Soil chemistry and land use
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Soil chemistry and land use Malcolm S. Cresser, Anthony C. Edwards and Zakia Parveen
As an empirical pseudo-science, soil chemistry was one of the oldest branches of chemistry, originating in prehistoric times. It continued in this low-key role until it evolved in the late nineteenth and early to mid twentieth centuries as a research discipline crucial to underpinning agriculture and the technical demands of increased food production. Over the past decade it has enjoyed a renaissance as its wider applications, especially in pollution science and hydrogeochemistry, and hence in strategic long-term environment planning, have been increasingly recognized.
Most people, if asked to describe the impor- tance of soil in the world today, would probably frame their answer in the context of plant growth and the need to feed a grow- ing population. If pressed, the more enlight- ened might venture beyond the demands of agriculture and horticulture and speak of the need for sustainability of forests, recreational land, and natural terrestrial ecosystems. Very few would extol the virtues of soil as a sink for global pollutants, or for its roles in the regulation of the air we breathe or of the quality of water many of us drink. Few, too, would consider its importance as a medium for the support of buildings and associated urban infrastructure components. However, it is these many facets of applied soil science which pose so many fascinating challenges to soil chemists at the present time.
The origins of soil chemistry The concept of sustainability in agriculture must, even if not recognised as such, be almost as old as agriculture itself. Removal of soil-derived plant nutrient elements from a site as part of the harvested crop yield would slowly result in a decline in natural
Malcolm Cresser, Ph.D., A.R.C.S., F.R.S.C., CChem.
Is Professor of Plant and Soil Science at the Univer- sity of Aberdeen. His research interests are primarily in the areas of soil fertility, pollution effects research, the link between soils and water quality, and environ- mental chemical analysis. His 180 publications include six books.
Anthony Edwards, B.Sc., Ph.D.
Graduated in Geology and Biology from Sunderland Polytechnic before obtaining a Ph.D. at Aberdeen University. After two years as a postdoctoral fellow at Aberdeen, in 1986 he moved to the Macaulay Land Use Research Institute. His research interests include soil fertility, especially aspects of phos- phorus chemistry in organic-rich acid upland soils, and the relationships between land use and river water quality.
Zakia Parveen, B.Sc., MSc.
Obtained both her B.Sc. and her MSc. from the University of Dhaka in Bangladesh. She is currently a Commonwealth Scholar at the University of Aberdeen, where she is just completing a thesis on zinc speciation and transformation processes in soil.
Endeavour, New Series, Volume 17, No. 3,1993. 0190.9327/93 99.09 + 0.00. Perganon Prow Ltd. Printed in Great Britaln.
soil fertility unless the nutrients removed were replaced. Nutrients such as calcium, potassium, magnesium, and phosphorus, and a range of trace elements, might be restored, at least partially, by natural bio- geochemical weathering of minerals in the soil parent material; nitrogen might be partially replaced by biological processes and inputs in rainfall.
How long such natural replenishment could sustain the adverse effects of crop harvesting depends upon the nature of the parent material. The materials with the most limited buffering capacities are quartzites and some old, quartz-rich sandstones. Soils derived from granites are marginally better. It has been estimated, for example, that it takes about 1100 years to strip all of the calcium out of half a metre depth of weather- ing granite in soil [l]. People settling on lower, gentle slopes near rivers could benefit from ease of water availability and in terms of soil fertility. Soils in such areas are often derived from aluvial deposits and fed with nutrient-enriched drainage water from up the slope.
Loss of base cations such as calcium, magnesium, and potassium in crops and drainage waters results in soil acidification if such ions are not replaced at an adequate rate . Such acidification could result, over
a few decades for a sensitive soil, in a decline in crop quality and yield to the point where a community might no longer have to be able to survive. Figure 1 shows an excavated dry stone wall in Arran, Scotland. There is strong evidence to suggest that use of the site for crop production was abandoned in the late Stone Age, soon after the wall had been constructed, almost certainly for the reasons described above. Soil from under large stones, which had not been used for cropping, and which had been protected from long-term deterioration to a substantial degree since the wall was first built, was still reasonably fertile . Thus even in pre- historic times, soil chemistry, if not a soil chemist, was influencing land use policy decisions.
Shifting cultivation is a modern day example of this situation which is widely practised in some tropical regions. It relies on exploiting the inherent fertility of a site after clearing and burning of the indigenous vegetation. The initial burn releases nutrients and may even increase soil pH (i.e. reduce soil acidity). Crop yields decline rapidly, however, over only a few years; a new site must then be prepared and the cycle repeated. Regeneration of natural vegetation or a long fallow period is necessary to restore the fertility of the initial site. This method is
Figure 1 An excavated dry stone wall in Arran.
extremely labour-intensive and requires substantial land areas. It may lead to severe permanent damage through erosion of the unprotected soil.
Once soil fertility had started to decline, even in prehistoric times it would not be long before it was noticed that plant growth was improved in the vicinity of manure and domestic waste deposits. From then it would be a short step to deliberate manuring. Mention can be found in Roman literature of 2000 years ago of manuring as if it was a long-standing practice . For much of the intervening period, and sometimes still today, trial and error and visible crop deficiency symptoms would dictate when and how much manure was applied. Thus early soil chemistry would have been very much an empirical science.
Soil fertility assessment The need to feed a growing world population stimulated chemists to investigate the pro- perties and functioning of soils, especially in the context of plant nutrient availability. The stimulus was exacerbated by the signifi- cant increases in crop yields which have occurred during recent decades, as a result of both improved soil management and the efforts of plant breeders. Higher crop yields have resulted in an increased demand for nutrients from soil. Current demands on soil are so intense that the majority of soils would be unable to sustain these levels of productivity without nutrient element sup- plementation. Fertility was once maintained largely through management practices, such as crop rotation, where a variable period of grass/clover (legume) pasture was included. The requirement for systems of this type has declined as a result of the extensive avail- ability and use of chemical fertilizers. Globally a total of 82 x lo6 t N, 41 X lo6 t P,O,, and 28.5 x lo6 t of K,O ferti- lizer are currently applied annually, although only about 20 per cent of arable and 3 per cent of grasslands are in fact fertilized. It is possible to improve greatly the nutrient status of soils through various approaches, including the use of fertilizers. For many years, therefore, assessment of soil nutrient status and fertilizer requirements has been a major preoccupation of soil chemists.
While fertilization for optimum yields might be regarded as the ideal situation, considerable areas of the world still suffer from severe soil fertility problems, and because of economic and availability con- straints, alternative approaches, including selective breeding of more suitable plant species, must be adopted. Where improve- ments such as irrigation schemes have been made, increasing soil salinity is a cause for concern, with up to 35 per cent of the irrigated area suffering from suboptimal productivity. Soil acidity and its associated problems also limit fertility on a widespread scale, affecting, for example, 40-50 per cent of the tropical land area.
Knowing how much fertilizer to apply involves much more than just replacing what has been removed by the harvested crop.
There are many other potential losses to be considered, including leaching and the chemical/biological transformations to non- plant available forms. The most convenient way of assessing fertilizer requirements in the majority of cases is through soil analysis. Samples are collected, extracted, and classi- fied in terms of their potential to supply particular nutrients to a crop over the grow- ing season. The range of chemical extractants used is substantial and this reflects historical and local preferences of the laboratories involved. In all cases, however, it is essen- tial that a complete balance of nutrient elements is achieved, as an insufficient amount of an individual nutrient will result in inefficiency in the whole system. Gpti- mum growth conditions vary greatly between regions and crop types; consequently, extract- able values are calibrated against experimen- tally manipulated fertilizer response field trials. However, due to the expense and labour involved in running field experiments, extrapolations have to be made when giving advice. Soil extraction values are classi- fied into a number (often five) of broad categories, and fertilizer prescriptions assessed after taking into account important local factors such as climate, crop/soil type, and previous cropping/fertilizer histories .
Land capability classification Because of the relative ease with which plant-available nutrient element concentra- tions and soil acidity may be adjusted by the use of fertilizers or liming materials, soil nutrient element status at any particular moment in a soils history is of minor impor- tance in the assessment of land use capability. Other factors, such as climatic conditions, slope, daylight hours, exposure, and stoni- ness, which are far more difficult (or even impossible) to modify, are therefore given much more weighting when potential of land for agriculture and forestry is being assessed.
It might be supposed, therefore, that the soil chemist has a minor role to play in the assessment of land capability and land use planning. This is increasingly becoming far from the truth, however, primarily because soil and land are no longer just perceived as key elements in food production or for the support of buildings and urban infra- structure or for recreational use. Increas- ingly, soils are also being perceived as a sink for a wide range of pollutants, either from the atmosphere or from direct dumping, and as having a crucial part to play in the regula- tion of freshwater quality [4-61. Thus soil critical load maps, which map the distribu- tion of soils with respect to their capacity for absorbing atmospheric pollutants without being damaged [7-91, must also be regarded as land capability classification maps. So too should maps showing sensitivity of ground waters to acidification where these are based upon the long-term buffering capacity of soils or bedrocks [lo]. In maps such as these, a sound understanding and quantitative know- ledge of soil chemistry and microbiology is essential.
Soil change One consequence of the dramatic growth in interest over the past decade in the link between pollution, soils, and surface and ground waters has been the realization of the need to study soil chemical change on much longer timescales than a single growth season or crop rotation cycle. This has resulte4l over the past few years in reanalysis of old, stored samples alongside analysis of fresh samples collected from the same sites to quantify key parameters in forest soils such as the rate of acidification [ 11, 121, the rate of lead accumulation at the surface [ 131, or the rates of carbon and nitrogen seques- tration [ 141. While such studies are impor- tant in pollution effects research, they are also very important in assessing land use effects, especially afforestation effects, on soils and associated drainage waters at the fundamental process level [15, 161.
Unfortunately, samples which have been carefully stored for many years and for which the sampling points are very precisely known are rare. In some studies of long- term change it has, therefore, proved neces- sary to compare new analytical data for fresh samples with old data for samples which have been lost long since [ 17, 181. An alter- native approach to reanalysis is to subject ecosystem microcosms to simulated precipi- tation representing diverse pollution climates. The advantage of simulation is that it allows very reliable controls (subjected to pollution- free rain) to be used for reference purposes, although the experimental approach is very time-consuming if run for a long time. Inter- change of microcosms between polluted and unpolluted sites is a useful approach, but care must be taken then to make sure that any changes observed are not attributable to climatic or other site differences.
Soils and freshwater eutrophication Land use can, and often does, have a substan- tial effect upon the chemical and biological quality of lakes and rivers in a region. Figure 2, for example, shows a scientist in Thailand making dissolved oxygen measure- ments in a shallow lake subjected to substan- tial loads of pollution from the adjacent land area. The aquatic vegetation and algal growth are so prolific that the water shows a diurnal swing of 2 pH units and a corre- spondingly large swing in dissolved oxygen as a consequence of the equally dramatic daily fluctuation in photosynthetic carbon dioxide consumption.
The link which may occur between the use of nitrogenous fertilizers iu a catchment and the amount of nitrate in its river water is now well established . Soil chemists and biologists have an important role to play in recommending fertilizer and land manage- ment practices and patterns of land use which minimize surface water and ground water contamination without resulting in unacceptably large losses in yield. This involves a fundamental rethink compared to the days when yield optimization was the driving force to fertilization strategy. There is au increasing need in many countries at
Figure 2 Measuring dissolved oxygen in a shallow lake exhibiting eutrophication in Thailand.
the present time to consider water resources alongside crop production when planning land use.
Until very recently, only losses of nutrient elements in inorganic forms tended to be considered in the eutrophication context. While this may be appropriate for agricul- tural land, it has recently been realized that losses of nutrients in organic forms from forest or upland moorland soils may be equally or more important to sustainability and to water quality, especially in the case of phosphorus  and nitrogen . How- ever, as yet very little information is avail- able upon organic N and P fluxes from soils under diverse land uses or about their significance in streams and lakes.
Afforestation effects on soils There has been much discussion over recent years about the effects of afforestation on soil chemical properties. Even as early as 1957 it was realized that acidic soils could be further acidified as a consequence of tree growth 1221. The process occurs over several years as a consequence of the rate of base cation removal from soil outstripping the rate of replenishment by biogeochemical weathering. Thus it is unlikely to be signifi- cant for soils derived from base-rich parent materials. A number of authors have lost sight of the importance of soil chemical characteristics when assessing afforestation effects on soils and associated drainage waters, and have compared soils under dif- ferent tree species without taking the trouble to ensure that the soils would not differ anyway, even if under similar vegetation. This has resulted sometimes in apparently conflicting results.
Because of their rather different litter chemical qualities and differences in tem- poral distribution of litterfall, deciduous and coniferous tree species tend to have slightly different effects. Decomposing litter from coniferous species is often appreciably more
acidic than that from hardwoods . Trees which retain their foliage in winter also have different pollutant trapping and precipitation interception loss characteristics. The soil acidification caused by the growth of trees is included in models such as SMART and MAGIC which predict the effects of atmos- pheric pollution of soils  and freshwaters , respectively. It is also a key component in the equations used in many countries for calculations and mapping of maximum tolerable acidifying pollutant deposition loads.
The growth in use of modelling The logical step for soil chemists, and indeed for environmental scientists generally, when pressed to make quantitative impact assess- ments of specified What if .? pollution, pollution abatement, or land use scenarios is to turn to modelling [26,27]. Soil chemists in u-my countries have come under pressure from funding agencies to develop usable models on timescales which are sometimes disturbingly short. Once developed, the models must be validated, and, if necessary, refined. Validation of long-term predictive models is particularly difficult, and often, as a surrogate, reliance is placed upon strong supporting circumstantial evidence.
One of the greatest dilemmas facing aspiring (or reluctant) modellers in this context is deciding what and how many eco- system components and processes should be incorporated into the model. If too many are used, data collection becomes prohibitively expensive, and the model is unlikely to be used on a wide scale; too few, and the model is unlikely to be robust. Often, by way of compromise, groups of processes are lumped together to reduce data input requirements.
Modelling water quality Until relatively recently, most models of water quality were catchment-specific, being
based largely upon a detailed knowledge collected over a year or more of the way a particular stream responded to changes in precipitation parameters such as solute com- ponent concentrations or amount, intensity, and duration of rain. Soil effects were relegated to the level of one or more black boxes, which in reality did little more than provide token justification for some of the empirical constants built in to the model to make it work.
Recently, however, progress has been made in the development of a model for prediction of key water quality parameters which makes use of fundamental soil chemi- cal properties, such as the percentages of soil cation exchange sites occupied by species such as Ca2+ or H+ , and the spatial distri- bution of soil types within a catchment . The advantage of this approach is that it allows prediction of water chemical quality for a hitherto unstudied catchment using only soil maps and the results of a limited amount of soil chemical analysis. Thus the model is readily transferable between catch- ments. This type of model could be very readily extended to the quantification of land use and soil management practices on water quality in any specified catchment.
Soil sustainability Mention has already been made of the key role played by the need to maintain soil fertility in the development of soil chemistry. Adequate amounts of a wide range of major and trace nutrient elements must be provided by soil season after season to sustain healthy crop growth. Failure adequately to maintain the required balance of essential nutrient levels ultimately results in poorer growth, often with characteristic deficiency symp- toms. Figure 3, for example, shows the lime-green discolouration and necrotic stripes typically associated with manganese deficiency. Such symptoms have great diag- nostic value to the experienced agricultural adviser.
It is often imagined that nutrient deficien- cies are likely to be a problem only for agri- cultural soils, as a consequence of continual removal with harvested crops, but this is not the case. Timber removal, especially whole- tree harvesting, results in substantial and permanent nutrient element losses. More- over, atmospheric pollution may disrupt natural element cycles, also resulting in increased risk of some elements becoming deficient. The exponential increase in atmos- pheric carbon dioxide is also a cause for concern at the present time. It is generally supposed that overall effects of this enhance- ment will be beneficial to plants, increasing photosynthetic yield and water use efficiency . However, enhanced growth will result in increased nutrient depletion rates from soils. For unfertilized, nutrient-poor soils, such as those in most moorland ecosystems, the soil may not be able to provide the extra nutrient elements required, resulting in growth problems. Figures 4 and 5, for example, show Culluna vulgaris (heather) plants grown on a typical nutrient-poor
Figure 3 Typical manganese deficiency symptoms in a cereal.
Scottish peat in ambient air and in air with the carbon dioxide concentration increased by 100 ppm, respectively . The reduction in shoot extension is very apparent.
Unfortunately, although much more is known now about long-term rates of acidifi- cation and decline in base cation status in soils, we still have relatively little informa- tion about long-term rates of decline in trace element availabilities. There is a real need for the collection of such information, and for the mapping of susceptibilities to trace element deficiencies under diverse land use and soil management regimes. This is especially true because of the compound- ing effects on yield enhancement of plant breeding, fertilizer use and elevated carbon
Figure 4 Calluna vulgaris growing in peat in normal air.
Figure 5 Calluna vulgaris growing in the same peat as in figure 4, in air in which the carbon dioxide has been enriched by 100 ppm.
dioxide. Effects of probable chemical changes on soil physical and microbiological properties also need to be assessed.
Developments in soil analysis Determination of total amounts of elements present in a soil sample is relatively simple conceptually, but is of limited use for fertility assessment, its value being confined largely to long-established mapping/classification purposes. It is much more difficult to deter- mine that fraction which is chemically active, although this is essential in terms of assessing bioavailability or toxicity. A wide range of chemical cocktails is used for extracting soil nutrients, most depending upon ion exchange, chelation, or hydrolytic reactions to function.
As a consequence of the general signiti- cance of exchange-type reactions in terms of general soil processes and plant uptake in particular, there has been considerable interest recently in the use of ion exchange resins for assessing the plant availability of a range of nutrients . Soil samples are usually extracted overnight with both cation and anion exchange material in the soil/ water suspension. The resin is separated, and the ions eluted for analysis. Generally an improved relationship with plant ion uptake is obtained compared with those for more established chemical extractant tech- niques. This procedure becomes especially attractive when combined with a multi- element technique such as inductively coupled plasma optical emission spectrometry [3 11. Such ion exchange-based procedures may also be successfully employed in situ in the field environment .
There has been a growing awareness over recent years of the importance of the chemical forms of elements in soils and surface waters, because tbe form may sub- stantially influence availability and toxicity. Characterization of elements according to their distribution between different forms is generally termed speciation. Most soil and
sediment speciation studies involve sequen- tial extraction using a series of selective extractants. Typically, for example, for a cationic element like zinc, the sequence might follow the order water soluble, exchangeable, organically bound, manganese oxide-bound, amorphous iron oxide-bound, and residual. In practice, totally specific extraction of one fraction at a time is not possible, although the procedures are highly selective. The amount found in each fraction should therefore be regarded as operationally defined, rather than as a specific fraction.
An application of this approach in the authors laboratory is illustrated in figure 6. The graphs show how much of the zinc in a sample of sewage sludge which has been labelled with a radioactive zinc isotope prior to disposal on soil remains in the readily available exchangeable form after one, seven, and 30 days incubation. The soils used were derived from the same parent material, but had been subjected to different land uses, namely: bracken and heather moor, forest, rough grazing, and improved grazing. Soil pH increased in a similar sequence. The zinc is increasingly rapidly transformed from the exchangeable form to other, less available forms as the soil pH rises.
There is still a need for better assessment of the biological activity of soil samples from chemically based extraction procedures. This is being achieved by the development, for example, of root bioassays. One such procedure, based upon root extension, has been suggested for quantifying the toxicity of ahuninium . Such studies are impor- tant in both the agricultural and the general environmental contexts. By using the effect directly as an analytical tool, and provided the test can be made sufficiently selective, the problems associated with chemical speciation techniques are removed.
Conclusion Soil chemistry at the present time plays a much more important role in the land capa- bility assessment than it did a decade or so ago. primarily this is because the term capability assessment is now being inter- preted in a much broader sense than hitherto. It is to be expected that this close involve- ment will last long into the future, because of the ever-changing nature and scale of environmental problems.
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