Keywords: Soil organic matter, additions and losses, equilibrium values, farming systems, benefits to yield, soil structure, nutrient availability.
Perceptions about the importance and role of organic matter in relation to soil fertility are not new and that there is an important relationship is a long-held belief among farmers. Maintaining soil fertility is crucial to maintaining sustainable agricultural systems but identifying and quantifying factors that contribute to beneficial effects is not easy. There is no one universal definition of, nor set of guidelines by which to assess soil fertility because it relates to the capacity of a soil to produce optimum yields and all soils differ in this. Although farmers may have concerns about the level of organic matter in their soils, until critical levels can be defined other issues take precedence in the daily management of their farms.
Until the early 1970s data from some of the long-term experiments at Rothamsted and Woburn showed that yields of many arable crops were independent of the level of soil organic matter (SOM), provided that sufficient plant nutrients were available in soil; these nutrients could be supplied by fertilisers. Somewhat earlier in the 1960s however, results from other experiments suggested that the amount of SOM could have beneficial effects and this led to much work, summarised here, testing the effects of SOM. Since the 1970s we have seen yield increases on soils where SOM has been maintained or increased compared to where it has not. This is because varieties with a greater yield potential have been grown and this potential has been protected by appropriate use of agrochemicals to control weeds, pests and diseases. Soils with a low SOM equilibrium may not allow full expression of yield potential from these varieties.
Most soil organic matter derives from the microbial decomposition of plant and animal material added to soil. When SOM is determined as percent carbon, the result includes the carbon in the organisms that live in the soil, the microbial biomass that is responsible for the breakdown of organic material. When SOM derived from inputs as diverse as farmyard manure and sewage sludge begins to decline when additions cease, the rate of decay is independent of the material added suggesting that SOM has a uniform composition.
The amount of organic matter in soil depends on the input of organic material from manures and plant residues and its rate of decomposition, the rate at which existing SOM decreases, soil texture and climate. All four factors interact so that the amount of SOM changes towards an equilibrium value specific to the soil type and farming system within a climatic zone. For any one cropping system the equilibrium level of SOM in a clay soil will be larger than that in a sandy soil and for any one soil type the value will be larger under permanent grass than with continuous arable cropping. Examples of these effects are given here together with benefits to crop yield. Also presented are some examples from field experiments of data on separating the effects of nitrogen from other factors. Improvements in phosphorus availability to crops due to better soil structure from SOM and the provision of low-energy bonding sites on organic matter are discussed.
Benefits from building up soil organic matter are bought at a cost, however. Large losses of both carbon and nitrogen from added organic manures have been recorded, often more than 50% of the carbon and nitrogen added. The decomposition of SOM in autumn leads to large quantities of nitrate in soil at risk to loss by leaching or denitrification at that time.
A E (Johnny) Johnston and Paul R Poulton, Rothamsted Research, Harpenden, AL5 2JQ, UK.
46 pages, 15 figures, 11 tables, 47 references.