The Online Gardener's Handbook 2010
Chapter 3: A Word about Soil
The Nature of Soil

Table of Contents

  1. Minerals and Soil Texture
  2. Soil Structure
  3. Organic Matter
  4. Clay Colloids
  5. Soil Fertility
  6. Soil pH
  7. Soluble Salts
  8. Learn More

Minerals and Soil Texture

The mineral component of a soil is made up of clay, silt and sand, often referred to as fine, medium and coarse size particles respectively. The percentage of each of these determines the texture of a soil.

  • Clay soils contain a larger percentage of clay (fine) particles.
  • Loam soils contain a balanced proportion of sand, silt and clay particles.
  • Sandy soils contain mostly sand (coarse) particles.

Soil texture is difficult to change. It is a basic soil property. However, if yours is a heavy clay garden soil, you can slowly alter its texture by adding sand and organic matter such as compost, manure or peat moss over successive seasons.

Soil Structure

Soil structure refers to the way soil particles cling to each other in clumps or aggregates. A well-structured soil contains small aggregates and is easy to work. Each aggregate holds water and nutrients, and the air between the aggregates allows excess water to drain away.

Poor soil structure reduces root growth of seedlings, making them more susceptible to drought. It also results in soil compaction after rain or when walked on, and this leads to poor water penetration and a lack of oxygen. Plants that grow in such soil are smaller, less vigorous, more prone to drought and lower yielding than those grown in healthier soil.

Good soil crumbles when forked or spaded. If a soil breaks into blocky clods with flat surfaces and sharp corners, soil structure is poor. You can improve your soil's structure by adding organic material such as peat moss, manure or compost. It is also a good idea to seed a garden with grass if it has been under cultivation for several years and the soil breaks into clods. Grass has an extensive root system that grows throughout the top 15 cm layer of soil and breaks up the clods. The roots also release substances that promote the bonding of smaller aggregates. In addition, grass roots, shoots and clippings provide plenty of organic matter to improve soil structure. To achieve the best results, a garden should remain seeded for two to three years.

Organic Matter

Matter in soil is mainly decayed plant material. In most soils, it accounts for up to 10% of the mixture. Soils with 17% or more of their weight as organic matter are called organic soils. Generally, the darker the soil, the higher the organic matter content.

Organic matter plays an important role in soil structure. It acts as a cementing agent to bind soil particles together. It is also the most chemically active component of soil and is a source of plant nutrients.

The surface of organic matter contains exchange sites that attract and hold plant nutrients. As nutrients are removed from the soil by plant roots, they are replenished by nutrients from these exchange sites. When fertilizer is applied, the exchange sites are saturated, creating a reservoir of nutrients for future use.

A garden that is cultivated year after year without the regular addition of compost or manure soon begins to lose organic matter. Over time, the soil becomes tight and compacts very easily. The quality of plant growth is reduced.

Clay Colloids

Like organic matter, clay colloids act as reservoirs of plant nutrients. The glue like minerals (colloid means glue) hold onto nutrients and release them into the soil as plant roots take up nutrients and water. A productive soil, therefore, should contain a balance of organic matter and clay.

Soil Fertility

Soil fertility is an important part of good lawn and garden management. Good fertility promotes plant growth, reduces the susceptibility of plants to insect and disease damage, improves drought tolerance and reduces weed populations in lawns. Usually, fertility problems are easily corrected. However to ensure successful gardening, it is also important to:

  1. select the most suitable plant varieties;
  2. control insects, diseases and weeds; and
    3. provide adequate water and light

Soil pH

Soil pH is a measure of the acidity or alkalinity of a soil. A pH of 7.0 is neutral. A pH below 7.0 is acidic and above 7.0 is alkaline. Soil pH has an important effect on the availability of many nutrients. As the soil becomes more basic (high pH), the availability of nutrients such as iron and manganese is reduced. Most plants grow well in soils with pH values of 6.0 to 7.5. However, some plants such as Rhododendron, Azalea, heathers, blueberries, and pin oak require acid soils and only grow well below pH 5.5. In most of Southern Ontario, soils are neutral to alkaline, though there are areas where soils are too acidic for optimum growth of most plants. Many soils on the Canadian Shield are low in pH.

You can get a general idea of your soil's pH by using pH strip papers that change colour to reflect pH levels. A low pH condition can be corrected by adding agricultural dolomitic limestone to the soil. Because of the chemical nature of high alkaline soils, however, it is very difficult to do the opposite and lower a high pH condition. Success can be achieved on medium and course textured soils with the addition of sulphur. If your soil has a high pH, and you wish to grow ericaceous plants and blueberries, it is best to build raised beds or to excavate the existing soil and replace it with a mixture of peat moss, composted bark or oak leaves and sandy topsoil. Sulphur should be incorporated into the soil mix and added yearly to keep the pH low.

Soluble Salts

Soluble salts are dissolved mineral ions in the soil. Some occur naturally, some come from fertilizers and some are the result of the decomposition of manure, mushroom compost or similar organic matter. Ontario soils are naturally low in soluble salts. Where the level of soluble salts is high, there have usually been excessive applications of fertilizers, manure or mushroom compost.

High concentrations of soluble salts can affect seed germination and plant growth. When soluble salts increase, water absorption by plant roots decreases, because the water is held in the soil with greater force. When salt levels are high, plants suffer abiotic drought.

Wilting of plants during the brightest and warmest times of day even though the soil may be moist is usually the first symptom of a high salt content. Plant growth will also be reduced, and leaves often appear small and dark green. As salt levels continue to increase, the tips and edges of leaves burn, and the root system is damaged. Plant growth is affected most under higher temperatures and low moisture periods. Damage from soluble salts does not include roadside salt spray damage to trees and shrubs.

Some plants such as begonias, impatiens, cucumbers and lettuce, are more sensitive to soluble salts than others. Seedlings are more sensitive than older plants, and will die if soluble salts are excessive.

Soluble salts can be measured in the laboratory by measuring the electrical conductivity of a soil-water slurry. Table 7 provides an interpretation of soil conductivity readings in a 2:1 water:soil paste, the procedure recommended by the OMAFRA accredited soil testing programme.

Table 7. Soil Conductivity (soluble salts) Interpretation

Conductivity mS/cm Rating Plant Response
0-0.25 Low Suitable for most plants if recommended fertilizers are used
0.26-0.45 Medium Same as above
0.46-0.70 High May reduce emergence and cause slight to severe damage to salt sensitive plants
0.71-1.0 Excessive May prevent emergence and cause slight to severe damage to most plants

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