By Ron Alexander

Clay is one of the three primary mineral components of soil, along with silt and sand. Clay soils contain a significant amount of clay particles, but also contain silt and sand particles at varying amounts, as well as some organic matter. Clay particles are the smallest of the mineral soil components. The US Geological Survey defines them “as materials having a particle size of less than two micrometers.”

Because clay particles are so small (fine in texture), clay soils possess primarily small pore spacing, which can make water and air movement difficult, and often leads to soil compaction. Soils containing a larger percentage of clay particles tend to be:

  • Sticky when wet
  • Dense – easy to compact
  • Difficult to moisten (hydrophobic) when dry, and
  • Highly erodible

Most plants do not thrive in clay soils, especially when they are compacted, as air and water movement – key to root growth – is compromised. Further, plants expel more energy to move their roots downward through a clay soil profile, than they do with loamy or sandy soils, which contain larger pore spaces. Although clay soils are considered by many to be difficult to work with, clay particles are an important component of good soil, as long as it is not its dominant component. Clay soils are good at holding water and nutrients as they possess a high cation exchange capacity. While nutrients tend to migrate (leach) out of coarse, sandy soil, clay soils hold onto them. Because of this characteristic, clay soils require less frequent fertilization than coarser soil types.

As a soil textural class, clay refers to soil material that is 40 percent or more clay, less than 45 percent sand, and less than 40 percent silt. Individual rock or mineral fragments in a soil that range from 0.05 to 2.0 millimeters in diameter.

Intuitively, many would think to add sand to clay-based soils to improve them. However, the organic matter found in soil amendments like Bloom can be more effective and economical as a solution. Remember that the goal is to improve soil structure to create larger pore spacing for air and water movement. Plant roots can also move more easily through soils possessing larger pore spacing. Through the addition of organic matter, the aggregation of smaller soil particles occurs, creating larger grouped particles. These ‘water stable’ particles can act more like sand particles in the soil, as larger pore spaces are formed around them, improving water and air movement. Note that while improved pore spacing can occur by physically placing organic matter particles in between clay particles, the biological activity that comes along with it (which creates protein-based soil glues), stabilizes those particles into larger groups. Clay soil particles tend to pack tightly together, rather than form aggregates. Notice how 2 Bloom products in Table 1 affect clay-based soils, improving pore spacing and therefore assisting in water infiltration (hydraulic conductivity).

Soil particle size comparison

Clay particles are about 1,000 times smaller than sand particles (A). Clays have many forms. For example, kaolinite has book-like leaves (B)

Product – BloomWater Holding Capacity
(mls/100g dry)
Hydraulic Conductivity
100% Clay soil, 0% Product


90% Clay soil, 10% Product24.85.6
80% Clay soil, 20% Product31.722
Product – Woody Blend
100% Silt/Clay soil, 0% Product37.63.2
90% Silt/Clay soil, 10% Product39.75
80% Silt/Clay soil, 20% Product45.222

Improving Clay Soils

In areas that are to be planted, apply organic matter-based soil amendments at an application rate of 10 to 30% by volume and incorporate them into the soil surface. Bloom should be applied at 10 to 20% by volume and both the Bloom Woody or Sandy Bloom blends at 20 to 30% by volume, based on the soil conditions and plants to be established. Where establishing turf, addition organic matter may be applied to clay soils through core aerification and topdressing, while in planting beds, organic matter can be applied at planting every couple of years (or as necessary). Remember, when incorporating soil amendments into clay soils, do not overtill, which can destroy soil structure, and do not incorporate in exceedingly wet or dry conditions, which also hurts soil structure. Even though Bloom and other soil amendments can reduce soil compaction in clay soils, the issue must be addressed on an ongoing basis for best results.