Evaluating Potential Rooting Ability
One of the most instructive things a water-wise gardener can do is to rent or borrow a hand-operated fence post auger and bore a 3-foot-deep hole. It can be even more educational to buy a short section of ordinary water pipe to extend the auger’s reach another 2 or 3 feet down. In soil free of stones, using an auger is more instructive than using a conventional posthole digger or shoveling out a small pit, because where soil is loose, the hole deepens rapidly. Where any layer is even slightly compacted, one turns and turns the bit without much effect. Augers also lift the materials more or less as they are stratified. If your soil is somewhat stony (like much upland soil north of Centralia left by the Vashon Glacier), the more usual fence-post digger or common shovel works better.
If you find more than 4 feet of soil, the site holds a dry-gardening potential that increases with the additional depth. Some soils along the floodplains of rivers or in broad valleys like the Willamette or Skagit can be over 20 feet deep, and hold far more water than the deepest roots could draw or capillary flow could raise during an entire growing season. Gently sloping land can often carry 5 to 7 feet of open, usable soil. However, soils on steep hillsides become increasingly thin and fragile with increasing slope.
Whether an urban, suburban, or rural gardener, you should make no assumptions about the depth and openness of the soil at your disposal. Dig a test hole. If you find less than 2 unfortunate feet of open earth before hitting an impermeable obstacle such as rock or gravel, not much water storage can occur and the only use this book will hold for you is to guide your move to a more likely gardening location or encourage the house hunter to seek further. Of course, you can still garden quite successfully on thin soil in the conventional, irrigated manner. Growing Vegetables West of the Cascades will be an excellent guide for this type of situation.
Deep though the soil may be, any restriction of root expansion greatly limits the ability of plants to aggressively find water. A compacted subsoil or even a thin compressed layer such as plowpan may function as such a barrier. Though moisture will still rise slowly by capillarity and recharge soil above plowpan, plants obtain much more water by rooting into unoccupied, damp soil. Soils close to rivers or on floodplains may appear loose and infinitely deep but may hide subsoil streaks of droughty gravel that effectively stops root growth. Some of these conditions are correctable and some are not.