Soil & Tillage Research, 8 (1986) 303--306 303 Elsevier Science Publishers B.V., Amsterdam --Pr in ted in The Netherlands

THE EFFECTS OF TILLAGE AND TRAFFIC ON SOIL STRUCTURE*

B.D. SOANE**

Scottish Institute of Agricultural Engineering, Bush Estate, Penicuik, Midlothian EH26 OPH (Gt. Britain)

F.R. BOONE***

Tillage Laboratory, Wageningen Agricultural University, Diedenweg 20, 6703 GW Wageningen (The Netherlands)

(Accepted for publication 27 November 1985)

INTRODUCTION

The Chairman and Rappor teur proposed that the effects of traffic on structure should be added to the original theme "Soil structure and tillage" since tillage and traffic were complementary components of the same system, and similar methods of measuring soil structure were used in study- ing both operations. A number of topics for discussion were suggested to the participants and the discussions are summarised in the following sections.

CHARACTERISATION OF SOIL STRUCTURE

This topic was clearly of major interest and it was thought to be im- por tant that participants of the workshop should be aware of the discussions held earlier during the conference on this topic by an informal group. P. Groenevelt summarised the proposals that were made. It was felt by the informal group, as well as by many participants at the Workshop, that an ISTRO working group or commission on tillage-related soil physical properties would be desirable. The Workshop agreed that those interested should be listed to aid the exchange of research reports, and proposed that a meeting of the working group be held during the l l t h ISTRO Con- ference in 1988. Several ideas for a central theme for this meeting were suggested. Announcements concerning the activities of the group could be made periodically in the ISTRO newsletter. For the time being, P. Groen- evelt (University of Guelph, Canada) and A.R. Dexter (University of Ade- laide, Australia) will act as Chairman and Secretary, respectively.

*Report of a workshop discussion at the 10th Conference of ISTRO, 8--12 July 1985, at Guelph, Ontario, Canada. * *Chairman. ***Rapporteur.

0167-1987/86/$03.50 © 1986 Elsevier Science Publishers B.V.

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STANDARDISATION OF MEASUREMENTS OF SOIL STRUCTURE

Several participants considered that the standardisation of measure- ments of soil structure was a major point to be discussed by the working group on tillage-related soil physical properties, but this view was not shared by everyone. It was mentioned that at least 2 books on physical assessment of soil structure exist (Black et al., 1965; De Boodt et al., 1967), and that a third will appear by the end of 1985 (Burke et al., 1986). It was felt to be important that during the next meeting, the methods described in these books should be discussed from the point of view of those working in tillage and traffic studies.

NEW METHODS AND TECHNIQUES FOR THE ASSESSMENT OF SOIL STRUCTURE

The following fields of interest were brought forward as having relevance to the need for new methods and techniques. (a) The dynamic aspects of the soil physical properties which depend on soil structure. Soil structure should be quantified in terms of the dynamic action of root growth, but also of subsequent dynamic root function. For example, soil aeration is a function of the supply and the consumption of oxygen, which are both t ime<lependent, dynamic aspects. (b) The characterisation of the direct effects of tillage tools on soil struc- ture. (c) The effects of changes in soil structure upon soil water matric potential. (d) A soil structure-rootability index dependent on soil aeration and mech- anical impedance, and related to soil water matric potential. (e) The measurement of penetration resistance by a technique better able to indicate the likely mechanical impedance encountered by growing roots. It was stressed that the methods presently available, although generally standardised, are not fully adequate for this purpose and have been devel- oped mainly from an engineering point of view. It was considered crucial that the methods used should be relevant to the objectives of the study.

STRUCTURAL HOMOGENEITY

The workshop recognised that soil structural variability is a very im- portant but complex aspect of soil structure. Recently, in soil science and soil survey studies, much attention has been paid to soil spatial variability. Various statistical procedures are available to define spatial variability of soil features or properties (Nielsen and Bouma, 1985). The observed variability is related to the dimensions of the undisturbed field-samples used in soil structural analysis.

A few new approaches and suggestions for further research were briefly mentioned. The use of micro-penetrometers and the application of sto- chastic analysis in root penetration studies seem to be promising. In the

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I.S.S.S. Subcommission B (Soil Micromorphology), at tention is being given to the variability of pore systems using thin section analysis. Anomalous results, like those on soil strength and soil porosi ty as influenced by previous soil management practices, and the existence of thixotropic effects also underline the necessity of a bet ter understanding of soil structure. Deter- mination of optimal soil structure for root growth and plant development and its variability was felt to be a key problem. Since the root system occupies a very small percentage of the total soil volume, only a small but well-disturbed and interconnected part of the soil needs to have a good rootabili ty. There is a clear role here for soil morphological measure- ments in tillage and traffic studies.

OTHER PROBLEMS AND FUTURE RESEARCH

Several participants no ted the important effects of root systems and other biological features on soil structural properties, such as infiltration rates and soil compactabil i ty. However, very little is known about the rate at which such improvements occur.

During the discussion, it appeared that basic problems are often tackled by sophisticated means, but that on the other hand, soil scientists and extension workers desperately search for rapid, simple and inexpensive tools in order to be able to advise farmers. A first step in translating re- search results into practical terms is to classify soil series, as defined in soil survey, into several soil structure classes which can be detected with simple techniques in the field. Measured water permeabilities are likely to be associated with each of the soil structure classes. Efforts should be made to develop a minimum set of measurements to be undertaken in order to be able to advise farmers about soil structural problems. Perme- ability measurement with dyes could be helpful because it reveals spatial variability in soil structure. The already mentioned stochastic approach to soil structure should be applied to heterogeneous situations and the method should be adapted for field application. Since many soil physical parameters are interdependent, special at tention should be paid to inde- pendent soil factors, of which at least one should be soil structural stability.

The anomalous results which are obtained are sometimes very valuable and should be analysed more closely. In these situations, mean values are less important than the information obtained, showing that some factor has obviously been overlooked or under-estimated. In the range of process- limiting values, insignificant changes in a certain factor, e.g. porosity, may have a large effect, for instance on soil aeration.

More should be known about the possibilities that waste materials, which sometimes are available at low cost, can offer in improving soil structure. In addition, there is a need for further information on the effects on soil

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s t ruc ture o f the dep th and in tens i ty of mixing organic m a t t e r into surface soil layers.

CONCLUSION

The 48 par t ic ipants o f 21 count r ies shared the view tha t : (a) more and co-opera t ive ef for ts should be made to develop new m e t h o d s

and cri ter ia fo r a m o r e adequa te charac ter isa t ion o f soil s t ruc ture ; (b) special a t t en t i on should be given in t ranslat ing research results fo r

pract ical field applicat ions.

REFERENCES

Black, C.A., Evans, D.D., White, J.L., Ensminger, L.E. and Clark, F.E. (Editors), 1965. Methods of Soil Analysis, Part 1. Physical and Mineralogical Properties, Including Statistics of Measurements and Sampling. American Society of Agronomy, Inc., Madison, WI, U.S.A., 770 pp.

Burke, W., Gabri~ls, D. and Bouma, J. (Editors), 1986. Soil Structure Assessment. EEC- Brussels. Balkema Publishers, Rotterdam, The Netherlands, in press.

De Boodt, M., De Leenheer, L., Frese, H., Low, A.J. and Peerlkamp, P.K. (Editors), 1967. West-European Methods for Soil Structure Determination. The West-European Working Group on Soil Structure of the International Soil Science Society. The State Faculty of Agricultural Sciences, Ghent, Belgium. (Loose-leaf + supplements I and II, 1970).

Nielsen, D.R. and Bouma, J. (Editors), 1985. Soil Spatial Variability. Proceedings of a Workshop of the I.S.S.S. and the S.S.S.A., 30 November--1 December 1984, Las Vegas, NV, U.S.A. Pudoc, Wageningen, The Netherlands, 243 pp.