The down and dirty on Soil

About this time 50 years ago, I was walking into a classroom at the University of Missouri. The class was Soils 101. This was a required course for all students in the School of Forestry. I wasn't too concerned about making a good grade. After all, we were only going to study dirt.

Within a few minutes the professor came into the classroom. He introduced himself. I thought something was up when I looked down at the big fat text book in my hand and realized the professor in front of me had the same name as the author. Maybe there was something more to dirt than what I had in mind.

The importance of soils was further ingrained in my brain by the first statement made by the professor that morning. "If any student in the class ever refers to soils as dirt, he (there were no girls in the class) will get an automatic F for the semester grade." Whoa, maybe this is a class that might be tough, but also the study of "soils" may be very important to me.

What is soil?

So what makes dirt a soil? You can think of dirt as what gets into the knees of your bluejeans when you kneel down in the garden to pull some weeds. Dirt is a combination of particles of maybe fine sand, loam or very fine clay. Soils on the other hand include not only dirt, but organic matter, fungi, insects, worms, roots and other biota. In a sense a soil contains living matter and it constantly evolves due to pressures of weather and how man manipulates it.

Paul Schnare shows a soil test kit that can be used to determine which soil amendments are needed to improve the physical properties of soil. Organic matter and either soil sulfer or lime are commonly used. (Fred Lynch)

Each component of a soil has a specific purpose. Briefly, sand consists of large particles of minerals that create space in the soil for air and moisture to move in and out. Both air (oxygen) and moisture are needed for plants and other biota to grow.

Clay on the other hand, is made up of small particles of minerals that are layered together like bricks in a wall. There is very little space between the particles. Plant nutrients such nitrogen, phosphorus and potassium are stored on the surface of these particles. These and other nutrients become available to plants when correct soil chemical conditions are met.

Silt is a soil particle intermediate in size with characteristics between clay and sand.

Advantage of loam

Probably the soil type most conducive to the broadest range of plant growth is a loam. This soil is comprised of equal parts of sand, silt and clay particles. The advantage of a loam is that it exhibits the best characteristics of each of the different soil particle sizes. It has a large water-holding capacity, along with a good nutrient-storage capacity. It usually has a lot of organic matter in it. A loam crumbles in your fingers.

Depending upon the soil in your garden, you can make additions to it to change the soil so that it has characteristics that are those most conducive for plant growth. The materials most commonly used as soil amendments (other than fertilizers) are organic matter (peat moss, compost, leaves, grass clippings) and either sulfur or lime.

Add organic matter to sandy soils to aid in moisture retention and nutrient storage. Organic matter added to clay soils will improve a soil's physical properties. That will result in a more porous soil that will allow moisture to move in and out of the soil more readily and allow plant roots to penetrate more easily.

Testing soil

The other soil characteristic that needs to be attended to is soil pH. Soil pH determines the availability of stored nutrients to plants for their use. Most plants prefer a soil pH of about 6.2 to 7.0. (Azaleas and rhododendrons are an exception to this rule. They prefer a pH of 4.0 to 5.0). You need to run a soil test to determine the existing pH of your soil. You can find soil test kits at your local garden center, or you can take a soil sample to your local university extension office and ask them to run the soil tests.

If your soil pH is lower than 6.2, amend the soil by adding lime to the soil. If the pH is above 7.0, apply soil sulfur to the soil.

The winter months are a good time to run soil samples. The results of these samples will allow you time to make the necessary amendments in order for you garden soil to be primed to produce the best results that can be had for the weather conditions that it will be exposed to during the 2014 growing season.