Soil Properties

Soil, like the world above it, abounds with plant and animal life. As everywhere in our living world, the key ingredients for the recipe are carbon, nitrogen, phosphorus, and water. Clearly soil and its properties are not the same everywhere. We all know this because we have experienced even in our backyards, how some soils pool water while other soils drain water as fast as we apply it. And if you think about it for a bit you probably have observed that some soils are red while other soils are as black as tar, that some soils produce plants that grow as tall as trees, and other soils grow nothing but stunted weeds. Why then are there such differences between soils?

Partly it is because of source material, partly because of how and where a soil is formed, and partly because of what happened to the soil after it was formed. A soil formed in a dry landscape with extremes of heat and cold will differ significantly from one formed from sediments deposited in the ocean. This in turn will differ from one formed in the depression of base rock from material deposited by water, such as snow or ice melt from glaciers. Each of these soils has different base materials and climates in which they formed. And to throw in one other factor, the level of the water table will play a role in determining the biotic component in the soil and can either inhibit or nurture plant growth and chemical processes.

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What are the important soil properties that make them a keystone for environmental processes? Soil texture, perhaps the most obvious of soil properties, is the proportion of different sized mineral materials (clay, sand, and silt). The size of soil particles affects the infiltration of water. As was discussed earlier, the size of a particle determines the size of the pores between particles, which in turn determines how rapidly water moves through the soil. The larger the pore is, the more rapid the downward movement of water. Soil texture establishes the capacity of the soil to retain moisture, as well as the nutrients present in water. Finally, soil texture, along with soil carbon, governs the stability (reflected in its structure as explained later) of soil aggregates(soil particles or clumps), and the propensity of a soil to erode.

This soil structure–its clumpiness–depends upon the site-specific conditions under which the soil formed. Soil structure is the grouping or arrangement of soil particles (Brady, 1974). Soil aggregates (cohesive groups of particles) are important for water infiltration into soils. The more that soil particles are aggregated, the larger the pores within the soil. To verify that increasing pore size increases water infiltration, conduct the following experiment.

Experiment 4.2:Remember our experiment in Chapter 2 where we poured water on clay and sand. You could modify that experiment and pore a gallon or liter of water on the ground and watch how much of that water flows downhill. Do this on sandy and clay soils and observe how much stays on the surface and how much infiltrates. It depends upon the soil’s properties, particularly pore size which is a function of soil texture and soil structure.

The stability of these cohesive soil particles is an important characteristic of the soil structure. If the aggregates break down easily, then the soil structure will not be maintained. The colloidal components of soil are the finer fractions of the soil that are the most chemically active. These materials when placed in water stay in suspension in solution or settles very slowly if at all. Soilcolloidsplay an important role in both binding soil particles together into aggregates and stabilizing them.

We need to take a little side trip here to talk aboutcolloidal particles. Colloidal particles in soils are either clay or humus. The particles are extremely small (less than 1.0 micron in size), have a large surface area per unit weight, and have surface charges that attract ions (atoms with an electric charge such as calcium (Ca⁺) or magnesium (Mg⁺⁺), or charged molecules like ammonium (NH4+) or phosphate (PO43−)) and water (Goldberg et al., 2012). For these reasons, they influence soil structure and soil chemical properties. The ability of these particles to attract and hold ions make them critical intermediaries in plant – nutrient interactions because the surface charges of colloidal particles function as the site of the carbon, nitrogen, and phosphorus chemical reactions and exchange. And, they are important for soil structure.‘Because of their surface charges, colloidal

particles also act as a ‘contact bridge’ between larger particles, helping them to maintain stable granular structure. The surface charges on colloidal particles make them the center of soil chemical activity and nutrient exchange.’ (Brady, 1974) What does that mean? It means these particles serve as the stage for some of the most important activity occurring in soils, including soil aggregation, nutrient retention and exchange, and water storage. The colloidal particles are where the carbon, nitrogen, and phosphorus action takes place and the gears of the cycles mesh.

Experiment 4.3:To observe colloidal particles put a tablespoon of the top layer of deep black soil into a glass of water. See what happens. Wait a day, the water in the glass will not be clear. It is the colloidal particles that are discoloring the water.

Soils with more colloidal material (clay and organic matter) have more stable clumps of soil. Because these colloidal particles are small enough to interact with other particles at the molecular level and because larger particles do not form these bonds, they can develop chemical bonds with other soil components to form larger clumps. These clumps (or aggregates) with high components of clay particles are generally more stable than aggregates in soils high in sand (An et al., 2010; Brady, 1974; Skidmore and Layton, 1992). Soils with the right amounts of colloidal particles are good for growing things, including crops. They can hold and store more water and the nutrients in water. And they enable roots to grow, benefiting plant development and health.

Soil organic matter, which contains much of the biologically available carbon, nitrogen, and phosphorus, has been discussed numerous times before. It generally comprises 0.5–10% of a soil, but can approach zero in highly eroded and arid soils (Brady, 1974; Haynes, 2005). It can be much higher in wetland soils.

Organic matter is made up of three components: small (fresh) plant residues and insects and microorganisms, decomposing (active) organic matter, and stable organic matter.

The portion of the organic matter of soils that is far less easy to breakdown (labile) is called humus. It does not turn over year to year (Natural Resource Conservation Service, 2020c). It is formed through the decomposition of larger organic material and can be broken down or destroyed more rapidly than clay (Brady, 1974). Humus is directly involved in developing and maintaining soil aggregates. Humus is less dense, is made up of generally smaller particles, and has a substantially higher number of binding sites for charged molecules like rN and P than clay. Its substantial colloidal component provides the glue that helps bind together soil particles, contributing to aggregate stability. Because humus slowly decomposes from constant interaction with the environment, continued deposition and decomposition of organic material is important to maintain the humus that is in the soil. As will be explained in the next section, its small size,

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lighter weight, and position close to the soil surface cause humus to be more easily removed through erosion.