Different types of soil retain water in different ways. Clay soils hold the greatest amount of water but they have too many fine capillary pores so plants cannot extract enough water for their needs. Sandy soils contain coarse pores so the water held within them is more readily available for the plants than in clay soils, but they drain quickly and there is little capillary movement of water sideways and upwards and the plants will suffer over dry periods. Loam soils are the most balanced soils as they contain a mixture of coarse and fine pores. Coarse pores will allow rapid drainage while fine pores will retain water, so the plants can benefit of it in dry conditions.
In soil, moisture rises towards the soil surface by capillary action from the water table below. Heavy clay soils may be saturated to 2 m above the water table, with some moisture available to plants roots as much as 3.5 m above it. Silt and most clay soils are saturated to 1.5 m with moisture available to plants roots to 2.5 m above the water table. Fine sand soils are saturated to 1.5 m with moisture available to 2.4 m above the water table. Coarser sand soils are saturated at 30 cm and with moisture available to 1 m. Gravel has no water rise at all. In may gardens the water table is too low to have any influence.
Is important to determine what type of soil you have in your garden in order to know how the water will be available for your plants and how you can help them develop well. This way you will know what plants to choose for your garden, or how you will need to improve your soil to suit your plants.