Watering the Garden
Adequate soil moisture is essential for good crop growth. A healthy plant is composed of 75% to 90% water, which is used for the plant's vital functions, including photosynthesis, support (rigidity), and transportation of nutrients and sugars to various parts of the plant. During the first 2 weeks of growth, plants are becoming established and must have water to build their root systems.
While growing, vegetable crops need about an inch of water per week in the form of rainwater, irrigation water, or both, from April to September. Keep a rain gauge near the garden or check with the local weather bureau for rainfall amounts, then supplement rainfall with irrigation water if needed. There are ways, however, to reduce the amount of water you have to add.
During dry periods, one thorough watering each week of 1 to 2 inches of moisture (65 to 130 gallons per 100 square feet) is usually enough for most soils. Soil should be wetted to a depth of 12 inches each time you water and not watered again until the top few inches begin to dry out. Average garden soil will store about 2 to 4 inches of water per foot of depth.
Reducing water demands
All of this water, however, may not be available to plants, particularly if the soil is a heavy clay. Clay particles hold soil moisture tightly. If, for example, there are 4 inches of water per foot of this type of soil, there may be as little as 2 inches available for plants. A relatively high level of humus in the soil, brought about by the addition and breakdown of organic matter, can improve this proportion to some extent. By causing clay particles to form aggregates or large clumps of groups of particles, humus also adds air spaces to tight clays, allowing moisture to drain to lower levels as a reserve, instead of puddling and running off the top of the soil.
The moisture-holding capacity of sandy soils is also improved by addition of organic matter. Though most soil water in sandy soil is available, it drains so quickly that plants are unable to reach water after even a few days following a rain. Humus in sandy soil gives the water something to cling to until it is needed by plants. Addition of organic matter is the first step in improving moisture conditions in the garden.
Mulching is a cultural practice which can significantly decrease the amount of water that must be added to the soil. A 4 to 6 inch organic mulch can reduce water needs by as much as 1/2 by smothering weeds (which take up and transpire moisture) and by reducing evaporation of moisture directly from the soil. Organic mulches themselves hold some water and increase the humidity level around the plant. Black or clear plastic mulch also conserves moisture but increases soil temperatures dramatically during the summer (to the detriment of some plants and the benefit of others) if not covered by other mulch materials or foliage.
Shading and the use of windbreaks are other moisture-conserving techniques. Plants that wilt in very sunny areas can benefit from partial shade during the afternoon in summer. Small plants, in particular, should be protected. Air moving across a plant carries away the moisture on the leaf surfaces, causing the plant to need more water. In very windy areas, the roots often cannot keep up with leaf demands, and plants wilt. Temporary or permanent windbreaks can help tremendously.
During those times when cultural practices simply aren't enough, when rainfall is sparse and the sun is hot, watering can benefit the garden with higher yields, or may save the garden altogether in severe drought years.