Gardening Technique Unit Screen//v.1

Water enters soil by moving into the pores between the soil particles. As water fills the pores, the soil becomes saturated, and water can move farther down into deeper air-filled pores. This wetting front cannot move down in the soil unless the soil above it is saturated. The rate at which the wetting front moves through the soil varies, depending on the texture of the soil. Water moves quickly through soil with large pores, such as sandy soil, and much more slowly in soil with tiny pores, such as compacted or clay soil. It is important to remember that each layer of soil must get very wet before the wetting front moves deeper into the soil. After the wetting front stops moving downward, the soil below it remains dry. There is no such thing as "keeping the soil slightly moist." Some of the soil always gets very wet. The more water you put on, the deeper you are watering. At each rainfall or irrigation, the soil gets as wet as possible, then it dries slowly until the next wetting. Soil moisture is in a constant state of flux.

Soil drainage: If you place a sponge in water and lift it out, water runs freely from it, pulled by gravity. Soil drains in the same way. Eventually the sponge will stop dripping. Gravity has pulled out all the water it can; the rest of the water in the sponge is held by capillary forces, the forces that make water stick to some surfaces instead of beading. When soil has finished draining, it is holding all the water it can against the pull of gravity. The amount of water in the soil at this point varies greatly from one type of soil to another. As water drains from the soil, air is drawn into it, so the amount of air in the soil also depends on the soil type. Sandy soils hold the least water and the most air; clay soils hold the most water.

Plants and soil water: If you squeeze a sponge that has quit draining, more water will drip from it. As you squeeze the sponge, it requires more and more effort to get it to drip, until you can't squeeze out more water, no matter how hard you try. But the sponge is not yet dry. In the same way, a plant can get water easily from very moist soil, but as the soil dries out, the water moves less and less freely into the plant, until finally the plant can't extract any more water at all. But even at this point, the soil still contains some water. Healthy plant growth requires free movement of water into the plant. When the soil is so dry that the plant cannot get enough water for its needs, the plant is under drought stress. The best time to water is when the plant has used about half the water available to it, before the plant is under any stress.

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