What is Soil Study Guide (page 2)

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Updated on Sep 26, 2011

Soil as Recycling Compartment for the Land

Because the decomposed products of life make soil so much of what it is, without life, there is no soil. With out life, the forces of weathering that reduce rocks to particles would wash those particles away. Without life and thus soil, every sunny day would be a drought, every rain a flash flood. Without life, particles of soil would quickly wash or blow away, to aggregate in valleys of deep sediment graveyards or tumble into the sea. Much of the continents would be as bare as today's fresh volcanic fields.

We will discuss how the soil's life is essential, not only in creating soil, but in functioning as a recycling process that allows plant life to flourish. But first, we will review some of the properties of soil that make it conducive for living things. In other words, what is a healthy soil?

A healthy soil strikes a balance between water infiltration and water retention (also called water holding). If the soil is hard clay, for instance, rain water will not be able to infiltrate it and will run off the surface to be wasted. Water needs to be able to move into the soil to become stored as the reservoir of soil moisture that plants can draw upon during days or weeks when no rain occurs. Thus, infiltration is crucial. The degree of infiltration is normally determined by the amount of sand in soil, which creates large pores for water to move downward.

On the other hand, a soil can be made of too much sand. Beaches, for example, do not make a productive soil. With too much sand, the water simply runs downward as gravitational water, goes into the groundwater system, and is lost to plants. Thus, a healthy soil will retain water as capillary water stored in microscopic spaces and on the mineral grains of the soil (already discussed). Clay and silt are good at retaining water; so is the organic humus.

A healthy soil also needs to allow enough air to circulate within it, as well as between it and the atmosphere. Aeration is obviously high in sandy soils and is inhibited by clay soils. Why is air needed? First of all, the roots of plants are just like us; they need to breathe air to get oxygen for their cells. Without a fresh supply of air from the atmosphere into the soil, plants can die. This is why it can be bad to overwater house plants. It's not the water directly that can kill the plants, but rather the fact that the water is preventing their roots from getting the needed air.

Except for the kinds of bacteria that thrive in the absence of oxygen (called anaerobic bacteria), all organisms in the soil must also breathe air for their oxygen. Air moves around in the soil because a healthy soil is porous. The porosity extends right up to the surface, from the A to O horizons, and allows air to move from the atmosphere into the soil, as well as from the soil into the atmosphere.

Also, the roots of plants and organisms in the soil give off carbon dioxide gas as a waste product from their metabolisms, the same metabolisms that use oxy gen. The waste carbon dioxide enters the pores of the soil and eventually moves up and out of the soil into the atmosphere. As a result of the breathing of soil organisms, the concentration of carbon dioxide is much higher in the soil than it is in the atmosphere.

A healthy soil also has a large capacity to hold nutrients. To some extent, this property is related to water retention because the nutrients that plants use are in the form of dissolved ions in the water. But a nutrient-holding capacity is also related to the kinds of particles in the soil. Not enough nutrients can be held by the water alone, and the water can sometimes be used up or fall to minimal levels during droughts. Nutrients are also retained on soil particles, and the best kind of particle for nutrient retention is clay.

Organisms are also crucial for a healthy, natural soil. Previously, we saw that countless numbers of tiny nematode worms inhabit soils, as well as other creatures such as ants, beetles, and earthworms. Another crucial type of organism is the fungi, whose microscopic white threads decompose organic materials within the soil. When it is time for some kinds of fungi to reproduce, they make mushrooms.

The soil is an entire ecosystem. Creatures run around (millipedes, centipedes, spring tails) and creep along (slugs, snails). Some of the soil's inhabitants are single-celled protozoans, seen under a microscope. Even smaller, visible only at a microscope's highest power, are the most important inhabitants of all, the bacteria.

Soil would not be soil without its bacteria. Although all the soil creatures participate in the break down of plant debris from the O horizon of litter, the bacteria are the ones that perform the greatest part of the final step in decomposition and return the elements in the organic debris into forms available again for the plant's roots to take in for the next round of growth. Bacteria are thus key to the recycling function of soil.

We will next see how the activity of bacteria determines the amount of organic material in the soil. Bacteria, which digest and therefore break down organic matter (say, from fallen leaves), are more active at high temperatures and less active at cooler temperatures. The climate of a region, then, can affect the activity of bacteria and thus the amount of organic matter in the soil. Very cold climates tend to have thick soils with a high content of organic matter. Famous for this are the peats of northern Canada and Siberia, in which the activity of bacteria is extremely slow.

At the other end of the climate spectrum are the tropics. What are tropical soils like? You might think they would be thick, given the abundance of vegetation and growth in the tropics, but no. Tropical soils, despite the rich vegetation, tend to be thin with low amounts of organic matter, because the breakdown (decomposition) is so rapid. Bacteria in the tropics digest at a high rate and keep the organic contents of the tropical soils low. That is why the tropical soils are often not very good for agriculture, after being used just a few years following deforestation.

The main role for bacteria is to recycle elements from the dead vegetation debris of the O horizon into dissolved ions in the soil water, because these ions are then available to plants for uptake and more growth. It is an important point that the processes of physical and chemical weathering are too slow and their products too little to supply the plants with the nutrients the plants need. Plants thus depend upon recycled nutrients.

An example of the importance of recycling can be seen from numbers of that key element for plants, phosphorus. Let us consider phosphorus requirements, summed over all the plants across all the continents. That need is about 40 times higher than the flux of phosphorus supplied to the soil from the break down of minerals by weathering. That means that when a plant takes in phosphorus through its roots, 39 parts out of 40 (close to 98%) is, on average, recycled phosphorus. This recycled phosphorus is, for the most part, derived from the actions of the soil bacteria.

A final property of a healthy soil that is related to the human use of soil for agriculture is what is called workability. Can the soil be plowed easily? Does it have all the properties of an overall healthy soil that can support intensive cultivation? Can it hold water, be naturally aerated, and retain nutrients?

Traditional agricultural techniques can create a loss of soil by erosion. If the A horizon (the topsoil) is lost, productivity plummets. Plowing also creates some soil loss by exposing loose particles to the eroding effects of wind and water. Plowing can also increase the activity of bacteria and thereby reduce the organic matter of the soil. In fact, on average, cultivated soils have about 25% less organic matter than they did before they were cultivated.

Solutions to the long-term maintenance of healthy soils for human use include better cultivation techniques. One promising technique that is rapidly growing in adoption is no-till agriculture. In no-till, the entire farm soil is not plowed. Instead, only a narrow strip where the seeds will be planted is worked by machinery precisely guided down the same path year after year.

Other advances are coming in irrigation, to attempt to use the minimum amount of water at precisely the time when the water is required. The precision application of fertilizers is also under way.

Practice problems of this concept can be found at: What is Soil Practice Questions

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