Soil & Tillage Research, 24 ( 1992 ) 101-106 101 Elsevier Science Publishers B.V., Amsterdam

Guest Editorial

Soil physics towards 2000

A.R. Dexte# and I.M. Young b aSoil Science Group, Silsoe Research Institute, Wrest Park, Silsoe, Bedford, MK45 4HS, UK

bSoil-Plant Dynamics Group, Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, UK

(Accepted 11 March 1992)

INTRODUCTION

Today there is increasing awareness that comprehension of the physical properties of soil (primarily soil water and structure) holds vital keys to the understanding of many processes of agricultural and environmental signifi- cance. This general acceptance that soil physics plays a fundamental role in the understanding of all soil processes presents great opportunities of which we, as soil physicists, must take full advantage. To do this we must examine carefully the types of research that we carry out at present, pinpoint any re- dundant areas, and emphasise new areas which complement other branches of soil and plant science.

Here we take the opportunity to examine specific areas of soil physics which we feel demand our greatest attention. By doing this we make a plea for greater focus in soil physics research. A focus which will help us to better understand the mechanisms which govern the fundamental processes in the soil and lead us to better manage that most important natural resource of Man--soil.

We have prepared this editorial in the light of a recent meeting of UK soil physicists on the future role of soil physics. Whilst we are responsible for the views expressed here, it is worth noting that similar views were expressed by other participants at this meeting on many of the following issues.

WHAT IS SOIL PHYSICS?

Soil physics deals with the energy status of the different phases (solid, liq- uid and gas) of the soil system and aims to quantify the fluxes of these phases that are produced by energy gradients. Central to soil physics is the study of

Correspondence to: A.R. Dexter, Soil Science Group, Silsoe Research Institute, Wrest Park, Sil- soc, Bedford, MK45 4HS, UK.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0167-1987/92/$05.00

102 A.R. DEXTER AND LM. YOUNG

the energy state and flux of water in relation to the spatial heterogeneity of the different phases. This heterogeneity is usually referred to as soil structure.

However, soil physics is concerned with much more than water. Fluxes of heat and gas through the soil surface and within the soil profile are also con- sidered. Mechanics is the oldest branch of physics and in the case of soil me- chanics deals with how soil systems change their size, shape and heterogeneity in response to different imposed mechanical potentials.

In all aspects of soil physics, the soil response is very strongly dependent on the soil structure. For this reason, the study of soil structure may also be con- sidered as a major branch of soil physics in its own right.

WHY IS SOIL PHYSICS IMPORTANT?

Water has a special place in soil physics because it can be considered as one of the principal driving functions for plant production and for life in general. The temperature and aeration status of the soil are also crucial for the proper functioning of soil as a chemical and biological reactor. The mechanical strength of soil influences the performance of the shoots and roots of plants and controls the energy requirement for all soil handling operations.

Soil structure controls the physical response of soil and consequently all the chemical and biological processes which occur within it. From the smallest size scale (nanometres) of pores between clay particles to the largest (deci- metres ) of the spacings between desiccation cracks, the geometrical structure defines the pathways for the fluxes, the places where mechanical energy den- sity will be concentrated, and the ecological niches for the huge range of biota which inhabit soil. It should be remembered that most of the world's biomass resides in the soil.

Soil structure and the energy levels and fluxes which are governed by it are very sensitive to soil management. Management practices such as tillage, wheel traffic and cropping can modify significantly the structure on which the per- formance of the soil depends.

PROBLEMS R E Q U I R I N G RESEARCH

Scaling laws

A principal objective in soil physics is to be able to perform experiments on one size scale (usually with samples with sizes of the order of 100 mm) and from the results of these to be able to predict the behaviour on other size scales which may be larger or smaller (by factors of up to 103 or down to l 0 -8, respectively).

In order to do this, it is essential that the scaling laws for soil physical prop- erties be determined. Classical scaling has been very successful in defining

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scale relations concerning certain properties in homogeneous porous media. However, inherent heterogeneity within soils may significantly limit the use of such scaling. Instead we have to develop and examine new techniques which can deal with soils exhibiting significant heterogeneity. Fractal geometry may prove to be a powerful method for the quantification of heterogeneity over structural size scales. Whether fractals have such potential or not, there is a clear need for scaling in soils to be adequately examined.

Inappropriateness of bulk measurements

During the last two decades, soil scientists have very slowly progressed away from a reliance on bulk soil properties, and we now accept that much more detail is required to adequately describe a soil. For example, it is now realised that the soil (ped) surfaces on which roots grow do not have the same com- position or properties as the bulk soil. This realisation of the differentiation between ped surfaces and bulk soil also helps to explain some of the unex- pected aspects of chemical and biological interactions between a soil and water flowing down macropores within it.

Soil structure

Soil structure, its architecture, stability and heterogeneity strongly influ- ence all soil processes. Comprehension of the processes behind the generation and stabilisation of soil structure and how this structure interacts with plants, microbes and solutes is one of the most important aspects of soil science. Soil physicists are presented with a great challenge and opportunity in tackling this problem.

For example, macropore spaces such as cracks and biopores have not been described adequately and yet they can have overriding effects on soil physical properties and on soil behaviour in general. A systematic study of the dynam- ics of soil macropore spaces is essential for the development of a soil physics which is able to predict the behaviour of real soil in the field.

Transport processes

Transport processes in real soils are not adequately understood. Although we can measure conductivities, diffusivities and permeabilities, these do not tell us how the fluid is moving in detail. The values which we obtain do not tell us directly the amounts of dead-end pores, the tortuosities, the residence times for fluid in certain types of pores, etc. Of course, we can calculate values for these different attributes if we use simplified models for soil structure. Although the results obtained show an encouraging consistency, our knowl- edge is still of a rather unsatisfactory kind. There is much scope for research

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on the prediction of the movement of fluids through realistic three-dimen- sional pore spaces.

Flow instabilities also need much more research. It is well known that, in certain circumstances, when one pore fluid replaces another, 'fingering' can occur with the formation of an unstable displacement front. This can occur even in relatively homogeneous materials such as sand. Similar instabilities can occur when water which is infiltrating soil crosses a boundary from a less permeable to a more permeable soil. These instabilities can have significant implications for the spatial distribution of the infiltrating fluid.

Soil as a reactor

Soil provides the environment for a huge range of chemical and biological processes. The soil can therefore be considered to be a reactor within which these processes take place. The structure of the soil influences its efficiency as a reactor.

The questions which soil physicists should be asking should relate not only to how soil structure appears on the human size scale but to how plants, mi- crobes and chemical species 'see' structure. For example, when water is added to soil, the pathways that this water will take are largely predetermined before it enters the soil structure, even though we are not yet able to predict what they will be. Microbes, however, may possess a motility which water does not and so their movements may be affected by different aspects of soil structure. Likewise, chemical or biochemical species can diffuse and react on the sur- faces of particles and therefore they are greatly affected by the morphologies of the surfaces as measured on size scales similar to themselves.

For each special process, we need to develop appropriate measures of soil structure. Because of the huge range of size scales involved, it is unlikely that any single method will have general applicability. However, fractal theory may be expected to provide useful links between results obtained at different size scales.

Ins t rumenta t ion and sensors

Instrumentation of increased accuracy and reliability will be required. This is because improved information about soil properties in the field will be needed both for research purposes and for higher levels of soil management. More replication of measuring equipment will be needed to cope with spatial variability and a greater frequency of measurement will be needed to cope with temporal variability.

In spite of considerable efforts over the last few decades, the problem of how to build cheap, reliable, rapid, non-destructive sensors for soil water has still not been solved. As water is the single most important factor in soil phys-

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ics and in soil management, the importance of developing better ways of measuring it cannot be overemphasised.

It is likely that remote sensing techniques will steadily get better although it is unlikely that the absolute accuracy of measurements made in this way will ever equal those from 'close-contact' measurements on samples. Most remote sensing techniques are sensitive only to the condition of the surface layer of soil. Problems of how to remotely monitor soil down to the rooting depth of crops (say 1.5 m) still remain.

Mathematics and modelling

We cannot claim to 'understand' a system unless we can predict the re- sponse of the system to a wide range of conditions considered as inputs. On the basis of this criterion, our understanding of the physical behaviour of soil is far from satisfactory. There is still a vast number of input conditions for which we are not able to predict soil responses with sufficient accuracy.

How do we improve our understanding? There are several approaches to this question. First, we can seek analytical solutions to problems. This ap- proach can be used only for cases where all aspects of the problem can be precisely defined. The approach involves the application of the known laws of physics to derive mathematical relationships between basic, measurable physical variables such as viscosity, surface tension and density. Second, for cases where the system is not so well defined, such as cases where biological processes are involved, it is possible to derive empirical relationships, such as regression equations, to describe the responses in terms of the inputs. Third and finally, for very complex systems such as the water balance of a soil over a season or the development of a crop, where much of the behaviour is non- linear and where several different processes contribute to the overall devel- opment of a system in time, it is necessary to run simulation models on computers.

If we are to increase our understanding of soil physical processes, then it is essential that we concentrate on the development and improvement of me- chanistic models based as much as possible upon the established laws of phys- ics rather than on 'black-box' or 'transfer function' type models. Although the latter have a prediction capability under special circumstances, they do little to increase our understanding.

We must improve the ability of soil physics to predict soil responses. This will require the development of improved models of all kinds of soil behav- iour by all three of the approaches described above.

THE FUTURE

It is an exciting time for soil physics. The once passive recognition of soil physics as having the potential to significantly improve our understanding of

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soil processes is being replaced by a clear acceptance of soil physics as an equal partner alongside the other branches of science being applied to agri- cultural and environmental problems. In many cases, such as pollutant trans- port through soils and microbial interactions in soils, the understanding of specific soil physical processes is the rate limiting step to an adequate com- prehension of these processes. Indeed without acceptance and greater under- standing of the natural heterogeneity present in soil and how it influences soil processes, the predictive power of models will always be suspect.

In this editorial we have tried to pinpoint specific areas which we feel will not only improve our understanding of real soils, but will also help in pro- moting a closer cooperation between the sciences. Many of the environmental problems facing us today are extremely complex and require multi-discipli- nary approaches. This requires a willingness of soil physicists to work coop- eratively with scientists from other disciplines. We must recognise that the days when it was possible to work in isolation have gone, probably for ever.

It is also vital for soil physicists to be much more active in promoting their subject than has been the case in the past. We must never miss an opportunity to point out what soil physics is and in particular what it can offer to further advance the knowledge of the soil's fundamental processes. To gain and sus- tain adequate provision of financial support for our work promotion of our subject is vital. One of our main aims must be to attain greater recognition by scientific administrators of the role that soil physics can and must take.

If we can promote our subject in this way and if we can continue to develop our science, then we shall enter the new millennium with a healthy, dynamic and evolving soil physics which will be well equipped to develop strategies for the proper management of the soil resource into an uncertain future.