The biggest difference between soil and rock is the presence of organic matter and the associated biological activity that takes place in the former. Soil organic matter is at the heart of healthy and productive soils.
Organic matter matters for maintaining soil health
The level of organic matter is highest in topsoil and with a few exceptions, it may decline dramatically with depth. New Zealand farmers and viticulturists respectively, regard the top 6 inches as their most productive soil. Topsoil is, therefore, a precious resource primarily due to the high level of organic matter and nutrients it contains relative to subsurface soil.
Once depleted, organic matter can take years to replace. With this in mind, adding natural residues and minimising topsoil loss through erosion and other forms of soil disturbance should be an important goal for every land manager.
Organic matter serves several functions, most importantly as an all-around fertiliser and soil conditioner and as a source of plant nutrients. Organic matter adds body to sandy soils and increases moisture and nutrient holding capacity. It promotes granulation in clay soils which helps with plant root penetration and maximises the ability for water and air to easily flow through the soil. It makes cultivation easier, resulting in better seedbeds, and reduces surface crusting that can affect new seedling growth.
The function of soil organic matter is broadly categorised into biological, physical, and chemical but they actually overlap and interact with one another.
Biological functions of organic matter
Organic matter is a food and energy source for soil organisms and a source of plant nutrients. Organic matter decomposition is a microbiological process that releases inorganic forms of nutrients such as nitrogen (N), phosphorus (P) and sulphur (S) which slowly become available for plant use.
The humus that develops as an intermediate product of these decompositions also acts as a store for nutrients. Soil organic carbon is generally highly correlated with total nitrogen. Therefore, the amount of N mineralisation (i.e. conversion of organic N compounds to ammonium-N) increases as soil organic carbon increases.
Organic matter provides active absorption sites for the deactivation of organic chemicals such as pesticides, particularly herbicides. Micro-organisms associated with soil organic matter may also rapidly decompose soil-applied organic chemicals.
Adding organic matter to the soil contributes a certain level of sequestration of carbon from the atmosphere. This is currently an active area of research by scientists concerned with increasing the carbon content of soils to mitigate the adverse effects of climate change and to potentially utilise it in emission trading schemes.
Physical functions of organic matter
One of the major effects of organic matter is to improve soil structure. Plant roots, earthworms, bacteria, fungi and other micro-organisms release organic compounds which help bind soil particles together to produce stable aggregates. this improves aeration and increases permeability which in turn makes the soil less susceptible to erosion. Stable aggregates also resist compaction caused by ploughing, and vehicle and animal traffic.
Organic substances have been shown to hold up to five times their own weight of water. This contributes to improving the available water-holding capacity of the soil, making growing plants less prone to short term droughts.
The dark colour imparted by organic matter in the topsoil increases the absorption of solar radiation. This may facilitate soil warming in spring and consequently seed germination and plant growth.
Chemical functions of organic matter
Organic matter possesses a high surface area and contains an abundance of negative charges. These negative charges contribute to the nutrient retention capacity of soils by attracting positively charged ions (cations) in the soil such as calcium, magnesium, potassium, ammonium and others. These would otherwise leach and get lost in the soil profile. It should be noted that negatively charged ions (anions) such as nitrate and sulphate are not held by negative charges or organic matter and are therefore subject to leaching loss.
Organic matter acts as a chemical buffer by resisting rapid changes in pH. On the positive side, this mechanism delays soil acidification, particularly in soils subjected to long-term fertilisation by urea and ammonium-containing fertilisers. On the negative side, one may need to apply larger quantities of liming material in order to raise the pH of an already acidic soil to a desirable level.
Organic matter complexes cations, particularly micronutrient cations such as iron and zinc, through a process known as chelation. The formation of iron-organic complexes makes insoluble iron more available for plant use.
