are
temporary and need frequent renewing.

Open-ditch drains are deeper, more permanent water passageways around
or across the fields.

Surface and open-ditch drains take only surface water. They also carry
off surface soil and manures washed into them. They frequently become
choked or stopped by trash and soil, and are in the way of cultivation
and harvesting operations.

Covered drains, under drains or blind ditches are water passageways
made of brush, poles, stones, tiles, etc. (Figs. 80-81), placed in the
bottoms of ditches and then covered with soil.

INFLUENCE OF COVERED OR UNDER DRAINS ON FERTILITY

_Influence on soil water._

Covered or under drains take not only surface water, but also remove
free water from the soil beneath down to nearly the level of the
bottom of the drains, and thus increase the area of film water.
Removing the free water enables the soil to absorb more readily rain
water falling on the surface and therefore checks surface wash and the
gullying of fields.

_Influence on soil ventilation._

Lowering the free water allows a deeper penetration of air and,
therefore, a deeper root development and enables crops to better
resist dry periods.

_Influence on soil temperature._

Lowering the free water in the soil influences soil temperature:

By diminishing the amount of water to be heated.

By checking evaporation.

By letting warm showers sink down into the soil.

By increasing ventilation and therefore permitting the circulation of
warm air in the soil.

The cropping season is lengthened by causing the soil to be warmer and
drier earlier in the spring and later in the fall.

_Influence on plant food in the soil._

Covered or under drains check losses of plant food that occur with
surface and open ditch drains. They render available more plant food,
for lowering free water and increasing ventilation:

Deepen the feeding area of the roots.

Aid the process of nitrification.

Aid chemical changes which make plant food available.

Check denitrification.

LOCATION OF DRAINS

As gravity is the force that is to take the surplus water from the
soil, the outlet of the drainage system should be at the lowest part
of the area to be drained.

[Illustration: FIG. 83.--CROSS-SECTIONS OF STONE-DRAINS.]

[Illustration: FIG. 84.
_A._ Cross-section of a pole-drain. _B._ Cross-section of a
tile-drain.]

[Illustration: FIG. 85.--A COLLECTION OF DRAINAGE TOOLS.]

[Illustrat