Achieving soil sustainability

By Joe A. Friend

AS judged by conventional resource and environmental textbooks, most of society still seems to believe that soils are a renewable resource. However, this is not academic opinion in some countries, such as Australia, where largely fragile, leached, or depleted soils have been worked hard for several generations.

The leading edge of academic thinking now is that a majority of world soils, apart from the deepest and most rapidly forming, are nonrenewable within a human lifetime. Estimated rates of soil formation are so slow as to be negligible in real terms or less than the rate of nutrient extraction from upper zones of the soil profile.

The question of renewability

The data, analysis, and proof that soils are, with few exceptions, nonrenewable can be used to show that short-term time frames are not really appropriate for sensible land use planning, the conversion of one land use to another, or monitoring land use factors. Land development planning has become burdened by the short-term political decision process that rests upon change taking place within a time span of three to five years.

The belief that soils are a renewable resource first appeared in the English literature in the 1970s (5, 24). Since then, many scientists and publications have spoken of soils as "renewable" (3, 4, 6, 12, 20, 22). However, no authors define a time frame for renewability.

Many soils are formed within time spans of 100 to 10,000 years because it takes about 100 to 200 years to form each inch of soil, on average, and most soils are 5 to 6.5 feet deep. On the other hand, it is well established in many parts of the world that soil loss rates are one to two orders of magnitude greater than average soil formation rates, from 1 to 10 years to lose a single inch of soil. Soil loss is defined as the aggregate net loss of solum depth due to nutrient extraction "mining," erosion by wind and water, and other factors.

Naturally, deeper soils on flatter country and in well protected positions due to natural forest cover may well be forming faster than their loss rate, but because real soil formation rates are rarely measured, no one really knows which soils are "renewable," depth-stable, or forming at a rate equivalent to the measured soil loss rate. In Australia, Grierson (8) probably provided the first indication that soil scientists need to measure soil formation rates, by classifying soils as either renewable or nonrenewable.

If people continue to think that soils renew themselves and do not need help to regenerate parts of their solum to reconstitute total soil depth, then little is going to change in today's agricultural world. Farmers may still go on mining the soil and reducing solum depth at faster and faster rates, with no knowledge that future generations may be in jeopardy because of shallower to critically shallow soils. Obviously, this issue is most important on shallow soils. Reganold and co-workers showed that up to 6.7 extra inches of soil depth can be lost within 30 years.

At current rates of nutrient extraction and exhaustion, bearing in mind that most cropping and pasture systems have root depths down to perhaps half the total depth of most profiles, a majority of world soils are being mined faster than they are or could be, regenerated. They are intrinsically nonrenewabIe and may become completely unusable within a few generations.

Perhaps the first researcher to give a high profile to the question of soil renewability was May (13). This still remains the only instance in a scientifically credible journal where soil has been deliberately labelled nonrenewable.

Connecting renewability and sustainability

Any soil may be renewable or not within a human time span. Factors such as proximity to mulch or organic manures influence renewability, but these factors are not readily measured or measurable, while soil renewal can be measured.

Monitoring soil depth within cropping systems or forests will help scientists to gauge accurately a soil's base ground renewal rate. This information is a prerequisite to understanding whether soil is being formed faster or slowerthan it is being lost. It is possible to know whether soil formation can be increased, and measuring soil formation rates over significant periods will help scientists begin a proper analysis of these issues.

Debate regarding what constitutes sustainable development at both national and international levels appears to have entirely disregarded the difficult issue of time frame. Yet there is sufficient data to determine what average soil renewal rates are worldwide. At the very least, a basic background figure of 150 years per inch worldwide (with a range of 100 to 200 years per inch) can be assumed. On that basis alone, any land or development that will mine or result in the loss of more than 0.06 inch of soil per year could be considered nonsustainable.

So-called universal soil loss equations, such as the universal soil loss equation, must be disregarded because soil depth may be lost faster as a result of nutrient extraction in various industries, such as forestry.

Another area of future research is the role deep roots in breaking rock down into soil. This is an area of concern because land once covered by deep-rooting trees is now farmland, and we do not know what functions may have been lost. This research could be coordinated with soil depth and soil formation rate studies.

Different scales of time and space

Although society is bound bv the need to produce and research everything within political time spans of three to five years, there is clearly a need for longer time periods to again become optional. This is particularly appropriate for land use planning and biological research and for most researchers exiperimenting with long-term effects. In general, such research can be completed within 11- to 15-year time spans, which for purposes here I have termed "sustainable time spans." In a nutshell, 11 to 15 years does appear to be an estimate of the sort time period essential for full testing of any complex biological theories, indeed any changes to land and land biology that must have solar, lunar, or any other 10-plus-year cyclical variables discounted.

Worldwide soil formation rates by direct determinations
Soil type Generic name location general notes age of material (years)* formation rate (years/inch) Reference
Alluvium Colorado, US   2000 271 11
Alluvium Pakistan   12000 190 10
Alluvium St. Vincent Volcanic ash 4000 44 10
Alluvium Wisconsin, US Decalcified loess 8000 203 18
Argiaquoll Oregon, US A-1 133 21 7
Entisol Hawaii, US Azonal, volcanic ash 45 3 14
Alluvium Iowa, US Grassed loess 100 8 19
Ferosol Senegal Laterite 35 6 2
Hapludalf Iowa, US Podsol on loess 4000 102 18
Hapludalf Iowa, US A-1, A-2 † 2500 211 16
Hapludalf Wisconsin, US A-1 † of 265 97 15
Hapludalf Iowa, US A-1 † only 400 30 21
Mollisol Iowa, US Alluvium 110 76 19
Oxisol/s Africa 3-foot solum 75000 1905 1
Spodosol Europe Podsol in glacial sand 1200 53 23

* The age of soil material is as determined by paleological and/or chemical techniques, assuming "maturity" of soil has not been reached, except in the case of the African oxisol/s, which have been deleted from the primary calculation of soil formation rate.

† Not full profiles/solum

That short-term political time spans have become almost mandatory within our research system, such that long-term monitoring and analysis of complex biological changes of an intergenerational nature are excluded, is an indictment of the present political process. It does, however, help to explain why genuinely difficult ecological research has been virtually absent in our scientific institutions, while loss of species and diversity continues unabated in the world.

Because we are an Earth-bound civilization, it appears incumbent on us as scientists, especially those in soil science who "support the rest" so to speak, to begin the task of accurately defining and specifying what is sustainable in the context of soil management and land use.

Establishing a sustainable time span as well as defining a time span to separate renewables from nonrenewables can help people to better understand both soil and land the hitherto poorly defined and misunderstood resources. A preliminary analysis of available world data on rates of soil formation indicates that a likely, satisfactory time period to monitor gross changes in soil or land is from 11 to 15 years. It is probable that scientific monitoring of soil formation over such sustainable time spans can yield statistically reliable data to verify whether soil is forming faster or slower after changes to management or treatments have been incurred.


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