Not all soil is equal.
Dozens of factors, like colour, mineral content, salinity, and pH level, all affect the productivity of soils and create unique agricultural challenges for farmers and gardeners around the world. One particularly useful characteristic used to classify soil is texture. Identifying soil texture gives valuable information about how to condition the soil for different types of crops.
When profiling soil texture, the key determining characteristic is particle size. Sand particles are the largest, ranging from 0.05 – 2mm. Silt particles are between 0.002 – 0.05, while clay particles are nearly microscopic at less than 0.002 mm.
Particle size has direct effects on the water infiltration and retention of soil. The large particles in sandy soil are porous and drain well but don’t retain water. Conversely, the tiny particles in clay soil trap water between them preserving moisture —this is why wet clay is moldable. However, soil with too much clay will be very dense and easily waterlogged which limits root growth in plants.
Most soils are a mixture of particle sizes and can be categorized into one of a dozen different types. Texture is determined as a measure of the percentage of sand, silt and clay in the soil. This is illustrated by what is called the “Soil Texture Triangle”:
While any soil texture in the center of the triangle is suitable for agriculture, most farmable soils consist of a blend of particles sizes called loam. Loam is a rich, black soil which contains a balance of sand, silt and clay. However, not all loam is created equal; higher quantities of one particle size will affect the characteristics of the soil.
For examples, loam with 25-40% clay is considered a clay loam. Clay loam is generally preferred by plants with shallow roots as the compact soil structure provides a strong anchor for root systems. Clay particles are a crucial element of productive soils by providing two basic functions in the soil. The tiny particle size traps moisture which can develop into a capillary-like network for distributing water through the soil. Additionally, clay particles attract and capture both micro and macro nutrients found in the soil through a process called cation (pronounced ‘cat’ + ‘ion’) exchange. While this process involves the rather complex exchange of elements on a molecular level, we can think of it as the process by which loose nutrients in the soil are attracted to clay particles and stored in a manner that is accessible to the root system. However, soil with too much clay can become compacted which stifles root growth and reduce oxygen in the soil.
Alternatively, a sandy loam contains roughly 60% sand, 30% silt and 10% clay. This type of soil is preferred by plants that like semi-dry conditions. The loose soil structure means root vegetables like carrots, radish and beets thrive in sandy loam as the root can easily penetrate the soil which allows for larger bulb growth. However, due to the low levels of clay in the soil, sandy loam often leaches nutrients and will require fertilization to maintain productivity.
In large scale production agriculture, information about soil texture is used to help decide yield potential. As noted by Trent Hilderman of Prairie Son Acres, “Soil texture is the basis to all yield potential per field. It directly relates to nutrient availability, water holding capacity, cation exchange, etc.”.
More directly, soil texture is also used to help determine fertilizer rates. For example, if two fields, one sandy loam and one clay loam, both have a yield potential of 60 bushels per acre (bu/ac), a farmer may choose to apply 60 bu/ac of equivalent fertilizer that would be used to the sandy loam soil but only 50 bu/ac for the clay loam with the expectation that a 10 bu/ac will become available in the soil.
This is echoed by Hilderman who states, “[You] can count on some nutrients being mineralized or more available in certain soils, so [you] can adjust accordingly”.
Ultimately, soil texture is just one of many characteristics used to classify soil. However, particle size is one of the most basic and straightforward metrics used to identify soil type, which, in turn, can help determine the best farming practices for a specific plot of land.