Cap­tur­ing soil mois­ture

Sum­mer pro­vides precious rain

The Northern Star - Northern New South Wales Rural Weekly - - COLUMN - PAUL MCIN­TOSH

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.


SOIL MOIS­TURE: A pad­dock in­un­dated with sum­mer rain.

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