Soil Quality

Pramble

Presented here is a summary of the concepts of soil, soil quality and soil fertility. These pages have been adapted from and update an essay originally submitted as part of Honours Degree in Habitat and Soil Management at the University of Reading, Berkshire.

The essay further explored the potential to then use these definitions to develop a self referential mechanism for relativistic assessment of the extent to which a soil fulfilled its potential.

Land ‘V’ Soil

Whilst aspect, slope and elevation are important factors in the concept of land the concept is essentially two dimensional having only width and length. Whilst Land may be covered by soil it may also be covered by exposed rock, scree or water. Thus land as a concept is two dimensional and refers to everything not covered by the oceans.

Soil whilst requiring land in order to form has the additional dimension of depth, it is three dimensional. As a consequence of this third dimension soils are or have the potential to be biologically and chemically dynamic: thus soils contain what are potentially more dimensions. However with land being the essential prerequisite for the formation of soil the concept of soil has invariable become confused with the concept of land.

Soil

Described by White (1997) as “the material that forms at the interface of the atmosphere and the lithosphere which is capable of supporting plant growth” soil is the product of past and present chemical, physical and biological processes which have combined to influencing the appearance and character of the material. The determining factor, the ability to support plant growth has traditionally been the measure of soil quality and or fertility. However a soil that has measurable inherent ‘quality’ may, for external reasons (i.e drought), be be incapable of supporting plant growth. Conversely many tropical forest soils, which support some of the Worlds richest and most dynamic flora and fauna communities, do so on soils that are often thin and impoverished. Quality and fertility and what constitutes ‘quality’ rather than fertility in a soil is somewhat blurred.

Whilst both are potentially measurable by their ability to support plant life; soil quality could be regarded as more of an inherent property (or properties) that is not solely responsible for supporting plant life. Fertility however refers more specifically to the ability to support plant growth and is more of an emergent property. In this respect soil quality can be regarded as the measure of fixed physical characteristics which are usually difficult to change whereas fertility is as a consequence of the management of variables that can be changed. Thus it could be argued that we should develop a quantitative index system for determining soil quality based on the measured inherent properties (the soils potential) and then a secondary qualitative system for assessing the fertility that reflects the state of the variables.

Origin of Soil Science

The formation of soil was first explored by the Russian geologist Dockuchaiev (1846 – 1903), who determined that soil formed over time as a consequence of climatic, mineral and biological processes which he demonstrated using the soil forming equation:

Soil = f(C, PM, O) x time
(where C = climate, PM = parent material, O = biological processes)

In 1941 the Swiss scientist Hans Jenny (1899 – 1992) expanded Dockuchaiev’s equation by adding relief/topology as a factor and separating the biological processes into two distinct groups, fauna and flora and coming up with:

S = f(C, PM, R, O, V,) x time
(where C = climate, PM = parent material, R = relief/topology, O = fauna, V = flora)

As the temporal aspect of time is important so it must be argued is the spatial requirement. This can be reflected ib the equation as thus:

S = f(C, PM, R, O, V,…) x time
Land

In this way soil formation can be shown to be a process that occurs upon and as a distinct process from the land itself. In separating the land out we are now better able to determine what we mean by soil quality as it relates to land.

Soil Quality

Doran & Parkin (1994) proposed a definition of soil quality as “The capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health” whilst the SSSA (1987) propose the definition of soil quality as; “Inherent attributes of soils that are inferred from soil characteristics or indirect observations”. Other workers have elected for more simple definitions such as “Fitness for Use” (Pierce and Larson, 1993) or “the capacity to function” (Karlen et al.. 1997).

As soil is not the only factor in plant growth we cannot simply assess soil quality on the basis of plant or animal health as these are not solely dependent on soil quality. Furthermore in assessing soil quality we are not assessing the whole environment, only the soil within that environment. It is therefore imperative that the measure of soil quality be a factor or degree of a universal soil function that relates only to soil properties. The ability to support plant life, which is dependent on a number of other factors, relates more to the soils fertility than it’s quality.

Given the huge variability in soil textures, biology and chemistry and the complex relationships between them it is unlikely that we can derive usable indexes from a comparative assessment of soil attributes that does not distinguish between inherent and variable qualitative aspects. This route could also result in simply developing another classification system. What is therefore required is a measure of soil quality based on the soils potential to function.

“The capacity to function” (Karlen et al 1997) and it’s quantitative measure expressed as a potential rather than a realized state may well permit the formulation of the concept of soil quality as an exclusive function of soil rather than as a measure of productivity. Thus: Soil Quality could be regarded as:

the inherent properties of a soil as they relate to the potential of the soil to function

Soil Fertility

Soil Fertility however refers to the rate and degree to which the soil fulfils this potential to function. Unlike Soil Quality which is largely the measure of ‘constants’ and the potential that these constants offer, Soil Fertility is more the measure of the state of the ‘variables’ and the degree to which this potential has been realised. These variables are the rate and degree to which processes and exchanges occur within the soil environment. They are not inherent properties nor are they the product of the inherent properties but are the consequence of the efficient management of resources within the constraints of the inherent properties.

For Soil Fertility in this context is “the degree to which the biological process operate within the constraints of the inherent properties” and whilst Quality is or has the potential to be a universal measure, Fertility is one that is relative to the specific local constraints.

These biological process are themselves measurable properties but unlike the inherent properties of Soil Quality the properties of Soil Fertility can be manipulated. So as quality is an inherent property so fertility becomes an emergent one dependent on the management of the biological variables.

Thus soil quality is more the reflection of inherent properties such as soil texture, depth, aspect and slope whilst fertility is more the degree to which biological process can operate within the constraints of these properties.

Thus we now see that soil, the product of past chemical, physical and biological processes has a quality that is a reflection of the extent to which these processes have added ‘depth’. This depth has a potential that can be be universally qualified and indexed from the quantitative measure of inherent properties to provide a qualitative index based on potential. Soil Fertility is then the measure of the degree to which this depth is or can be subsequently utilized. In this way fertility becomes a measure of efficiency based on the degree to which a given soil performs against it’s own potential rather than as a measure and league table of unrelated soils.

next: soil fertility as a function of soil quality

References

Karlen, D.L., Mausbach, M.J, Doran, J.W., Cline, R.G., Harris, R.F., Schuman, G.E. (1997). Soil Quality: A concept, definition and framework for evaluation. 61: 4-10 Soil Science Society of America, Madison.

Soil Sc. Soc. of Am.(1987). Glossary of soil science terms. SSSA, Madison, WI. [cited in Doran, J.W., & Parkin, T.B. (1994). Defining and assessing soil quality. In Defining soil quality for a sustainable environment (SSSA Special publication No 35). Soil Science Society of America, Madison].

Parr, J.F., Papendick, R.I., Hornick, S.B. & Meyer, R.E. (1992). Soil quality: Atributes and relationships to alternative and sustainable agriculture. Am. J. Altern. Agric. 7:5-11. [cited in Doran, J.W., & Parkin, T.B. (1994). Defining and assessing soil quality. In Defining soil quality for a sustainable environment (SSSA Special publication No 35). Soil Science Society of America, Madison].

Pierce, F.J. and Larson, W.E. (1993). Developing criteria to evaluate sustainable land management. p.7-14 In J.M. Kimble (ed.) Proc. of the 8th int. Soil Management Workshop; Utilization of Soil Survey Information for Sustainable Land Use. May 1993. USDA-SCS, National Survey Center, Lincoln, NE. [cited in Doran, J.W., & Parkin, T.B. (1994). Defining and assessing soil quality. In Defining soil quality for a sustainable environment (SSSA Special publication No 35). Soil Science Society of America, Madison].

White, R.E. (1997). Principles and practice of soil science. 3rd edition. Blackwell Science ltd, Oxford.

Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011

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