1.2: How is Soil Made?

Before we consider what makes a soil rich or poor, we should learn how it comes into existence. Soil consists of four parts: solid mineral particles, water, air and organic matter. The particles are generally of sand, silt and clay size (and sometimes also larger fragments) and were derived from weathering of rocks or deposition of sediments. They mainly consist of silicon, oxygen, aluminum, potassium, calcium, magnesium, phosphorus, potassium and other minor chemical elements. But these elements are generally locked up in the crystalline particles and are not directly available to plants. However, unlike solid rock, soil particles have pore spaces in between them that allow them to hold water through capillary action: the soil can act like a sponge. This is an important process because it allows the soil water, with the help of carbon dioxide in the air, to very slowly dissolve the mineral particles and release nutrients—we call this chemical weathering. The soil water and dissolved nutrients, together referred to as the soil solution, are now available for plants. The air in the soil, which is in contact with the air above ground, provides roots with oxygen and helps remove excess carbon dioxide from respiring root cells.

What role do plants and soil organisms play? They facilitate the cycling of organic matter and of the nutrients, which allows soil to continue supporting life. Plants’ leaves capture solar energy and atmospheric carbon from carbon dioxide (CO₂) through photosynthesis. The plant uses this carbon to build the sugars, starches and all the other organic chemicals it needs to live and reproduce. At the same time, plant roots absorb both soil water and the dissolved nutrients (nitrogen is added to soils or directly to plants through associated biological processes). Now, the mineral nutrients that were derived from the soil are stored in the plant biomass in organic form in combination with the carbon from the atmosphere. The seeds tend to be especially high in nutrients, but the stems and leaves also contain important elements. Eventually plants die and their leaves and stems return to the soil surface. Sometimes plants don’t return directly to the soil surface, but rather are eaten by animals. These animals extract nutrients and energy for themselves and then defecate what remains. Soil organisms help to incorporate both manure and plant residues into the soil, while the roots that die, of course, are already in the soil. This dead plant material and manure become a feast for a wide variety of organisms—beetles, spiders, worms, fungi, bacteria, etc.—that in turn benefit from the energy and nutrients the plants had previously stored in their biomass. At the same time, the decomposition of organic material makes nutrients available again to plants, now completing the cycle.

But is it a perfect cycle? Not quite, because it has not evolved to function under intensive agricultural production. The chemical weathering process that adds new nutrients into the cycle continues at a very slow pace. On the other end of the cycle the soil captures some of the organic matter and puts it “in storage.” This happens because soil mineral particles, especially clays, form bonds with the organic molecules and thereby protect them from further decomposition by soil organisms. In addition, organic matter particles inside soil aggregates are protected from decomposition. Over a long time, the soil builds up a considerable reservoir of nutrients from slowly decomposing minerals and carbon, and of energy from plant residue in the form of organic matter—similar to putting a small amount of money into a retirement account each month. This organic matter storage system is especially impressive with prairie and steppe soils in temperate regions (places like the central United States, Argentina and Ukraine) because natural grasslands have deep roots and high organic matter turnover (Figure 1.1).

Figure 1.1. Soils build a storage reservoir of carbon and nutrients in organic matter, and can also hold water and air. The organic matter builds up from decayed plant material and accumulates mostly in the dark root zone under the surface. Photo by USDA-NRCS.

In a natural system this process is quite efficient and has little nutrient leakage. It maximizes the use of mineral nutrients and solar energy until the soil has reached its maximum capacity to store organic matter (more about this in Chapter 3). But when lands were first developed for agriculture, plowing was used to suppress weeds and to prepare the soil for planting grain crops. Plowing was also beneficial because it accelerated organic matter decomposition and released more nutrients than unplowed land. This was a major rift in organic matter cycling, because it caused more organic matter to be lost each year than was returned to the soil. In addition, a related rift occurred in nutrient cycling as some of the nutrients were harvested as part of the crop, removed from the fields and never returned. Other nutrients were washed out of the soil. Over time, the organic matter bank account that had slowly built up under natural vegetation was being drawn down.

However, until organic matter became seriously depleted, its increased decomposition through tillage helped to supply crops with released nutrients and these rifts did not cause widespread concern. On sloping lands these losses went much faster because the organic matter near the surface also eroded away after the soil was exposed to rain and wind. Only in the past century did we find effective ways to replenish the lost nutrients by applying fertilizers that are derived from geologic deposits or the Haber-Bosch process for producing nitrogen fertilizers. But the need to replace the organic matter (carbon) was mostly ignored until recently. The organic matter in the soil is more complex and plays many important roles in soils that we will discuss in Chapter 2. Not only does it store and supply nutrients and energy for organisms, it also helps form aggregates when mineral and organic particles clump together. When it is made up of large amounts of different-sized aggregates, the soil contains more spaces for storing water and allowing gas exchange, as oxygen enters for use by plant roots and by soil organisms and the carbon dioxide produced by organisms leaves the soil. So in summary, the mineral particles and pore spaces form the basic structure of the soil, but the organic matter is mostly what makes it fertile.