That is, once clay soils are saturated, they retain moisture longer than other soil types. Plant roots growing in clay can be injured by excessive soil moisture during wet weather or by excessive irrigation. Soil modification in the planting hole will not have long-term benefits. Water from the surrounding area will more easily enter modified soil, flooding the root system. Improving clay soils is difficult and usually not worth the effort.
Sandy soils drain very well but retain little moisture for plant growth.
Adding organic matter will improve the water-holding capacity of sands for a while, but the organic matter will decompose over time and the soil will again hold less moisture. As with clay, it may be easier in the long run to select plants that will adapt to the growing conditions on the site rather than try to change them. Drainage problems can be related to the grade or slope of the site. Both dry and wet situations can occur. Plants on the top of a berm or slope may suffer from dryness.