The Science of Soils

If you have plants that are stunted or just won’t grow, perhaps it’s time to take a closer look at
your soil. Garden soil is essentially made up of differing ratios of silt, clay and loam.

A soil texture triangle is used to classify the texture class of a soil. The sides of the soil texture triangle are scaled for the percentages of sand, silt, and clay. Clay percentages are read from left to right across the triangle (dashed lines). Silt is read from the upper right to lower left (light, dotted lines). Sand is from the lower right towards the upper left portion of the triangle (bold, solid lines). The boundaries of the soil texture classes are highlighted in blue. The intersection of the three sizes on the triangle will give the texture class. For instance, if you have a soil with 20% clay, 60% silt, and 20% sand it falls in the "silt loam" class.

Improving the structure of your soil is one of the most important aspects of soil care, and adding organic matter is the most effective way to accomplish this. Organic matter also helps maintain the pH balance of the soil and adds nutrients.

Good topsoil is:

Organic matter in the soil ensures a continuous food source for soil organisms. As the organisms decompose the organic materials, they help maintain good soil structure, making the soil a more favorable place for roots to grow. However, organic matter cannot be built up permanently in the soil because it continually decomposes and disappears; soil building must be a continual process in the garden.

The first step to improving your soil is to have your soil tested for pH and nutrient levels. Soil pH is a measurement of the alkalinity or acidity of soil. pH is measured on a scale of 1-14, with 7 as the neutral mark, anything below 7 considered acidic and anything above 7 considered alkaline. Knowing the pH will help tell you whether your soil is alkaline or acid because certain nutrients can only be accessed by plants when the soil pH falls into an acceptable range.

Most plants prefer a somewhat neutral pH, anything from 6.2 to 7.0. However there are many plants that are more specific in their pH needs, such as blueberries which like a very acidic soil and lilacs that prefer a more alkaline soil.

The soil test report will tell you whether your soil needs lime or nutrients or both. If the soil test recommends lime to make the soil less acidic, apply it by working the lime into the soil with a spade or a tiller to a depth of about 6 inches. Lime changes the pH balance of the soil, which is critical to absorption of nutrients by plants.

Homeowners can choose from four types of ground limestone products: pulverized, granular, pellet form and hydrated. Pulverized lime is finely ground. Granular and pellet form lime are less likely to clog when spread with a fertilizer spreader over turf areas. The finer the grind of the limestone the faster it will change the soil pH value. Hydrated lime should be used with caution since it has a greater ability to neutralize soil acidity than regular limestone.

Many ornamental plants and some fruit plants such as blueberries require slightly or strongly acid soil. These species develop iron chlorosis when grown in soils in the alkaline range. Iron chlorosis is often confused with nitrogen deficiency because the symptoms (a definite yellowing of the leaves) are similar. Iron chlorosis can be corrected by reducing the soil pH value.

Two materials commonly used for lowering the soil pH are aluminum sulfate and sulfur. These can be found at a garden supply center. Aluminum sulfate will change the soil pH instantly because the aluminum produces the acidity as soon as it dissolves in the soil. Sulfur, however, requires some time for the conversion to sulfuric acid with the aid of soil bacteria. The conversion rate of the sulfur is dependent on the fineness of the sulfur, the amount of soil moisture, soil temperature and the presence of the bacteria. Depending on these factors, the conversion rate of sulfur may be very slow and take several months if the conditions are not ideal. For this reason, most people use the aluminum sulfate.

Both materials should be worked into the soil after application to be most effective. If these materials are in contact with plant leaves as when applied to a lawn, they should be washed off the leaves immediately after application or a leaf burn may result. Take extreme care not to over-apply the aluminum sulfate or the sulfur.

To improve the soil structure, add compost. Yard waste such as shredded leaves, crop residues, straw, and similar materials should be tilled into the soil in the fall to allow decomposition through the fall and winter. At the same time, grass clippings, manure, or fertilizer should be incorporated to provide the extra nitrogen required to help break down the dry organic materials. Many extreme soil conditions can be corrected through the addition of organic matter.

For soil that is primarily clay or sand, or that has a hard surface crust (water runs off), or is compacted, incorporate 2 to 3 inches of organic matter in the top few inches of soil and mulch liberally for several years. The worms will pull the organic matter down below the soil line and will do your digging for you.

Remember if you bring in top soil, you’re not bringing in fertility or organic matter (the lifeblood of the garden) all you’re doing is bringing in material to raise the level of your soil line.

Do your garden a favour...feed the soil, not the plants.