Soil texture determines how easy it is for plant roots to access the water and nutrients they need to survive and thrive. Soil texture also determines a soil’s ability to hold on to water and nutrients.
Soil texture is a measurement of the relative proportions of the sand, silt, and clay minerals found in a soil sample. Each of these words actually refers to the particle size of these soil separates, and not to any particular chemical property:
To put these numbers into perspective, imagine that a dime is a particle of clay. If that were true, silt would be about the size of a softball and a grain of sand would be the size of a bicycle wheel. Of course, there are no bicycle wheel sized grains of sand, but they can be seen with the naked eye. Silt particles can be seen with a standard, classroom microscope. To actually see clay particles, you would need to use an electron microscope, they are so small! Also, while most soil particles tend to be spherical, clay forms plates that generate up to 100,000 times more surface area. These means that there are many more potential points of attachment for nearby nutrients and water molecules.
Soil texture classifications
In the U.S., we use 12 different classifications to talk about soil texture. Each class has specific characteristics based on its feel, its tendency to crumble, and its likelihood of leaving a ‘stain’ behind on your fingers when handled:
You may also hear someone talk about soil that is fine textured or coarse textured. Finely textured soil contains more clay, while coarsely textured soil holds more sand. Soil texture is not the same thing as soil structure. Soil structure refers to the way minerals, air, water, microbes, and everything else is arranged into clumps, called aggregates.
Soil texture and drainage
The large and small spaces between these particles are called macropores and micropores, respectively. These spaces are what allow water, air, nutrients, and roots to move through the soil. In most cases, those spaces are similar in size to the particles they are around. Since water molecules tend to stick better to smaller particles, such as clay, and the micropores are smaller, clay has a far higher nutrient and water holding capacity than loam or sand. Clay is also more prone to drainage and compaction problems. Clay is also harder for roots (and shovels) to move through. The larger macropores and bigger particle size of sandy soils make leaching and nutrient loss more common problems. Erosion is another factor to consider when it comes to soil texture. Heavy sand and sticky clay are less likely to blow or wash away, while loam is more vulnerable to erosion.
Soil texture and nutrient retention
The nutrients used by plants are ions of specific elements. Ions are atoms or molecules that have either a positive or negative charge, due to the addition or loss of an electron. Cations, such as calcium and potassium, are positively charged and attracted to negatively charged soil particles. Anions, such as phosphorus and sulfur, are just the opposite. The ability of a soil to hold onto a positively charged mineral ion is known as its cation exchange capacity. Organic material and clay tend to have negative charges, while water and sand tend to be positively charged.
How to identify your soil’s texture
Use these steps to determine your soil’s texture:
For example, a 10” sample has 1” sand (10%), 7” silt (70%) and 2” (20%) clay:
Creating better soil texture
Ideally, your soil will be 50% macropores and micropores (containing 50% air and 50% water), 2 to 5% organic material (more is better), and 45 to 48% minerals. In a perfect growing medium, those minerals would be 40% sand, 40% silt, and 20% clay.
Rather than trying potentially disastrous quick fixes* to improve your soil’s texture, adding small amounts of organic material, such as aged compost, over a long period of time, will improve your soil’s ability to retain water and nutrients, while still allowing air, water, and roots the spaces they need to move freely.
* The most common quick fix for soil compaction, adding sand to clay, actually makes the problem worse, because the tiny clay particles fill the spaces between the sand particles. [Sand + clay = cement]
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