Topography

The phrase “lay of the land” refers to the site’s topography—how steep it is, which way it faces, whether it’s in a river bottom or on a hilltop. The topography affects the soil, water drainage, air drainage, and the site temperature.

Topography and Soil

Hilltop soil is usually shallower than valley soil; bottom land (valleys and along creeks and rivers) probably has deep and fertile soil. Hillside soil is apt to be very shallow—steep hillsides often have exposed bedrock.

Over time, erosion has washed soil from hilltops and slopes and accumulated it on bottom land. Much bottom land is alluvial—it was deposited by a creek or river. Alluvial soil is usually very deep and rich.

Topography and Water Drainage

Sloping land drains well because gravity pulls the water through the soil. The steeper the land, the better the drainage. This drainage can cause erosion problems if the slope is too steep. It may be necessary to install terraces and retaining walls to slow the flow of surface water.

Water moves through the ground by seeping through the soil. If the soil is uniform, it seeps evenly through all the soil. However, most soil is not uniform—some places offer less resistance to the flow of water than others. These places are called aquifers. An aquifer is an underground zone that remains saturated with water. If a hole is dug into an aquifer, the hole fills with water. A well is a hole dug into an aquifer.

Some aquifers emerge on the surface of the land on hillsides and at the bottoms of slopes. Springs are formed as water flows from the aquifer at this point.

One type of drainage problem occurs when an aquifer is close to the surface of the soil. If it’s within a few feet of the surface, the soil remains wet or boggy all the time. This problem is usually referred to as a “high water table.” It can only be solved with drain lines, or by putting in a bog garden in that location. High water tables are usually encountered in bottom land, close to creeks, ponds, or rivers.

Topography and Air Drainage

In the evening, the cooling earth cools the air in contact with it. This cooler air flows downhill just as water does, accumulating in low spots. These low spots often form frost pockets, places where frost is more severe than surrounding area. If you are gardening in a frost pocket, your beans might freeze weeks earlier than those of your neighbors on the slope above you. Gently sloping hillsides avoid frost longer than either low spots or level land.

This flowing cold air acts much like flowing water; it accumulates on the uphill side of an obstacle, then flows around the obstacle, often bypassing the downhill side. The uphill side of a hillside house may have later frost in the spring and earlier frost in the fall than the downhill side.

Topography and Sun Direction

The compass direction a slope faces influences how much it heats during the day. North of the equator, gardens on the south or west side of a hill face more directly toward the sun and absorb more heat during the day. Gardens that face north or east absorb less heat, and are likely to be cooler and moister than their neighbors on the other side of the hill.