So, after week two of my class, an overarching theme has appeared: Surface area per unit soil volume is important.
Source
Soil Particles
Let’s start with the basics. There are three general types of soil particles: sand, silt, and clay. The ratios of their relative diameters and specific surface areas (surface area divided by volume) are listed below.
| Particle |
Relative Diameter |
Specific Surface Area |
| Sand |
10,000x |
1x |
| Silt |
100x |
100x |
| Clay |
1x |
100,000x |
While sand is the particle with the largest diameter, it has the smallest surface area per unit volume. This means that for a fixed volume, a soil composed of all sand particles will have a smaller total surface area than a soil composed entirely of clay particles.
So, why is this important? Well, in the post last week we learned that plant roots need three things: water, nutrients, and air. Both water and nutrients are held on the surface of soil particles, and the air is found in the spaces between the particles. A soil’s composition and particle size directly affects the amount of water, nutrients, and air in the soil. The surface area also affects the internal drainage, runoff potential, and how herbicide is applied.
Aggregates
If the smaller soil particles have better nutrient and water holding capacities but not a lot of space between them for air, and the larger particles have more space between them, but not as good nutrient and water holding capacities, how can we have the best of both worlds? If your first though was like mine, you answered compromise and choose a medium size particle and would have been wrong. The answer is soil aggregates.
A soil aggregate is a group soil particles that are associated with each other in more or less stable packets (clumps) of soil. If each grain of sugar is a soil particle, then a sugar cube is an aggregate. Aggregates allow for the benefit of a high surface area (from all their smaller components) while allowing the space for air that a larger particle would have. A well aggregated clay soil, with aggregates the size of sand particles will have more water holding capacity, more nutrient holding capacity, and the same amount of aeration as a sand soil of the same volume. Organic matter acts as the “glue” that makes the aggregates possible, which is the real reason why it is so important to add to soils (Until this class I thought it was for the nutrients).
Okay, well that concludes this week’s soils lesson/summary of what I’ve learned. Stay tuned next week for hopefully more (potentially on aeration and drainage of soils).
