You Pay for Drainage, Whether You Have it or Not

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• You Pay for Drainage, Whether You Have it or Not
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Introduction

WATER is the key to high yielding crops. That is the one thing that yield maps keep telling us over and over, especially in dry years. If that is the case, then why do we spend so much time and money trying to get rid of it?

Ironically, the reason for tile drainage is to make more water available to the roots of the crop.

To understand this, you have to understand a bit about how water behaves in the soil. The soil is like a giant sponge, full of holes of different sizes. Water drains out of the large holes very easily, but the small holes hold on tightly to the water. As the soil dries, the water is held in smaller and smaller holes, and more and more tightly. Water won't flow from a small hole into a large one, unless there is pressure from more water pushing it.

A tile drain is like an extra large hole in the soil. When the ground is saturated, the pressure of the water in the soil pushes it into the tile, where it can be carried away. The flow of water in the tile does "pull" some water out of the soil, but this only acts to speed up the drainage of excess water, not to remove more water from the soil.

As the excess water drains into the tile, the remaining water is held more tightly. Eventually, the water is held tightly enough to stay against the pull of gravity, so that no more will flow into the tiles. This point is called field capacity, and represents the maximum amount of available water in the soil. The tile doesn't remove any available water, but only the excess.

To see this for yourself, get a sponge and a bucket of water. When you first pull the sponge out of the bucket, water drains freely out of the large holes, then slows down to a few drips, and eventually stops. At this point, there is still lots of water in the sponge, but it is held more tightly than the force of gravity is pulling it.

Roots will pull water out of the soil from field capacity down to the point where the remaining water is held more tightly than the pull of the roots. This is called the wilting point, because that is what the plants will do if the soil gets that dry. The difference between field capacity and the wilting point is the amount of available water in the soil.

When the soil is saturated, that water is not available to plants. Roots need both moisture and oxygen to grow (unless it is a specialized plant like rice, which pipes its own oxygen down to the roots), so roots won't even grow into a saturated zone. In a poorly drained soil, the saturated soil is close to the surface of the soil, so a shallow root system develops. As the water drains away over the summer, the crop is left with roots only in the top few inches of soil, which quickly dry out.

When the field is tile drained, the saturated zone is much deeper in the soil, so the roots can grow through a much larger volume of soil. This soil still contains the maximum available water for the crop, so the total water available is greatly increased.

Of course, when the rooting zone is enlarged, the ability of the crop to take up fertilizer is also increased. Along with that, the soil will warm up faster in the spring, so the crop can be planted earlier, and the risk of soil compaction is reduced. It's no wonder that tile drainage pays!

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