Storing 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:

• Won't filling up the soil during the winter flush water-soluble micronutrients too deep for roots to reach them?  (I can taste the micronutrient deficiency in certain crops after extended bouts of heavy rain already.)
  
• Doesn't my soil already get saturated during the winter?  (I know it does in certain areas.)
• Won't saturated winter soil kill perennials?
  
• Doesn't the water slowly drip down to the groundwater layer and then trickle out into nearby creeks and springs?
  

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.

But 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....