereal country. No imagination is needed to assume that from December 2017 to now, the weather conditions have been very dry and very windy, as well as a hot end to summer.
Now we know that stubble country is good for allowing infiltration of rain fall into the profile however weeds, run-off, deep drainage as well as evaporation, can lose quite a deal of this stored moisture. The full profile you had at last December, can now be as low as 20 to 30 per cent remaining in your soil.
Assuming weeds have not been overly present, then evaporation can easily account for up to 60 per cent of soil water loses in many over summer situations. I have read of CSIRO research which says up to 12mm of free water can be lost per day during summer.
So with many of our best black soils water holding capacity of 180mm plus in a metre core, the soil can certainly dry out fairly quickly with these high evaporation rates. Of course it gets worse in lighter soils where the actual evaporation depth can be down to 50cm.
There is no doubt stubble cover improves water infiltration, however it does not eliminate evaporation entirely, especially after those miserable little showers of only 10mm or so. It slows down evaporation levels but does not stop it.
Standing stubble still attached by roots to the soil is more effective in controlling erosion in our summer storm seasons. It is still a positive pointer and of course with our modern day planters in zero till situations, is less likely for planters to block up with stubble and stubble pinning.
One reason of why you get big yield expectation gaps, I have observed, is the overestimation of stored soil water. Rain gauges and rainfall wall charts are good however you really need a soil probe or much better, a small hand corer.
A ground truthing walk then to clarify what your charts and water apps may tell you. A simple method, it lets you determine, much more accurately, the amount of moisture you have in your soil profile.
This will give you a far better perspective on planting decisions, such as we are experiencing now, with the recent rain and storm fronts we have had in the last two weeks. It also tell where your moisture profile starts and finishes through the profile.
With the substantial cost and short supply of any summer hybrid planting seed, you need to be confident in your moisture levels, both at the surface and at depth as well, along with the overall paddock consistency for a sensible planting decision.
❝ There is no doubt stubble cover improves water infiltration...
— Paul McIntosh
SOIL EROSION: Bad erosion after heavy rain on a cultivated paddock in early 2018.