Poor drainage can be the result of the soil type or the grade. Excessively drained soil fails to retain moisture. Plants must tolerate dry conditions or be given additional water.
Clay soils drain poorly.
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.