The type of soil has to be born in mind. For example a well-drained soil needs more frequent watering than a clay or loam soil. A common mistake with watering is to assume that if the top of the soil looks wet the job has been done. In fact, as a rough guide, the soil needs to be saturated to a depth of 1 and a 1/2 inches.
The watering requirements of different crops varies and getting it wrong can have a disastrous effect on the quantity and quality of the harvest.
Brassicas (Brussels Sprouts, Cabbage, Cauliflower, Kale etc)These vegetables can tolerate slightly dry conditions but they must be well watered when they are transplanted to their permanent position. If they are allowed to dry out at this stage their growth will be checked and they will bolt. Split heads will occur in cabbages if they are subjected to dry conditions and then sudden watering.
Legumes (Runner Beans, Peas, French Beans, Broad Beans etc.)Early varieties of legumes which flower and fruit in early to late spring are not really affected by dry conditions. Runner beans and late peas however, if allowed to dry out at the roots, will drop large amounts of blossom and develop small pods.
french beans will fruit well in dry soil conditions.
Root Crops (Carrots, Parsnips, Swedes, Turnips etc.)Deep rooted root vegetables like carrot, parsnip and swede, can withstand dry spells. Shallow rooted root vegetables such as turnips should be watered when the soil is dry to a depth of 1 inch. Sudden watering of dry soil can lead to the roots swelling up and then splitting.
Onion Family (Leeks, Shallots, all Onions)All the onion family need plenty of regular watering. Too little water makes onions small and over-strong in flavour. Leeks need enough water but are less of a problem because they are not harvested until very late in the season when the soil is naturally moist.
Cuccurbits (Cucumbers, Marrows etc.)All of the cuccurbit family require plenty of moisture. If the soil is too dry blossom can be lost or the fruits will be small and misshapen.
Salad Stuff (Lettuces, Radishes, Celery etc.)All salad stuff requires plenty of water. Dryness results in lettuce bolting and tasting bitter. Radishes go "woody" when soil is over dry. Celery must be kept moist or it will be tough and stringy.
Tomatoes (all varieties)If tomato roots are allowed to dry out the flowers will drop. This is especially important when the tomatoes are planted out. The root ball must be kept moist.
Potatoes (all varieties)Very small potatoes will result from dryness at the roots of potato plants.
Vegetables do not thrive if they are under or over watered. Give careful attention to watering regularly and it will pay dividends.