How do soils develop? Why are they disappearing? What does this mean for agriculture?
Soils do not just appear out of thin air. They were once large bodies of rock, formed under heat and pressure in the lithosphere. As uplift brought them towards the surface, the stable environment they once knew of high pressure, high temperature, and low oxygen, was replaced by variable tectonic stresses, quick temperature changes and low pressure. The minerals in the rock each reacted differentially in this new environment, expanding at different rates and creating microfractures and other weaknesses. These processes, which cause the once monolithic rock to weaken and break into smaller units, are called mechanical weathering. As rain and groundwater flow over and through the rock, they bring dissolved carbon dioxide (from decomposed surface plants) in the form of carbonic acid. It seeps into rock fractures and deposits protons that displace cations. This weakens the chemical structure of the rock, causing the changed rock to fall apart into secondary minerals. Mechanical decomposition increases the total surface area available for the acid to act on, hastening this process of chemical weathering. The type of soil developed depends on the type of chemical weathering that occurs, and the composition of the bedrock acted upon.
Soil is more than an abiotic exercise in entropy. As the decomposed rock, or saprolite, reaches the surface, it is exposed to the biosphere. Animals burrow and dig, while plants grow roots to exploit the weaknesses that mechanical and chemical weathering introduced. This is bioturbation breaking the saprolite up. As these organisms die, they add carbon and nutrients back into the soil. This is soil as we know it: a relatively small-grained, hydraulically conductive mixture of minerals and biotics that can store nutrients and water in a manner that allows primary producers to access them.
The decomposition of plants and other organic matter releases complex soluble organic molecules into the environment. Some of these molecules have chemical structures that interact favorably with metals, facilitating the creation of molecules that combine carbon and metals in a process known as chelation. These molecules can generally interact with organisms. This is beneficial when it involves metals such as manganese and iron, since many organisms require them to function. Chelation is bad for organisms when it mobilizes metals such as arsenic or mercury, which interact with cells in toxic ways when attached to these soluble organic molecules.
The rock that comprises the Amazon Basin has been there for 10 million years. It is low and flat, and the sediment in the basin does not travel far as a result. It has been decomposing into saprolite for quite some time, in a hot climate with high precipitation rates and an abundance of organic matter. This climate facilitates speedy chemical reactions, causing organic matter to decompose quickly, releasing a high amount of carbon dioxide. This does not leave a large window of time for chelating molecules to store necessary nutrients, as they are merely one step in the organic decomposition process. Instead, the carbon dioxide leaches minerals out of soils by the process described earlier, rinsing the cations that would make a soil fertile out of the underlying saprolite. The plants that exist on these soils are locked in a self-preserving cycle -- they die and contribute their nutrients back to the soil. But if you were to cut down these plants and remove them, trying to grow new plants in their stead, you would find a thin topsoil that cannot sustain crops.
Since bioturbation has “fluffed up” the saprolite into soil, there has been an increase in the material’s porosity. This means that there is more room between grains to store water. Plant roots exert osmotic pressure on the soil to absorb the water present in these pores. Unfortunately, the strength of plants is not limitless. The surface area of soil particles interacts with the water through hydrogen bonding, creating a force that holds water in the soil pores. This is a capillary force. The plants can only absorb water until their best effort is exceeded by the capillary force. The larger the pores in the soil, the smaller the relative capillary force, allowing the plant to extract a higher proportion of water from the soil.
There is an abiotic mechanism that keeps cations from simply being rinsed through the soil by precipitation or groundwater. These positively charged ions are attracted to the surfaces of negatively charged clays and other secondary minerals in the soils. As such, they are bound up and remain localized. Plants have a mechanism to release cations from these soil-storage reservoirs. Plant roots can acidify their environments, effectively donating positively charged protons to displace the cations on the clays. This frees up the cations that the plants require to be absorbed by the plant root.