Archive for March, 2013

I’ve been away for a while.  Mostly because work is busy but also because I haven’t had much new to report.  But in the last couple of weeks I managed to get some more data on some recent mulch trials and have written it up and presented it to a group of people at a meeting.  The most interesting thing to come out of it was that you CANNOT rely on winter rains to wet up the soil profile.  Even after 600 mm of rain over winter, at 20-30 cm depth the soil was still dry!  And of course the thicker the mulch layer the worse the problem.

The other things that’s apparent is soils are highly variable.  Lots of preferred pathways exist in soil so, in these trial for example, there was a high degree of variability between the three replicate treatments in all cases.  When a significant rain event occured (33mm) the response under the unmulched soil was less than under the other treatments – probably because of runoff due to non-wetting.

While some of the mulches maintained the moisture content in the soil below over summer most were so dry that it really didn’t matter!  They were drought stricken.  This raises an interesting point.  What is the point of amending soil to hold more moisture when there isn’t any!  Certainly, in discussions with someone that conducted trial on a range of soil amendments recently, under the Water Corporation watering regime there was absolutely no difference between treatments. Drought is drought!  If plants are water stressed it doesn’t matter a damn about the improved water holding capacity of the soil if there’s no water in it they can’t access it anyway!  And in fact the situation could be made worse because clay, while holding more water, holds much of it a higher soil moisture tension (ie its harder for plants to extract it).

So this whole issue is complex.  But the main thing to think about is whether or not the soil profile is wet up throughout the root zone over winter.  And due to preferred pathways that exist in soil, you will need to dig down in more than one place to find out!  If the soil largely remains dry in the root zone (20-30 cm) at the end of winter then you need to think about how you apply mulch, especially if you are a) overhead watering and b) only watering 2-3 times a week.

I would dearly love to do some research on this topic but alas funding for “home garden” type issues doesn’t exist so we are reliant on the bits and pieces done here and there, often by institutions like TAFE.  And extrapolations from the commercial stuff done by eg DAFWA.

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