Activity – Water Stable Aggregate Test (Slake Test

Supplies

• Clear plastic or glass jar(s) filled with irrigation water

• Wire mesh (hardware cloth) or window screen formed into a basket to suspend from the rim of the jar, with its bottom about 2 inches into the water

• A small shovel or other tool for collecting your sample

Instructions

1. Collect several soil clods or aggregates and air dry them for at least 1 week. Depending on initial moisture and soil texture, drying may take up to 1 month.

• You may want to collect clods or aggregates from various soil types or soils under differing management regimes to compare them.

2. Set up jars with your irrigation water.

• Note that the use of irrigation water differs from the NRCS protocol, which calls for distilled water. Using irrigation water will give a better idea of how the soil will behave when irrigated. If you don’t have irrigation water available, distilled water is fine. Be sure to record what kind of water you have used.

3. Place one or more soil clods or aggregates into the wire basket in each of your jars.

Make sure they are fully submerged.

4. Observe the soil clods or aggregates over the next five mnutes - what percent of the clod remains intact?

What did you observe?

Interpretation

What was the topography like from where you collected the sample? What was the soil depth?

What was the vegetation in that area? What has the management been in that area? These are just some examples of things to think about, observe, and compare. Based on what you know about your property, can you come up with some of your own?

Pore Space

Glossary

Pore Space

part of the soil that is either air or water.

Pore space is affected by the amount of clay and organic matter a soil contains, which are key in aggregating solid soil particles and thus making space for air or water. There are two types of pore space in soils, and every soil likely has a mix of both.

Macropores

are generally greater than 0.08 mm in diameter. They are related to increasing soil organic matter and aggregation, as well as to the activity of soil organisms and roots. They can also form between sand grains, which gives sandy soil its well-drained and aerated property.

• Increasing macropores leads to increased water and air movement. Micropores

are generally less than 0.08 mm in diameter. They are largely formed in clayey soils and are characterized by slow air and water movement.

• Micropores are usually filled with water, but this water is not available to plants because it is held too tightly.

• And they are too small to allow much air movement.

When thinking about pore space, it is important to consider not only the total collective amount of pore space throughout a soil, but also the size of individual pore spaces. Keep in mind the different behaviors of water and air in macropores compared with their behaviors in micropores.

Discussion – Pore Size Distribution

What management activities might cause a relative decrease in macropores and a relative increase in micropores?

Bulk density

Bulk density is an important characteristic to consider when gauging the ease or difficulty with which water will flow through the soil or plant roots will grow into the soil.

The volume of the cube shown here includes pore space and solids (mineral and organic matter components). The mass of the soil solids and air would be used to calculate the soil mass.

Glossary

Bulk Density

describes the dry mass of both solids and pores (everything but the water) in a given volume of soil. It is often expressed in grams per centimeter cubed (g/cm3).

Bulk density is affected by:

• Texture (proportions of sand, silt, and clay particles)

• Structure (aggregation and pore space)

• Depth in the soil profile

• Management