Page 336 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

7.30 THE WORK

and 100 the other, or at the intersection of diagonals across the original squares. Any changes of
grade necessary at this stage can usually be made by the drag level.
If a small error has been made in balancing cut and fill, the gradient in the lower part of a field
or bench may be increased or decreased slightly to change the quantity needed to balance. Large
errors may require lifting or lowering the whole field, or one of its benches, or making arrange-
ments for disposal or borrow outside the plot.
When the entire field, or a large section of it, has been brought as near the finish grade as is
practical, all stakes are removed and the job is finished with a land plane. See Chap. 17. This will
flatten ridges and hollows around stakes, plane off spill windrows, piles, and track marks, and
even local inaccuracies at hitting the grade.
Planing also serves as a maintenance operation and is sometimes repeated after each harvest.
Distribution pipes are usually laid immediately after completion of leveling. Ditches should
not be dug until the pipe is on the job, as drifting soil can fill it very rapidly. It is usual to lay and
cement the pipe, to partially cover it by hand, and to allow it to cure. This fill is removed by hand
where standpipes are installed. The ditch is then backfilled and graded over by machinery.

IRRIGATION DRAINAGE

Alkali. All soils contain some salts, in both soluble and insoluble forms, and these are necessary
for plant growth. In rainy climates the soluble salts tend to be leached out of the soil and carried
away in underground water about as rapidly as they are made soluble by plant action and weath-
ering, or added as fertilizer.
In dry climates where there is not sufficient rain to leach them effectively, they accumulate in
the soil, accounting for the great richness and productivity of the land. However, in flat low areas
and some other places, the salts, then known as alkali, may be concentrated so heavily that they
kill plants instead of aiding them. Alkali may also appear as a surface crust where groundwater
comes to the surface and evaporates.
Underground Pools. Where soaking rains are rare, natural underground drainage tends to be
poorly developed or nonexistent. If such an area is irrigated, water absorbed in excess of that
required by the crops will accumulate in a stagnant underground pond whose top may rise close
to the surface.
This water will dissolve minerals on its way down and while lying underground, and usually
becomes so alkaline that it injures or kills plants which absorb it. If it does not become loaded
enough to do this, it still may injure plants by drowning their lower roots. Also, when the water
table is near enough to the surface that capillary attraction will lift it to the surface—a short dis-
tance for granular soils, a long one for fine-grained soil—its evaporation will form an alkali crust.
When such a stagnant or semistagnant pool forms, the land above it usually becomes unfit for
crops; and even if irrigation is stopped and the water slowly drains away, the alkali deposits in the
soil may render it unusable. Artificial leaching would reestablish the underground water.
Drainage. The area can usually be put back in production by the installation of an adequate sys-
tem of drains. These will serve to lower the water table below the trouble line, or to give the water
enough flow toward the drains that it will not stay in the soil long enough to become alkaline.
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Such drains preferably are deep, 6 to 7 ⁄ 2 feet being usual, and spaced from 75 to 800 feet.
Close spacing is for impervious soils, wide for granular ones. However, for subdrainage purposes,
the porosity of the soil cannot be judged from casual inspection, or even by analysis of samples.
Heavy impervious clays often respond readily to tiling because they are filled with fissures, either
open or sand-filled, which conduct the water. Many really tight soils will not require drainage
because of their refusal to absorb the irrigation water.
Some irrigated lands are composed of alternating layers of heavy and porous soil, which are in
the form of lenses tapering to nothing on each end, so that natural drainage must move through
both types of soil. Ditching cuts and drains the porous lenses.