Every gardener has heard the term "water deeply; that it is better to water deeply once a week than to water shallowly more often. What does it mean to water deeply and how can you tell if you've accomplished it?
What Does Watering Deeply Mean?
There is no hard and fast definition for watering deeply, but it generally means that the water is able to soak at least 8 inches below the soil surface. The point behind this is that most plant's roots are not sitting close to the soil surface. They have worked their way down into the soil, in search of water and nutrients. This helps protect the plant in times of drought, because the soil surface will dry out much quicker than it will below ground, where the soil is cooler. Since you cannot control the rain, there will be weeks when your garden will get much more water than it needs and weeks when it will be your responsibility to see that your garden is watered.
Another common gardening recommendation is to make sure your plants get at least 1 inch of water every week. One inch does not sound like a lot, and it isn't. That's a minimum. It is better for the plants if the soil gets a good soaking down to at least the 8 inches mentioned above. That's because i inch of water will evaporate or dissipate quickly, whereas a thorough soaking several inches below the soil surface will linger long enough for your plant's roots to get a good drink.
You could try to get around this by giving your plants a little bit of water daily, rather than a good weekly soaking. If you have a drip irrigation system where you are guaranteed that the garden really will get a daily watering, that's fine. However it is not a practical plan if you are watering by hand or hose. Plants that are used to getting frequent water will not develop the deep root system that is needed for the plant to survive periods of drought, so making your plants dependent on daily watering and then missing a few days will cause long term problems. Once a plant is water stressed, it can take weeks to recover, and in the case of annuals and vegetables, every week counts.
How to Test How Much Water Your Garden is Getting
How quickly water runs through soil and how much is absorbed for the roots to access will depend on what type of soil you have, the weather conditions, and how fast the water is being applied.
Water runs through sand much more quickly that it penetrates clay. That's why it is advised you amend both type of soil with organic matter, which is great at holding onto water just long enough for the plants to get at it. A 3 - 4 inch layer of mulch will help conserve what ever moisture is there.
However, there's a simple test to get an approximate idea of how much water is falling on your garden. Water your garden by what ever method you have and then wait a half and hour. At that time, dig down into the soil with a trowel. If it's not wet 8 inches or more below the surface, it might be that you haven't watered enough or it could be that you watered too fast and the water ran off elsewhere. It is probably both.
Next time, try a gentler stream of water for a longer period of time. A gentle soak for an hour or 2 is better than puddling soil around your plants and moving on. It primes the soil to absorb more water and to allow the water to spread out in the soil.
It can take a few tries to get it right and you don't have to be obsessive about getting exactly 8 inches. The point is to make sure the soil is absorbing and holding the water long enough to hydrate the plants. Once you master the concept of watering deeply, your plants will stay healthier in whatever weather comes their way.