Writing The Future

Soil is an arrangement of minerals, biological material and empty space, or pore space. Pore space is important in air and water exchange between the atmosphere and soil, which affects root growth, water movement, and water storage. As stated in lecture, an ideal soil with good structure will be about 50% total pore space by volume and will be filled with both air and water (50/50 ideally). Soil texture (sand, silt and clay fractions) and structure control the amount of pore space in a soil. Organic matter is also important because it helps provide better structure by aggregation. You already know the texture of your soil and soon you will know the porosity, which, together will tell you an enormous amount about it’s aeration, water-holding capacity and suitability for plant root exploration.

As we do things to alter the pore space, the bulk density changes. For instance, the compaction of a soil with heavy equipment would reduce pore space and increase bulk density; adding organic matter will increase pore space, causing bulk density to decrease. The practical part is that when bulk densities reach about 1.5 g per cubic cm root growth starts to become limited; and at bulk densities of 1.8 g per cubic cm or greater, root growth stops.

Here’s a brief math refresher. When we refer to grams (whether megagrams, kilograms, micrograms, etc.) we are talking about mass, ie weight of the soil solids .

When we talk about cubic centimeters (cm³) we are talking about volume—how much space it takes up. The calculation for Bulk density is simple: