Soils can be dry for any one of several reasons:
The soil is sandy and doesn't retain water.
Rainfall is minimal.
Areas under the canopy of trees where rainwater doesn't penetrate to the ground underneath. This type of position often has two other problems - these being shade and a dense mat of tree roots that penetrates the soil and competes with any plants you try to establish there.
Water is not absorbed easily. This could be for a number of reasons such as poor soil structure (eg: hard packed clays), the presence of quantities of rock, or water repellent soils such as some fine sands.
The soil is under the eaves of buildings.
OVERCOMING DRY SOILS
There are a variety of techniques that can be used to improve dry soils. These include:
1. Incorporating organic matter into the soil. This will improve the ability of sandy soils to retain moisture and help 'open up' clay soils (ie: improve its structure).
2. Mulching soils will help retain moisture near the soil surface, as well as help buffer sudden changes in soil temperatures, help control weed growth and help control loss of soil through erosion. Mulches can be either inorganic materials such as rock or gravel, or organic materials such as pine bark, compost, rotted manures, grass clippings, etc. Organic manures have the advantages of providing nutrients and helping to improve soil structure, while inorganic mulches generally last longer (ie: don't decompose).
3. Soil ameliorants such as gypsum can be used to 'open up' hard packed or poorly structured soils to improve penetration of water into the soil. Water repelling soils can be treated with wetting agents that help increase water penetration.
4. Sprinklers and trickle irrigation systems can be installed to increase the amount of water reaching the soil.
5. Use plants that will grow successfully in such conditions.
DROUGHT TOLERANT PLANTS
(Tolerates dry periods but grows best with some wet periods)
Callistemon (most varieties once established)
Cassia (Most types)
Cercis siliqustrum (Judas Tree)
Dodonaea (most species)
Ficus (Common fig)
Tecomaria capensis (Tecoma)
PLANTS FOR DRY PLACES (Grow well in soils which are normally dry)
Acacia aneura (Mulga)
Hawthonia -small succulents, the hardiest are H. retusa varieties which tolerate wet periods & full sun as well as dryness & heavy shade. Leaves are fleshy, pointed, triangular and green sometimes with reddish or bronze colouring. Other shade hardy species include H. cymbiformis, H. magnifica, H. mirabalis, H. mundal, H. retusa and H. paradoxa
Gasteria -larger succulents with more attractive flowers than Hawthonias (generally pink tube flowers on long stems). Particularly shade hardy species include G. armstrongii, G. batesiana, G. liliputana and G. verruculosa.
Sempervivums -Best in cold areas (not good in nth coast NSW in full sun -that is too hot). Grow very well in shade, some of the best species are S. arachnoideum & S. tectorum
Plus a range of full sun succulents