Soil Air and Temperature Chapter 7

The above reaction can be split into a oxidation ½ reaction and a reduction ½ reaction. This concept is important to understanding soil aeration as shown a few slides later.

Above ground Below ground Thus, soil air is typically lower in oxygen but higher in carbon dioxide and water vapor than the above ground atmosphere. This is due to soil respiration and slow gas exchange. Some mass flow due to changes in pressure, too.

It occludes soil pores, slowing the already slow process of diffusion (and mass flow) through soil pores. Diffusion of gas in water is orders of magnitude slower than in air.

This is measured using a platinum electrode in conjunction with a reference electrode. The platinum electrode develops a potential that depends on the chemical environment in the soil, either tending to be oxidizing or reducing, i.e., relatively high or low potential. Biological activity in the soil largely controls this chemical environment.

This is the Nernst equation. It can be derived from free energy relations and says that the potential depends on the standard potential (reactants and products in their standard states) and the relative concentrations of oxidized and reduced forms of a redox couple, like iron (below). Clearly, if [oxidize] > [reduced] the Eh > E o and conversely. So, if larger values for Eh (which is measured by the Pt electrode) indicate an oxidizing environment and lower values, a reducing environment. Oxidizing can be equated with well-aerated and the opposite.

As far as microbial respiration goes, there are bugs that can couple the oxidation ½ reaction with a different reduction ½ reaction, one not involving oxygen as the terminal e - acceptor and get along just fine. This is their natural way of respirating.

This the ½ reaction for oxidation of oxygen in water (-2 state goes to 0 state). More O 2, higher Eh (see Nernst equation, below). Make sense?

This is Fe 3+ reduction, the Fe 3+ being in the solid form.

When oxygen is depleted by aerobic respiration, the aerobes shut down. But there are microbes that can use oxidized N in NO 3 - (nitrate) as The terminal e- acceptor in their respiration, reducing it to N 2 (or another reduced form). However, the supply of nitrate is finite, these guys use it up and then shut down. During their activity the chemical environment becomes more anaerobic and the Eh decreases. Similarly, there are bugs that can use Mn 4+ and Fe 3+ in their respiration. They become active once the Eh becomes sufficiently low for these ½ reactions to occur. The C reducers are active when the Eh is really low.

These conditions all favor development of anaerobic conditions. Clearly, high water content does because it impedes gas exchange with the above ground atmosphere. Small pores slow drainage, keeping a soil wet once it becomes so. Obviously, aeration is poorer deeper in the soil than near the surface. The effect of temperature is to increase biological activity. Thus, if the soil is wet, what oxygen that is there is more rapidly depleted when the soil is warm than cool, bringing on anaerobic conditions faster and quicker progression through the series of terminal e- acceptors, thus, more intensely reducing conditions.

With poor aeration, things go anaerobic and this kind of metabolism in the soil is adverse to plants for several reasons. Here are some.

As for the matter of nutrient availability, consumption of nitrate is undesirable because it is a major source of N for plant uptake. On the other hand, reduction of iron or manganese, both essential micronutrients, may in some cases be a bad thing even though neither are depleted. This is because both elements in their reduced forms are more soluble, potentially excessively soluble and available for plant uptake, causing toxicity. Too much of a good thing, you know.

You can drain land (provided it’s legal) or you can grow a variety that is perhaps more tolerant of wet soil conditions. We have recognized for quite some time now that wetlands are not wastelands but areas that are critical to the functioning of the overall environment.

More organic matter where wet due to slower decomposition where less aerated. Recall that the gley condition indicates presence of chemically reduced iron, Fe 2+. Mottling that indicates concentrations of oxidized iron, Fe 3+, in a matrix that is other- wise depleted in iron suggests alternating anaerobic and aerobic conditions. When the soil is wet, the more soluble Fe 2+ is produced and partially washed out of the profile and when dry, it oxidizes back to Fe 3+, pre- cipitating in concentrated zones. This condition supposes iron was initially present but re- duced, solubilized and washed out. Chroma 1 or 2, i.e., gray, or maybe bluish.

Neglect air because its specific heat is very small. Since water has a large specific heat (~ 1 cal g -1o C -1 ), increasing water content substantially increases specific heat. Clearly, a soil when wet warms more slowly than when dry.

It takes heat to evaporate water and this heat comes from the medium where water evaporates, tending to cool it.

This is a proportionality factor analogous to hydraulic conduc- tivity, Q = K T (T Hot – T Cold ) / L Where q is heat flux, T is temperature and L is distance. The greater the contact of soil solids, the greater the K T. Since water increases thermal contact increasing water content increases thermal conductivity. Air is a poor conductor.

This phenomenon is due to non-instantaneous heat flow, i.e., it takes time for heat to be conducted from relatively hot to cold.

The same thing is seen on an annual basis –less variation deeper in the soil and out-of-phase fluctu- ations.

So, a mulch keeps the soil cooler in summer and warmer in winter. You can adjust water content or use a mulch.