University of Illinois Extension
Soil - Site Assessment - Stepping Stones to Perennial Garden Design - University of Illinois Extension

Soil

Soil is another important factor to assess before planting your garden. In fact many argue it is more important than sunlight. Regardless of where your opinion lies, checking and amending existing soil is so much easier before you plant than after you plant.

Soil is essential for plant growth for several reasons. Soil provides plant roots with nutrients, water and air. Soil also serves to anchor the plant. There are different types of soil, and it is very common to find the foundation soil around your house to differ from your property line soil.

Soil is comprised of three mineral particle sizes: sand, silt and clay. Large, course mineral particles are sand, fine mineral particles are silt, and very fine mineral particles are clay. An ideal soil is composed of 45 percent mineral (sand, silt and clay), five percent organic material (humus or plant debris and soil organisms), 25 percent water and 25 percent air. The composition of the mineral portion determines the soil type. Soil type influences the availability of air, water and nutrients to the plant. Sandy soil is a loose soil, made up of larger particles creating more air spaces and thus has very little moisture-retentive abilities. Not only is water unavailable but neither are nutrients that have to be in a liquid form for the roots to take them up. Silt soils have less air space and clay soils offer the least. These soils have the ability to hold water, but often it can be too much, eliminating air that is required by roots for nutrient and gas exchange.

Ideally, equal amounts of sand, silt and clay particles are desired. Soils with all three particles present in significant amounts are called loam. To find out what type of soil you are dealing with, there are local soil testing labs that will tell you. They will also test your soil pH and various nutrient levels. Ask for their home and garden test. If not, your analysis will be per acre rather than per 1,000 square feet.

Perennials will not thrive in wet soils. Good drainage is important. Many plant guides list a moist, well-drained soil as desirable. What does this mean? A moist, well-drained soil has both air and water present. Soil particles will hold water and any excess drains away, leaving air spaces. You can feel moisture in the soil, but it is not saturated. We all have seen, heard and smelled water logged soil - a heavy shovel full sputters like a wet sponge. Recognizing this problem before planting allows you to amend the soil and alter this condition.

Soil Particles
Clay Silt and Sand Comparison

Think of clay as the size of a pea, silt the size of a grapefruit, and sand the size of a basketball.

Clay Versus Sand Comparison

Fill a bathtub up with peas and water. Pull the plug. The water drains out very slowly due to the small spaces between the peas. There are forces holding that water in place. This is the same thing that happens in a clay soil.

Fill that same bathtub with basketballs and water. Pull the plug...and whoosh! The water drains quickly. There is nothing there to hold on to the excess water. This is what happens in a sandy soil.

Diagram of Loam

What you want is a mixture of peas, grapefruits and basketballs, or clay, silt and sand. This is called loam. You have small spaces to hold water and larger spaces to drain excess away. This creates a moisture retentive yet well-drained soil.

It is important that you evaluate your soil and accept the fact that it cannot be greatly altered - only modified. It took millenniums to form and one generation of gardening is not going to drastically alter it.

A sandy soil is a loose soil with little moisture retentiveness. A clay soil is tight, with small air spaces that hold moisture longer resulting in poor drainage. Organic matter will help improve both soil types. It fills the air spaces in sandy soils increasing moisture retentiveness by giving moisture something to adhere to. Organic matter loosens clay soils, opening up air pockets to increase drainage. Incorporating aged manure from any grain-eating animal such as horse or cow, or compost into the upper four to six inches of the soil is recommended.