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Storing water in soil

Storing water in soilStoring water in garden soil sounds too good to be true.  Is it?

The idea is that you can channel excess water (such as the output of your gutters or the overflow from your rain barrel) into the garden.  This Mother Earth News article provides an example of a very simple-to-implement system (shown to the right) and this information on Steve Solomon's website provides more technical details on the soil side.  You can recharge the soil during winter rains, then your plants will be subirrigated during summer droughts.

But does soil water storage work?  I've run across lots of people mentioning the concept, but no one who shows a multi-year system in action.  Here are my reservations:

I decided to crunch some numbers and see if my qualms had any basis in reality.  Solomon's information suggests that two to three inches of rainfall can be stored in the top foot of your soil (with the exact amount depending on your soil type --- clay holds more, sand holds less).  Since some plants can access moisture ten feet deep, we'll work with a value of 25 inches of plant-available water stored in the soil, which equates to 15.6 gallons of water per square foot of surface area.

Water in soilBut here's the trouble: we receive an average of 49 inches of rain per year, which means normal precipitation would fill the soil bank up twice without adding any extra water from our gutters.  Yes, there's a lot of transpiration and evaporation happening during the growing season, but I'm pretty sure that there's not much (if any) room left for water in the soil at the end of a normal winter in our area.

In drier parts of the world, filling up your garden soil bank by diverting excess flow from your roof probably makes more sense.  But I think we'd need to get creative if we wanted to put any more water into our soil during the winter.  Perhaps filling the aisles in our terraced back garden with wood chips would do the trick (but I never have enough wood chips to "waste" them on a nonessential use like that).

Back to the drawing board....

Our automatic chicken waterer keeps chores to the minimum so you can spend your time enjoying your backyard flock.

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I was intrigued by the article too, but did not think about micro-nutrients. I am trying to think if there is a way to measure the water content at various depths and times, to test the theory? It's one of those things that I would need to see for myself or see data to believe.
Comment by De Wed Sep 19 08:48:26 2012

The standard way is to take a soil sample, weigh it, dry it and weigh it again. The difference in weight is the amount of water (and in theory other volatiles, but I don't think soils contains much else).

Drying can be done by putting the soil in an oven at >100°C until the weight doesn't decrease anymore.

Comment by Roland_Smith Wed Sep 19 13:46:31 2012

Roland and De --- I think you're right that some scientific measurements are probably in order. Roland's method is the tried and true way, but I think there might be some simpler methods that would still give us semi-solid data.

They make moisture probes that you can stick a certain distance into the earth and see how saturated the soil is, but the little cheap one I have doesn't go very deep and doesn't seem very accurate. Another method would be to simply dig a hole and do a squeeze test to see how damp the soil is.

My tried and true method of telling that the ground is saturated is to see whether the water stays puddled on the surface a day after heavy rains. :-)

Comment by anna Wed Sep 19 16:00:10 2012

It sounds to me like you don't really need to store any more water in your soil, but rather need to find a way to drain the top foot or so- at least around your trailer. If you can make a pond that would be ideal.

A few siphon linked IBCs should provide you with irrigation water if you suffer a dry spell. When they drop to a certain level, you can reconnect them to the downspout. When they are full, the spouts can drain far from the house.

Comment by Eric in Japan Thu Sep 20 08:20:27 2012
I really, really want to believe that soil water storage is useful, but I think that with anything, there are caveats. For the amount of rain you receive, I question the utility somewhat. I also question how useful it is for annuals. In my dry climate, I think storing moisture from the winter snows is very helpful to the perennials, and means I have to irrigate those perennials a lot less before the monsoons come (usually July). For some of my perennials, I do not need to water them until sometime in June. However, for my annual vegetables where the roots are not that deep, I find I still have to irrigate them. Because most of my vegetable beds are double dug, I find that about June, most of the moisture that was present in the soil is gone (but that is in the unamended soil). Fortunately, often the recipe for improving drainage or retaining water is the same, adding more organic matter and both dry and moist soils need mulch.
Comment by Tisha Thu Sep 20 13:35:43 2012

Eric --- I'm like Tisha --- I really, really want to believe that soil water storage is useful. Unfortunately, the realist in me agrees with you --- I think that in our climate, we'd probably be better off just shunting the water off out of the way (or making a pond if we decided to get fancy.)

Tisha --- It sounds like your climate has significant dry periods, which is probably what it takes to make soil water storage worth its salt. We get so much water that I generally don't water perennials at all (although I keep meaning to hook up a system I can use during droughts).

Comment by anna Thu Sep 20 15:00:25 2012

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