Sustainable Soil Organic Matter Options Land Management 34 “Soil organic matter is a of productivity and to minimise the Soil organic matter is a dynamic substance and a process. It is negative environmental impacts of mixture that reflects the balance a mixture of materials – plant farming activities. between additions of new organic matter and losses of organic matter and animal remains and What is soil organic matter? already in the soil. Soil organic products of decay processes matter exists in various forms that have been going on for Soil organic matter is the fraction which differ in their biodegradability months or years.” of the soil that includes: plant and or resistance to decomposition. It is F. E. Broadbent animal residues at various stages generally divided into three pools: of decomposition; plant roots; cells active, intermediate or slow, and Introduction and tissues of soil organisms; recalcitrant or resistant. The active and substances synthesised pool includes microbial biomass The biggest difference between by the soil population. Fresh or and labile organic compounds that soil and rock is the presence of undecomposed plant materials make up less than 5 percent of the organic matter and the associated such as litter and straw and animal soil organic carbon. The slow pool biological activity that takes place dung lying on the soil surface usually makes up 20 to 40 percent in the former. Soil organic matter are not included in this definition These three pools have different is at the heart of healthy and although upon decomposition, rates of turnover. The active pool productive soils. Although organic these will eventually become part ranges from months to years, the matter typically makes up a small of the soil organic matter. Organic slow pool in decades, and the percentage of most mineral soils matter is largely made up of recalcitrant pool in hundreds to (less than 10 percent by weight), the elements carbon, hydrogen, thousands of years. it is very important because it oxygen, and nitrogen and also positively influences or modifies the phosphorus and sulphur. It is The active and part of the effects of almost all soil properties. expressed as a percentage intermediate pools are involved in Understanding the role of organic of the soil mass less than 2 mm nutrient supply and in the binding matter in maintaining a healthy soil in diameter. of small soil particles together is essential to ensure a high level to form larger structural units called aggregates. Aggregation is important for water infiltration, The formation of soil organic matter aeration and drainage, and reduces the soil’s susceptibility to erosion. On the other hand, the recalcitrant pool or humus possesses a large quantity of negative charges and contributes largely to the nutrient- holding capacity (cation exchange Cow dung capacity) of the soil. It also imparts a dark colour to topsoil. Soil rich in organic matter Why soil organic matter matters? Grass Organic matter is critical for Partially decomposed maintaining soil health. The level of organic matter is highest in the topsoil and, with few exceptions, Soil which has little declines dramatically with depth. organic matter New Zealand farmers regard Litter the top 6 inches as their most productive soil. Topsoil is therefore Decomposition of plant residues and animal manure added to the soil produces organic matter that is a precious resource primarily due beneficial to soil health and plant growth to the high level of organic matter 34 Land Management Soil Organic Matter and nutrients it contains relative to subsurface soil. Once depleted, organic matter takes many years to replace. With this in mind, adding plant and animal residues and minimising topsoil loss through erosion and other forms of soil disturbance should be an important goal of every land manager. Organic matter serves several functions, the most important of which are as a soil conditioner and as a source of plant nutrients. Organic matter adds body to sandy soils and increases its moisture- and nutrient-holding capacity. It promotes granulation in clayey Topsoil is a precious resource primarily due to the high level of organic matter and nutrients it soils which helps in plant root penetration and entry of water of sequestration of carbon from lots of negative charges. These and air into the soil. It makes the atmosphere. This is currently negative charges contribute to the cultivation easier, resulting in better an active area of research nutrient retention capacity of soils seedbeds, and reduces surface by scientists concerned with by attracting positively charged crusting that can adversely affect increasing the carbon content of ions (cations) in soil such as the emergence of seedlings. The soils to mitigate the adverse effects calcium, magnesium, potassium, functions of soil organic matter are of climate change and to potentially ammonium,etc. These would broadly categorised into biological, utilise it in emission trading otherwise leach and get lost in the physical and chemical but they schemes. soil profile. It should be noted that actually overlap and interact with negatively charged ions (anions) each other. Physical such as nitrate and sulphate are Biological One of the major effects of organic not held by the negative charges Organic matter is a food and matter is to improve soil structure. of organic matter and are therefore energy source for soil organisms Plant roots, earthworms, bacteria, subject to leaching loss. and a source of plant nutrients. fungi and other micro-organisms Organic matter acts as a chemical Organic matter decomposition release organic compounds which buffer by resisting rapid change is a microbiological process help bind soil particles together to in pH. On the positive side, this which releases inorganic forms produce stable aggregates. This mechanism delays soil acidification of nutrients such as nitrogen (N) improves aeration and increases particularly in soils subjected to phosphorus (P) and sulphur (S) permeability which in turn makes long-term fertilisation by urea and which slowly becomes available the soil less susceptible to erosion. ammonium-containing fertilisers. for plant use. The humus which Stable aggregates also resist On the negative side, one may develops as an intermediate compaction caused by ploughing, need to apply larger quantities of product of this decomposition and vehicle and animal traffic. liming material in order to raise the also acts as a store for nutrients. Organic substances have been pH of an already acidic soil to a Soil organic carbon is generally shown to hold up to five times desirable level. highly correlated with total their own weight of water. This nitrogen. Therefore, the amount of Organic matter complexes cations, contributes to improving the particularly micronutrient cations N mineralisation (i.e. conversion available water-holding capacity of organic N compounds to such as iron and zinc, through of soil, making growing plants less a process known as chelation. ammonium-N) increases as soil prone to short-term droughts. organic carbon increases. The formation of iron-organic The dark colour imparted by complexes makes the insoluble Organic matter provides active organic matter in the topsoil iron more available for plant use. absorption sites for the deactivation increases the absorption of solar of organic chemicals such as radiation. This may facilitate pesticides, particularly herbicides. How is soil organic soil warming in spring and matter measured? Micro-organisms associated with consequently seed germination soil organic matter may also rapidly and plant growth. In the laboratory, organic matter decompose soil-applied organic is actually measured as total Chemical chemicals. carbon by high temperature Adding organic matter to the Organic matter possesses a combustion analysers which raise soil contributes a certain level high surface area and contains the soil sample to a temperature Sustainable Options Land Management 34 Soil Organic Matter of about 1,300°C and the total Soil organic matter serves several functions carbon is measured as carbon dioxide. Total carbon is expressed as a percentage of the soil weight. Physical Biological The measurement includes both Stabilises soil structure Source of energy for organic and inorganic (carbonates) biological processes carbon. However, since most New Improves soil water retention Supplies plant nutrients Zealand soils contain very little (N, P, S) inorganic carbon, total carbon is Improves drainage and aeration Deactivates organic a good measure of organic matter chemicals (e.g. herbicides) content. To obtain the percentage Reduces susceptibility Contributes to of organic matter, total carbon is of soil to erosion Functions sequestering multiplied by a conversion factor Imparts dark of soil carbon from the of 1.72. This assumes that organic colour to soil organic atmosphere matter is 58 percent carbon. In matter reality, the carbon content of organic matter varies and so it is better not to convert and just report the measured total carbon as is. Chemical Ratings of total carbon for New Zealand soils are as follows: Contributes negative charges to reduce leaching of nutrient cations (Ca++, Mg++, K+, NH4+) Level Total carbon (%) Resists rapid change in soil pH Very low Less than 2 (delays soil acidification) Complexes cations Low 2-4 Medium 4-10 High 10-20 What factors influence decomposing organic matter. The Very high Above 20 organic matter levels optimum soil temperature