How Much Water is Enough?
Although watering outside plants is probably not high on your priority list at this time of the year - especially after our recent precipitation - I want to return to our discussion of plants and water this month. Specifically, I would like to discuss how to know when you have applied enough water to your plants.
As I discussed in earlier articles, water is stored in the spaces between soil particles. As water falls on the soil and soaks in, it progressively fills the spaces between the particles. Gravity pulls some of the water deeper into the soil but some is so tightly held by the soil particles that it does not move downward. When gravity has removed all the water it can, the soil is said to be at field capacity. Plants can pull more water out of the soil than gravity but some still remains attached to the soil particles after the plants have taken what they can. When plants have removed all the water they can, the soil is said to be at the wilt point. The water content of the soil between field capacity and the wilt point is called available water.
The amount of available water depends on the type of soil. Sand particles in sandy soils do not hold on to water molecules very tightly. This allows more water to be drained away by gravity leaving less available water for plants. Conversely, clay particles hold water molecules more tightly than sand particles, but some molecules are held so tightly that plants cannot remove them. This means that although there is more total water in clayey soils at field capacity, there is less available water for plants. Loamy soils, have the best water properties. They can hold a lot of water against the pull of gravity, but the water is not held so tightly that plants cannot remove it.
Available water can leave the soil in two ways. It can be evaporated directly from the soil surface, or it can be taken up by plants and moved to the leaves where it is transpired and evaporated to keep the plant cool. The combination of evaporation and transpiration from the leaves is called evapotranspiration. After the available water is removed from the soil, plants begin to wilt. At first, the wilting is only apparent during the hottest part of the day but plants recover overnight. As the soil moisture level drops, plants stay permanently wilted and will ultimately die if not given additional water.
The frequency with which you must water is determined by the type of soil you have, the amount of precipitation that has fallen, the temperature, the relative humidity, the wind, and the number of plants you have. One way you can tell when it is time to water is to watch your plants. When they begin to show signs of wilting, it is evident that they are losing water through evapotranspiration faster than they can take it up from the soil. Another way is to use a soil probe.
A soil probe is simply a metal rod you can push into the ground. A screwdriver will do in a pinch, but something longer is needed to check soil moisture at greater depths-24 to 30 inches is a good length. To check soil moisture conditions, you push (or attempt to push) the probe into the ground to the root depth of your plants. If it pushes in easily, there is probably enough moisture in the soil for your plants. If it is difficult or impossible to push into the ground, it's time to water.