Because gardeners are in a hurry to plant seed in the Spring, we sometimes overlook the importance of first examining the soil we are going to work with. Soils vary considerably from location to location and even so within a location. For example, on a single farm it is often the case that said property will contain soils of varying textures and productivity. Even though most vegetable gardeners have a small area to work with, and most likely the plot will be uniform for soil type, it still remains important to become familiar with your garden’s soil. This topic will address soil texture which can have a profound affect plant productivity.
A single definition of soil is being borrowed from a USDA publication located on the Internet as follows:
soil – (i) The unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants.
Going a step further, a soil used in agriculture is said to be made up of the following constituents; Porosity (air) – (25%), Water holding capacity – (25%), Minerals – (45%) and Organic matter – (5%). The organic fraction is typically comprised of Humus – (80%), Roots – (10%) and Organisms – (10%). However, please use this as a guide only since many arable soils have 1 to 3% organic matter content thus reducing their water holding capacity below 25%.
Within the soil’s aforementioned “mineral fraction” there are three classes of soil particulate; sand, silt and clay. Sand is the largest and clay the smallest in diameter. The soil’s textural classification is based upon the quantity of each type of particulate. For instance, a soil containing 15% sand, 50% clay% and 35% silt is classified as a clay soil. This was determined by plotting the respective percent particulate content with the use of a soil textural triangle (Soil Conservation Service, USDA, Soil Survey Manual, Agricultural Handbook, No. 18 (1951). However, there is good news ahead, when you perform a soil test with a University Extension Laboratory or private environmental lab you will typically receive analyses of macro and micro soil nutrients, pH, CEC (Cation Exchange Capacity), organic matter (%) and a soil texture classification.
OK, so what’s the big deal about soil texture and what can be done about it should you want to make improvements to your garden’s soil? For instance, if you learn that your soil is 50% or greater in clay content you may be wise to add generous amounts of well-decomposed organic matter in addition to building raised beds. Adding some excellent quality topsoil* would prove useful as well. Working with a clay soil is possible but for the less experienced gardener it can prove to be disappointing. A clay soil is tuff to dig, turn and/or hoe especially when it’s wet or dry. Additionally, clay soils typically do not have good soil structure thus reducing good porosity and water relations between soil and developing root systems. On the opposite side of the soil textural scheme, having a soil high in sand content also offers challenges to support a highly productive garden. For example, sand has poor water holding capacity and will retain less plant nutrients to its colloidal surface.
In my opinion, soils with the following textural classifications; clay loam, sandy clay loam and silt clay loam are ideal garden soils. These three soil types have a nice mixture of sand, silt and clay making them easy to till and generally have good retention of nutrients and excellent water holding capacity.
*Caution, avoid adding sand to your clay soil as this actually worsens the situation by creating concrete like structures in your clay soil.
There’s a lot more to be said on soil texture, so, if you have interest to dig deeper, I recommend that you locate a few used textbooks and/or search the Internet. Good luck!