A sneaky soil problem - hydrophobia
Updated: Aug 14, 2018
No, not rabies. The "fear of water" I'm talking about is a common property of very dry soil, a condition which causes the soil particles to repel water and resist wetting. You can stand there and pour water on a bed for half an hour and somehow it just disappears. Maybe the surface looks wet, but scratch a bit and find it's hardly deeper than paint.
This is a common problem with drought-tolerant plantings. You can let the soil dry out but when the plants really need water, you have a problem getting it down to the roots.
The cure is small, frequent, gentle applications of water. Think a misty rain for at least a couple days. Or, if you're short of that, a brief shower with a hose over and over and over, letting the soil absorb what it can between each dose. A drip hose may work if you coil it around a shrub or tree with only a few inches between turns.
Bare soil often develops a crust, a mix of large and small soil particles, that hardens and repels water. That can be broken up, at least until the next shower of drops pushes the particles together again, but the only long-term solution is a mulch or covering of plants.
And, as a preventative, a thick covering of plants is the most effective. Mulch can dry out and become difficult to water itself, but roots give the water a network of channels to flow into and soil seems to dry out less quickly under a shady, humid covering of leaves.
Peat moss is quite hydrophobic and will take hours to wet completely. Likewise, potting soils containing peat can dry into monsters that leak water out the bottom of the pot while protecting their root-killing drought.
There are wetting agents to be applied by watering can. A mild dose of soap, agar-agar (a gelling agent made from seaweed) or, if you haven't planted an area, hot water.
And while organic matter isn't a cure all, the more you have in the soil the less likely it is to become hydrophobic. Rotting leaves and other materials hold lots of water and the end product of decay, the dark sticky material called humus, can hold as much as 90% of its weight in water.
Not much help when you have an already established garden, but a good incentive to go to trouble of adding lots and lots and lots of organic matter before you plant.