Nee­dle scan­dal’s toll

Stub­ble im­proves wa­ter in­fil­tra­tion

Warwick Daily News - South West Queensland Rural Weekly - - News - PAUL MCIN­TOSH

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.

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