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A review has been undertaken into how soil organic matter affects a range of soil properties that are important for the productive capacity of the soils. The potential effect of varying the amount of soil organic matter in soil on a range of individual soil properties was investigated using a literature search of published information largely from Australia, but also including relevant information from overseas. The soil properties considered included aggregate stability, bulk density, water holding capacity, soil erodibility, soil thermal properties, soil colour, soil strength, compaction characteristics, friability, nutrient cycling, cation exchange capacity, soil acidity and buffering capacity, capacity to form ligands and complexes, salinity and the interaction of soil organic matter with soil biology. Overall this review concentrated on the soil physical properties and results from some of the field studies on soil organic matter and soil properties and so is intended to compliment the earlier review by Krull et al (2004) and another on the role of soil carbon in nutrient cycling (Macdonald and Baldock 2010). Some conclusions on the effects of soil organic matter on several soil properties were made. Soil organic matter had clear effects on water holding capacity, cation exchange capacity, aggregate stability and buffering capacity to acidification. Soil organic matter also had a definite effect on the compaction and strength characteristics of soils which in combination with friability can determine how the soil responds to traffic and tillage. Soil organic matter was an important factor in providing a nutrient supply and in nutrient cycling, especially of nitrogen, but also of significant proportions of phosphorus and sulphur and other micronutrients. The relative importance of soil organic matter to provide certain functions in soils varied with texture, with soil organic matter generally being more critical in soils with lower clay contents. Different soil organic fractions had different properties and varied in their importance for different soil properties. While data was generally lacking, it appears that the humus fraction is more important for the functions requiring chemical activity such as cation exchange capacity and pH buffering capacity and the particulate organic carbon is more important for aggregate stability of the larger aggregates, the rapid recycling of nutrients and as a food and energy source for the soil microbial population. The level of soil organic matter and the levels of certain factions can influence the biological activity and microorganisms in the soil. The development of new methodologies such as the measurement of microbial biomass, substrate analysis, phospholipid analysis, DNA analysis and enzyme evaluation has given improved insight into many of the biological processes in soils. This has shown that land management and even different plants or crops can change the biological populations. However, a more complete understanding of the overall effect some of these changes can have on overall productivity and nutrient cycling requires a higher level of understanding then is currently available. There were some values for the levels of soil organic carbon required to maintain the soil properties in a functional form and the value of 2% soil organic carbon occurred a number of times as a useful guide. One important conclusion is that by increasing soil organic matter it is possible to improve several soil properties simultaneously and so have a cumulative effect on productivity. A final part of the review was to consider the capacity of the dry land agricultural systems to achieve levels of soil organic matter that could improve soil properties. This was done using soil carbon data from across Australia from a range of sources including the National Soil Carbon Project. In general there was a percentage of soils that had soil organic carbon levels sufficiently low as to affect soil properties. These also seemed to occur in areas where available land management practices could increase the levels of soil organic carbon above the measured levels and so there is a potential to improve productivity in these areas by increasing levels of soil organic carbon. A large proportion of the soils also had higher levels of soil organic carbon which although not ideal for soil properties, could be more difficult to increase under standard land management systems. A small percentage of soils had high levels and probably indicate what is possible under the more specialised land management systems. One conclusion was that for many soils, increases in soil organic matter have the capacity to strongly influence only the surface soils, perhaps only the top 10 cms and the top 20 cms at most. This limits the capacity of soil organic matter to influence soil productivity. Even so the top 10 to 20 cm is a critical zone for the soil. It is the interface where seeds are sown, germinate and emerge. It is where a large proportion of plant materials are added to the soil for decomposition and the recycling of nutrients and where the rainfall either enters the soil or runs off. So the potential to improve soil condition in the top 10 to 20 cm is still critical for plant productivity. A deficiency in the studies on the effects of soil organic matter on soil properties and productivity is the need to consider the impact of soil organic matter across a wide range of seasons and soil types. The means to address this deficiency is perhaps a program of applying biophysical models using the known effects of soil organic matter on the soil properties.
