Commonly, there is good soil moisture at the point of harvest where rainfall has been above average in the past couple of months of a winter crop.
This is the case in many cropping areas of NSW this current season. Looked after, this stored water can go a long way to helping the 2022 crop, especially should it be a below average season.
Storing soil moisture, plus efficient capture of rain from now to sowing, in so many years is the difference between a good crop and a poor one - an early insurance policy for the next crop. The amount of soil water stored depends on many factors, including soil type, with some soils able to store 300 millimetres of available water (deep clays) and lighter soils around 100mm. Whatever the soil type, that stored soil water is going to be a valuable part of crop yield almost every year, especially when we have below average rainfall. Stored soil water level also helps for decisions like double cropping.
SoilWater app estimates stored soil water in grain crops and fallows across Australia. It is based on a large amount of research and commonly provides similar soil water estimates to installed moisture probes. It is worth keeping a check on paddock soil water levels as the fallow season progresses and the app can estimate what soil water would have been present at harvest as well as during progress of the fallow season.
Currently, many soils across the state are at 50 per cent and in some cases close to 100pc soil water storage capacity at harvest. Some areas have accumulated more since harvest finished. Clearly it is critical to keep as much of this soil water as possible and to capture more as the fallow season progresses.
The amount of water a soil can store is very much dependent on soil type. Deep, self-mulching, cracking clays with crops able to root 1.8 metre deep, especially if slower maturing varieties are sown early, can potentially store around 300m of plant available water.
At the other extreme, a sandy surface soil overlaying a sodic clay may only be able to store around 90mm to a likely root depth of 0.8. But even in these soils, early-sown roots can go considerably deeper, accessing additional water.
Normally, stubble cover plays an important role in capture of fallow rainfall. Research at Tamworth for example, after the breaking of the drought in 2020, showed how good groundcover captured most of rain that fell whereas most ran off in a bare soil situation. Other research has shown that if after harvest there is little plant cover, for example a pulse crop in a dry spring, a short season cover crop, like millet, may be worth a try. While growing the cover crop will use some soil water, its ability to better capture rain after it is terminated can more than compensate for its water use.
Good stubble cover also helps in slowing surface evaporation of fallow soil moisture, as well as keeping useful amounts of it nearer the soil surface. That can help in timely sowing, again often an important agronomic advantage.
Also important is timeliness of control of post-harvest fallow growth. It may be weeds or self-sown crop from harvest loss. It may seem a logical aspect of farming, but so often around harvest, it's busy with less than timely weed control. Weed growth can use up to 6mm of soil water a day, which quickly leads to big losses.
Research has also shown that allowing weeds to get away in fallows take up a lot of soil nitrogen, commonly a big limiting factor in non-pulse crops. While this nitrogen commonly returns to the soil nitrogen pool it is not available to the next crop when needed as it takes time to break down.
It is worth testing SoilWater app on some of your paddocks, especially if you don't have moisture probes. For example in our area, a clay loam soil, a crop rooting to 140 centimetres depth, is capable of storing around 150mm of available soil water. At mid-November SoilWater app estimated post-harvest soil stored water would be 90 mm.
SoilWater App, using BOM rainfall data for any cropping area across Australia, was designed by Dr David Freebairn and a team of researchers from the University of Southern Queensland. It can be downloaded free from the Apple App store.
- Next week: Galvanised burr control achievable with planning.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
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