Paul McIntosh discusses controlling run-off in cultivation during storm season
Summer provides precious rain
WELL, it is storm season again and this old photo could be typical of your paddocks if the rain comes hard and heavy, as it can in our northern summers.
However, if you look closely you will see the left side is awash with surface water or run-off water and the right side looks relatively dry and only the wheel tracks are inundated. What is the difference between the two parts is that the left side had been cultivated for extra weed control with a disc chain set-up and the right side had been zero tilled. It does not take much imagination to observe what will be the best result as far as moisture capture in our soil (or our water tank) for the future does it.
In dryland farming country, the soil is our water tank for providing moisture through the following crops’ life. We farm moisture and that simply means we do everything in our power to have any of these pre-plant rain events leave a build-up of soil moisture through the profile. The more moisture our soils store then the more certain we are of achieving a profitable crop.
As you can observe in the photo, this harvested wheat crop stubble has been left about 8 to 12 inches high and, along with our other popular winter cereal in barley, are considered to be the kings of stubble.
When we talk about fallow efficiency we mean the proportional amount of water left in the soil profile (plant available water or PAW) after a fallow period.
Unfortunately this fallow efficiency number is usually only about 20 and 30 per cent, although this can vary greatly, depending on rain fall event size and timing in the fallow period, stubble cover, infiltration capacity, evaporation and weed presence. All things we know about, but nothing like refresher points to us all in our quest to retain moisture in our soils for our next crop.
I want to chat about the infiltration capacity of our rainfall events. The perfect water-capture scenarios in our paddocks can all be very different and seeing how we certainly do not have control of the weather patterns, then our receival platforms eg. the soil surface, needs to be seriously considered.
The easiest scenario for soil surface preparation is the after-harvested winter cereal stubble of a 4-6 tonne per hectare grain crop with deeply cracked ground and a low sodium percentage of cations from your soil test (no surface crusting) and other good structural characteristics in the top 30cm of soil. In other words, no compacted layers or bare areas. You would not want to touch this sort of situation in an intended summer fallow. No matter how the rain comes, I would judge this soil will adsorb moisture like a sponge. Nutritional additions like solid fertiliser, Big N or manures/composts certainly need some thought and involve mechanical ingenuity to place these nutrition options in the appropriate spot for plant roots to uptake in the future.
The hard scenarios are those ones where the paddock is very bare of surface stubble, has been cultivated to a fine tilth, right down 75 to 100mm deep and then you strike a hard or plough pan. Include as well as having post rain crusting issues in the top soil, all adds up to not good farming scenario at all, but it does occur. In this situation if the rains do fall very gently, like 20mm every two hours, we might get some build-up of profile moisture for future planting and growing of a decent crop in this tough block of country. Heavy storm rain will eventuate in soil surface sealing, with massive water run-off starting a large soil erosion factor.
So what could you do here in this tough situation? My first thought would be to get the biggest chisel plough operating below normal plough layer and leave it very rough and cloddy. Not exactly an immediate planting situation, but at least any rain would work its way into the soil profile. Even pasture paddocks may need a renovation event or two to allow our probable heavy summer rain fall events to penetrate into the root zone and not just run-off. Narrow tynes on a row spacing of 50cm would be very advantageous in these pasture paddocks and of course performed at a very steady speed.
So consider your individual blocks for how much rain fall will be effectively stored in your soil profile and not be run-off or a future large evaporation statistic.
More about evaporation next week.