ReviewIs there a critical level of organic matter in the agricultural soils of temperate regions: a review
Introduction
It is well-known that additions of organic matter (OM), e.g. manures, composts, above-ground crop residues, or increases in soil organic matter (SOM), e.g. below-ground crop residues, microbial biomass, can improve soil properties. This was, and continues to be, the basis of much traditional farming, where large amounts of mineral fertilisers are either not available or are eschewed, or where tillage/short grass-ley rotations (with or without animal husbandry as part of the cycle) are preferred over long-term monoculture. In recent years, organic farming has increased, wherein a move from industrially produced fertilisers to ‘natural’ products is a central part of the farming system (see e.g. Lampkin and Spedding, 1990). An increase in SOM is also still seen, by many conventional farmers, as a desirable objective. Better plant nutrition (N, P, S, micronutrients), ease of cultivation, penetration and seedbed preparation, greater aggregate stability, reduced bulk density (Db), improved water holding capacity at low suctions, enhanced porosity and earlier warming in Spring have all been associated with increased amounts of SOM (Carter and Stewart, 1996). It follows from this that a decrease in SOM, e.g. by oxidation following cultivation, especially under long-term tillage, the withdrawal of grass-leys from rotations, the absence of animal or ‘green’ manuring, or long-term climate change, might adversely affect most, if not all, of these properties. This decline might occur even if, in the long term, SOM reaches an equilibrium with additions of OM in crop residues (Arrouays and Pelissier, 1994), composts, sewage sludge or other organic wastes (Mamo et al., 1999). There are clear relationships between these different perceptions of SOM and current views on definitions of soil quality (‘soil health’), and the identification and development of appropriate soil quality indicators and soil monitoring programmes (see e.g. Davidson, 2000).
Kemper and Koch (1966) indicated that many soils in the western United States and Canada suffered significant decline in structural stability if soil organic carbon (SOC) less than 2%. Similarly, Greenland et al. (1975) concluded that soils in England and Wales with <2% SOC were prone to structural deterioration. However, this 2% threshold was a ‘rule of thumb’ (sic) to indicate soil structural stability, not a measure of soil physical properties in the field. Nevertheless, this value of 2% SOC (equivalent to ca. 3.4% SOM1) is often taken by those with concerns over the impacts of intensive farming practices as a threshold below which the soil becomes physically unstable, more susceptible to cultivation damage and to erosion, and that society should be concerned when a significant proportion of soils are below this value (e.g. Pretty, 1998). Crop yields might suffer due to reduction in the soil’s capacity to cycle nutrients. This will, in turn, lead to reduction in the return of SOM to the soil via crop residues and the cycle of deterioration will intensify. The Royal Commission on Environmental Pollution (RCEP), in its report on ‘Sustainable Use of Soil’ (Department of the Environment, 1997), commented on the undesirability of allowing SOM to decrease too much, although it avoided recommendation of limiting values. Thus, a concept of a critical threshold for SOC has become widespread and bought to the attention of policy makers who have sought clarification of the scientific validity of this concept. Whilst soil scientists might look askance at the suggestion of a single threshold applicable to all soils and land uses, there is clearly a need to review the position, summarise the quantitative evidence for and against this, and promote the findings to a non-specialist audience. This review was commissioned by the then UK Ministry of Agriculture, Fisheries and Food (MAFF) to address these concerns and to assess whether 2% SOC is a critical threshold of soil quality.
Consideration of such critical levels involves assessment of the quantitative evidence, i.e. the nature of SOM and the properties it confers on soils, whether justifiable limits can be set for a range of soil types, climatic conditions, or land management/cropping practices and, finally, whether there are any dis-benefits from an increase in SOM levels in soils. In other words, are there critical upper levels of SOM? We emphasise, again, that we were considering the quantitative evidence; we have looked at much anecdotal and descriptive reporting, but these sources of information are rarely accompanied by quantitative data.
Section snippets
SOM—some general considerations
A detailed discussion of the physico-chemical nature of SOM and related factors, for which there is a very large literature (see e.g. Hayes and Wilson, 1997), is beyond the scope of this review. However, certain points are germane, and are discussed below. For many hundreds of years, much farming practice was based on the sound observation that additions of relatively fresh organic materials to soils improved tilth and crop yields, i.e. they affected soil physical properties and crop nutrition.
Influence of SOM on crop production
Allison (1973) summarised the role of SOM in crop production under four headings: crop nutrition, micronutrient availability, soil available water, and soil buffering capacity, viz:
- 1.
The ‘active’ pool of SOM, i.e. plant and animal residues added to the soil in the previous 5, or at most, 25 years, provides N for crop growth, and increases the availability to plants of many essential micronutrients. Older SOM makes almost no direct contribution to crop nutrition. This concept of ‘young’ OM has
Soil physical properties
The addition of OM is considered to increase the stability of soil aggregates, reduce Db, alter pore size distribution leading to greater water holding capacity, and reduce susceptibility to erosion (Carter and Stewart, 1996).
Conclusions
The greatly increased crop yields of the last half century have often been obtained despite decrease in SOC, often to levels below those posed as critical. The literature suggests that where reductions in SOC lead to reductions in yield potential these are small, and have not lead to a crisis in crop production. Some studies, usually on eroded soils, indicate that as SOC decreases it becomes increasingly difficult to obtain yields equivalent to those on un-degraded soils by the addition of
Acknowledgements
This review derives from Contract SP0306: ‘Critical Levels of Soil Organic Matter’, financed by the Ministry of Agriculture, Fisheries and Food, London, UK.
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