24 Apr 2015

The dark art of a soil water balance

Luckily understanding changes in soil moisture isn’t as difficult as this balancing act!  But it is all about balance.  A soil water balance (SWB) is a method of tracking how much water is in the soil available for plant growth.  If you know what the current soil water balance is for your farm, then you know if this is adequate – or limiting – for pasture growth.

A SWB works on some simple principles.  Imagine your soil is a glass of water.  The bigger the glass the more water can be stored – this is same as your soil type.  A loamy soil can store a lot more water than a light stony soil.  When the glass is full, that equals field capacity.  If you put more water in a full glass it just spills out.  The same with a soil at field capacity – if more water is poured on, either from rain, effluent or irrigation, the excess has to go somewhere because the soil can’t hold it.  The only places it can go are run-off or drainage.

Getting back to your glass of water – when it is full to the top you have to be really careful with it.  Soil is the same.  At field capacity soil is vulnerable to treading and compaction damage, plus there are limited air pockets which crops need to grow well.  So you want to spend as little time at field capacity as possible.  Sometimes it rains and rains, and you can’t do anything about that – but you can control when you add those extras – effluent and irrigation.

So to drop the level of your glass of water you drink some – imagine this is through a straw.  This is what happens with soil too.  As the grass or crops grow, they draw water from the soil and transpire it out their leaves.  This is the only way water leaves the soil once it is at field capacity. And as you get down the glass, it becomes harder to suck up the water.  The same for grass – it is harder to get the second half of the water in the soil so the grass is under stress.  Also, when it is cold and damp the grass doesn’t grow as much, so it doesn’t use as much water.

So a soil water balance is like keeping track of how full your glass is.  Ideally you want it to be over half full, but not right to the top.  Regen keeps a soil water balance running for your farm every day. Way easier than a Cirque du Soleil balancing act.


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