Digging in two bucketful of organic matter such as well-rotted manure per square metre will add to the water-retaining capacity of the soil. As much as two weeks moisture for subsequent crops can be retained, as well as adding sufficient nutrients for good growth.
Cultivating soils after the beginning of April leads to severe moisture loss. Better to loosen any compaction by wiggling a fork and scratching a seedbed with a light hoe.
Fertilisers are very helpful in droughts. Adequate nutrient supply allows plants to make the most efficient use of available water. However, adding excessive quantities brings no extra benefits and may damage crops and the environment.
Sowing and spacing
Sowing early, before May where possible, will allow crops to root out into moist soil before severe drought arrives. Drought stress becomes severe in most dry years from June. This is especially applicable to crops such as carrots that will stand in the ground until ready to use, but less useful where successional sowings of spinach or salads for example are needed for continuity of supply.
Later sowings, typically after May, might have to be made into dry soil. Where this is the case applying water down the drill (groove cut in soil) before sowing is a water efficient way of ensuring good germination.
Successional sowings might be problematic if the soil is dry in mid-summer. Consider sowing more early crops and freezing or preserving the surplus for use later.
Spacing plants more widely, usually by about 50%, allows each plant a greater volume of soil to explore for water and therefore more drought resistant, at the cost of reduced overall yield. An example of this are cauliflowers planted at 30cm intervals in rows 45cms apart where irrigation is available, and at 45cm intervals in 60cm rows where water is unavailable.