Soil Organic Matter

Organic matter is widely regarded as a vital component of a healthy soil. It is an important part of soil physical, chemical and biological fertility. This information examines what soil organic matter consists of and how it can contribute to soil fertility. It also discusses how soil management can affect organic matter concentrations in the long term, and presents evidence that addresses the question "how much organic matter does my soil need?"

What is organic matter?

In its broadest sense, soil organic matter comprises all living soil organisms and all the remains of previous living organisms in their various degrees of decomposition. The living organisms can be animals, plants or micro-organisms, and can range in size from small animals to single cell bacteria only a few microns long.

Non-living organic matter can be considered to exist in four distinct pools:

• Organic matter dissolved in soil water
• Particulate organic matter ranging from recently added plant and animal debris to partially decomposed material less than 50 microns in size, but all with an identifiable cell structure. Particulate organic matter can constitute from a few percent up to 25% of the total organic matter in a soil
• Humus which comprises both organic molecules of identifiable structure like proteins and cellulose, and molecules with no identifiable structure (humic and fulvic acids and humin) but which have reactive regions which allow the molecule to bond with other mineral and organic soil components . These molecules are moderate to large in size (molecular weights of 20,000 - 100,000). Humus usually represents the largest pool of soil organic matter, comprising over 50% of the total
• Inert organic matter or charcoal derived from the burning of plants. Can be up to 10% of the total soil organic matter

When plant and animal debris is added to soil, it is broken down by macro- and micro-organisms, initially into particulate organic matter, and finally into humus. The raw materials can vary greatly in their resistance to breakdown. Woody organic substances like lignins are very resistant, while more simple compounds like sugars are readily utilised. Along the way, microbial populations increase. In the process they synthesise their own compounds which add to the diversity. In turn, these organisms die and are consumed by others.

Carbon dioxide is a by-product of this complex chain of processes (microbes breathe out CO₂ just like we do!). Over half of the carbon added to soil is lost as CO₂ during breakdown. Because of their varying reactivity, the turnover times for these different carbon fractions varies from a few months to tens of thousands of years.

Measuring soil organic matter

While living organisms, particularly the plants we grow, are of vital importance to us, it is the non-living organic matter that we measure as 'soil organic matter'.

The most common methods for measuring soil organic matter in current use actually measure the amount of carbon in the soil. This is done by oxidising the carbon and measuring either the amount of oxidant used (wet oxidation, usually using dichromate) or the CO₂ given off in the process (combustion method with specific detection).

Laboratories these days generally report results as soil organic carbon. Those that report as soil organic matter have usually measured carbon and converted to organic matter by multiplying by 1.72. However, this conversion factor is not the same for all soils, and it is more precise to report soil carbon rather than organic matter.

The amount of organic matter in soil

The amount of carbon (the measure of organic matter) in a soil depends on a range of factors, and reflects the balance between accumulation and breakdown. The main factors are:

• Climate - For similar soils under similar management, carbon is greater in areas of higher rainfall, and lower in areas of higher temperature. The rate of decomposition doubles for every 8 or 9^oC increase in mean annual temperature. Tasmanian agricultural areas have a mean annual temperature of 11-13^oC.
• Soil type - Clay helps protect organic matter from breakdown, either by binding organic matter strongly or by forming a physical barrier which limits microbial access. Clay soils in the same area under similar management will tend to retain more carbon than sandy soils. Hence the sandy sodosols of the northern midlands have less carbon than the clay loam ferrosols of the north west regardless of management (Table 1).
• Vegetative growth - The more vegetative production the greater are the inputs of carbon. Also, the more woody this vegetation is (greater C:N ratio), the slower it will breakdown. So, the crop system can strongly affect carbon concentrations.
• Topography - Soils at the bottom of slopes generally have higher carbon because these areas are generally wetter and have higher clay contents. Poorly drained areas have much slower rates of carbon breakdown.
• Tillage - Tillage will increase carbon breakdown. However, the impact of tillage is generally outweighed by the effect of management on the amount of carbon grown and returned to the soil. An exception to this is where tillage leads to increased erosion.

Table 1. Organic carbon concentrations % in Tasmanian soils subject to different management

Benefits of organic matter

Organic matter can be considered a pivotal component of the soil because of its role in physical, chemical and biological processes (Table 2). Many of these functions interact. For example, the high cation exchange properties of organic matter are a major means by which organic matter is able to bind soil particles together in a more stable structure. The reactive regions present in humus are numerous, and give these molecules a capacity to bind to each other and to mineral soil particles, and also to react with cations (positive charge, e.g. Ca2+, K+) in the soil solution.

The density of cation exchange capacity (CEC) of organic matter is greater than it is for clay minerals (Table 3). While a high CEC is an important attribute of soil organic matter, please note that organic matter does not have an anion (negative) exchange capacity, and is therefore not able to bind anions like phosphate and sulphate. However, organic matter is a substantial reservoir for phosphorus and sulphur, as well as nitrogen. These elements are bound within the organic structure, and are released to the soil solution when microbes break down organic matter.

The ratio of carbon:nitrogen:sulphur:phosphorus in organic matter is roughly 100:10:1.5:1.5. A hectare of soil 10 cm deep with a bulk density of 1 tonne/m3 weighs 1,000,000 kg. Therefore, soil with a carbon content of 3% would contain 3,000 kg of organic nitrogen, and 450 kg each of organic phosphorus and sulphur per hectare. Not all of this is mineralised each year, but there is considerable potential for nutrients in organic matter to contribute to plant requirements. These should be taken into account, particularly the nitrogen.

Table 2. Functions of soil organic matter

Table 3. Cation exchange capacity of different soil particles

Research in Tasmania over the last five years has shown strong associations between soil carbon and a range of soil physical, chemical and biological properties in all the main soil types (Table 4).

Table 4. Soil properties which have been strongly associated with soil carbon in Tasmanian soils