One of the skills of a gardener is knowing when to water. In general, the right time to water is when the plant has used about half the water available to it. Some plants, however, need a dry period to thrive, and others can’t tolerate even a little dryness, and need to be watered more frequently.
The Touch Test
The simplest and most common way of assessing the water level of the soil is to feel it. Since the surface soil dries out rapidly, and the plant roots are not at the surface, it’s necessary to check the soil a few inches below the surface, where the plant roots are. If the soil is loose enough, dig down 2 to 4 inches with your hand and feel the soil. If it isn’t loose enough, use a trowel or shovel to dig a small hole and feel the side of the hole.
At the right moisture level for average garden plants, the soil should feel cool to the touch, but should not wet your finger. At this point there is some water in the soil, but it’s time to water again.
At the right moisture level for plants that can’t tolerate dry soil (many house plants and shade plants), the soil should dampen your finger when you touch it, but not make it muddy.
At the right level for arid-climate plants, which need a dry period, the soil should feel dry. However, don’t wait too long after it feels dry — even desert plants do better with adequate water.
Soil Sampling Tubes
Rather than digging a hole, you can draw out a core of soil with a soil sampling tube (also called soil tubes and soil core samplers). A soil sampling tube is a hollow tube from 1 to 3 feet long with a wide slot cut along the side and a sharpened bottom. Most types have a T-shaped handle to push the tube into the soil. It is used by pushing it into the soil, then withdrawing it filled with soil. The core of soil can be knocked out of the slot on the side. With a soil sampler, you can easily feel the soil at various levels without digging a hole. These samplers are also useful for drawing soil samples to send to a laboratory for analysis.
For more about soil sampling tubes read: http://www.copernicus-psd.com/soil-sampling-tubes/
Many instruments are available to measure soil moisture. Most are expensive, and are made for farmers or groundskeepers who use precision irrigation methods. But some are inexpensive and simple enough for home use. These instruments fall into two categories: those that give a read-out, and those that automatically turn a watering system on or off.
A tensiometer is a long, thin metal probe with a small meter (much like a light meter) mounted on the top. When the probe is stuck in the soil, the amount of moisture at its tip (usually made of ceramic material) registers on the meter. Tensiometers measure the capillary action of the soil directly (the tension of the film of water), so give an excellent picture of the water available to plants.
Some tensiometers are made to be left in one spot for at least a whole season. They are buried, with the gauge above the soil. Others give an instant read. This type is most useful to home gardeners. You can push the tube into the soil, wait a moment, and read the tension of the water in the soil.
Using a tensiometer is a simple operation. The probe should be inserted 12 to 24 inches deep under shrubs and 24 to 36 inches deep under trees. The higher the reading (in centibars), the drier the soil. When the meter reads in the 50 to 70 centibar range (moderately dry), it’s time to water.
Check the moisture readings again, at the same depth, 12 to 24 hours after watering, to be sure you’ve watered enough. Recently-watered soil has a tension of under 10 centibars. Some tensiometers are designed to turn a watering system on and off instead of giving a read-out. This type of tensiometer is installed as part of the irrigation system.
Another type of soil-moisture instrument measures the electrical conductivity of the soil. Dry soil doesn’t conduct electricity. The wetter it is, the more electricity it conducts. Unfortunately, water conducts electricity better when there are more salts dissolved in it, so these instruments can be fooled by soils that are unusually high or low in fertilizer salts.
Most consist of two metal probes with a gauge at the top. The reading is instant when the probes are inserted into the soil. Some types make a clicking sound instead of having a gauge, with the clicks being faster in wetter soil.
Other Soil-Moisture Sensors
Many other types of moisture sensors are available. Most are made to be permanently placed in the soil. A few are portable.
One useful and inexpensive water-saver is an attachment to irrigation systems that consist of a small cup placed above ground. The watering system can only turn on if the cup is empty; if it contains rainwater, the system won’t come on until it evaporates or is emptied. This keeps automatic sprinklers from watering on rainy days.