Kinds of Soil
There are many types of soil, which may be narrowed down roughly to clay, sand, and loam types. Clay soils have a greater capacity for holding water than other types, but are rather difficult to handle. Their physical structure is improved by the addition of sand, humus, weeds, manures, and grass clippings. Sandy soils are easy to work, but they leach easily. They are improved for growing purposes by the addition of organic material.
Humus consists of organic material such as peat, leaf mold, and compost, and plowed-under cover crops (soy beans, alfalfa, and clover). A mixture of sand, clay, and humus produces a vegetation-sustaining loam. The addition of humus improves the structure and character of the soil as well as its water-absorbing capacity and its texture.
Drainage of the Land
The drainage of surface water from land often presents a problem. Good drainage is needed for the protection of the given site; for the comfort of those who are to use it; and, not least, for the good of the plants to be grown upon it. Few plants succeed in cold, damp, un-drained soil. Most plants require warmth and air at their roots.
Any water that falls upon the area must be kept moving, though at not too fast a pace. The land needs to be shaped so as to carry surface water away, and to prevent its collecting or standing in pockets. The water must be spread out or else controlled mechanically or structurally. Some soils, however, are so open and dry to pose no drainage problem.
The drainage of an area is facilitated by proper surfacing. But it can be assisted by underground drain tiles or pipes laid in lines from 20 feet to 40 feet apart and from 3 feet to 4 feet deep, depending upon the kind of soil and climate. For heavy soils, the drains must be closer together. The bottoms of the trenches for the tile must have sufficient fall throughout their lengths to provide ready flow to the outlet.
In general, the land near a house should slope away from the house at a rate of approximately one inch per foot. As far as possible, the existing drainage relations of an area, such as the points of inflow and outflow, should be preserved.
Terraces and Banks
Terraces and banks may serve a variety of purposes, and may assume a variety of shapes and sizes. Terraces should be almost level, with a pitch of not less than 1 inch to each 10 feet. Grass banks should pitch not more than 1 foot vertically for every 4 feet horizontally. Such banks, especially if they are of a light, sandy variety, may need to be retained with roots of vegetation. The contours of the slopes should have a smooth-curving flow.
Levels and Slopes
The level, or nearly level, plane is most suitable for areas where people gather together, such as a terrace, or for areas and courts where physical exercise and games are pursued.
Any grade below 3 or 4 per cent approaches a level plane. This implies less than 3 feet or 4 feet in a vertical lane for every 100 feet of horizontal distance. Slopes of 4 to 10 per cent make walking and running difficult. Slopes that are above 10 per cent are steep and usually require steps for their utilization and treatment. A hill site for a home therefore offers complex problems, but these are often compensated for by the sense of space that comes from the extended views.
Convex and Concave Surfaces
Besides ground levels and slopes, there are the variations in ground that come into play with convex and concave surfaces. The treatment of such surfaces passing from one plane to another with modulated gradations that shade into each other provides the landscape architect with one of his most interesting problems in design. These gradations are important on embankments, where, instead of abruptly joining planes there may be a blending of natural forms.
The necessary shaping of the surface can often be determined by eye with the use of a line and stakes. But, on other jobs, the aid of leveling instruments may be required in staking out the plot.