ENABLE
NEWSLETTER OF THE ASSOCIATION
FOR BETTER LAND HUSBANDRY
NUMBER 14, JANUARY 2002
Editorial
Switching Emphases
Articles:
Shifting Views on Land Degradation
– T.F.Shaxson
Conservation Calypso – John Linsley
Bookshelf:
‘Conservation Agriculture – a Worldwide Challenge’ – ECAF and FAO
‘Subsoil Compaction : Distribution, Processes and
Consequences’ – R.Horn et al.
‘Shifting
Ground : The Changing Soils of
‘Soil
and Water Conservation Engineering’ – R.Suresh
‘Dynamics
and Diversity : Soil Fertility and Farming Livelihoods in
Appreciation
ABLH-Kenya :
Milestone – T.F.Shaxson
EDITORIAL
SWITCHING EMPHASES
This issue contains materials which challenge what have
been common views about land degradation and the gloomy predictions about falling
productivity which have generally accompanied them. Is land
degradation is as rife as is commonly supposed? Peter Lindert’s
book, reviewed here, challenges the all-too-common assumption that signs of
erosion necessarily indicate that agricultural soils are degrading irreversibly
because they are non-renewable resources over the short term. The
paper ‘Shifting Views on Land Degradation’ complements this with an assertion
that if there is sufficient organic matter and organic activity in
the soil, soil may be forming more quickly from the top downwards -- by
biological means -- than from the bottom upwards by chemical/physical means.
Evidence from research and observations of successful direct-drilling systems
in
A second factor deserving more attention, also signposted
here, is how we interpret the apparent effects of observed runoff and
soil loss on yields. As long as ‘soil fertility’ is deemed to
inhere predominantly in the chemical characteristics of soil, then loss of soil
in runoff waters, together with associated nutrients, will continue to be
assumed to be the chief cause of yield decline. This may not always
be justified. Especially in the seasonally-dry tropical and
subtropical regions, short-term water-stress in plants, induced by insufficient
soil moisture which is readily-available to roots, can have perceptible effects
on plants’ function and growth within a matter of days, whereas supposed effects
of soil loss on yields may only be perceptible after a season or more.
Where are the research data that disentangle these two effects when runoff and
erosion are confounded together in results from e.g. erosion/runoff plots?
An implication is that the loss of potential soil moisture as runoff has a
so-far ignored effect which could be proportionately greater than that of loss
of physical+chemical components of soil fertility. Jon Hellin and
Martin Haigh have data from field plots in
The Think-pic on p.23 proposes that the difference in
yields between those ‘before’ and ‘after’ erosion is better explainable by the
differences between the soil conditions on these two occasions, as this affects
root-zone conditions, than by the quantity of eroded/lost soil collected in the
measuring device. The often lower-quality post-erosion (sub)soil
which has been exposed may be of poorer physical/hydric conditions as well as
being of worse chemical/biological conditions. But the converse can also
be true, where subsoil exposed by erosion may provide better conditions for
rooting than the lost topsoil did. Difference between
characteristics of a soil before and after erosion seems to offer a
more-logical explanation for yield differences than do the mental gymnastics
needed to model erosion/yield relations based on the prior
assumption that loss of soil is always the prime cause of yield decline.
The Editor.
.oOo.
SOIL AS AN ORGANISM?
“ I shall argue that
the key to a comprehensive theory of living systems lies in the synthesis of
... two very different approaches, the study of substance (or structure) and
the study of form (or pattern). In the study of structure we
measure and weigh things. Patterns, however, cannot be measured and
weighed: they must be mapped. To understand a pattern we must map a
configuration of relationships. In other words, structure involves
quantities, while pattern involves qualities.
“ The study of pattern
is crucial to the understanding of living systems because systemic properties
... arise from a configuration of ordered relationships. Systemic
properties are properties of a pattern. What is destroyed when a
living organism is dissected is its pattern. The components are
still there, but the configuration of relationships among them – the pattern –
is destroyed, and thus the organism dies ...
“ Whenever we
encounter living systems of organisms, parts of organisms, or communities of
organisms – we can observe that their components are arranged in a network
fashion. When ever we look at life, we look at networks”.
Fritjof Capra, 1996, ‘The Web of
Life’.
.oOo.
ARTICLES
SHIFTING VIEWS ON LAND DEGRADATION
(Prepared for 1st
meeting of the Technical Advisory Group and the Steering Committee of the
project
LAND DEGRADATION ASSESSMENT IN
DRYLANDS - L.A.D.A.
F.A.O.,
©Perrmission to make digital or
hard copies of all or part of this work for personal use is granted by FAO
without fee provided that copies are not made or distributed for profit or
commercial advantage and that all copies bear this notice and the citation.
T.F.Shaxson
INTRODUCTION
Clarifying our concern
Concern about land degradation in drylands can be restated
as being our increasing uncertainty about sustainability in future of (a) water
supplies – soil moisture, groundwater, streamflow – and (b) plants’ production
of biomass.
Even where the climatic pattern at a particular place may
not show signs of declining rainfall, sequences of vegetation surveys in most
situations of land degradation show a tendency towards increasing aridity and
more-xerophytic vegetation. This points to a problem of increasing
water deficit from the viewpoint of the plants.
We are all uneasy that past efforts to avoid or halt
degradation, to rehabilitate land, to safeguard soil fertility, and to make the
results sustainable have not been notably successful. They are not
generally underlain by a sufficiently sound basis for achieving
conservation-effective, productive and sustainable results in terms of
biomass and streamflow. We are still uncertain even after 80 years’
work. There is still too little critical discussion of ‘conventional
wisdom’, hallowed by years of
repetition and tramlined thinking. Farm
families are generally not enthusiastic about conventional ‘SWC’
recommendations, even though they are concerned about yields, stability of
production and availability of water.
Can our present answers work?
It is commonly assumed that ‘SWC’ - which is
commonly characterized in reality by a largely mechanical approach to stopping
runoff and soil loss - can solve the problem, and that the difficulty lies in
getting farmers to adopt and apply our technical recommendations.. It is
sometimes presumed that we non-farm agriculturists (NFAs) can design better
production systems for farmers to improve matters.
Improvements in institutions, marketing, policies, and further applications of
significant amounts of money are often proposed, based on the prior assumption
that runoff and soil erosion are the prime causes of the problem.
We should be bold enough to question this. In what ways can the
greater precision of LADA results contribute to lasting improvements if
these basic assumptions are incorrect?
We should acknowledge that there is a ‘log-jam’ in
thinking, and a need to review and re-appraise current bases and
directions of approach to reversing the trends.
From unjustified assumption to ecological realities
There is still a widespread assumption, evident in current
literature, that land degradation is some sort of invisible land-damaging
monster, in which soil erosion is the main dynamic: ‘the onward
march of land degradation’, ‘ land degradation is rampant’, ‘the scourge of
land degradation’, ‘threat of soil erosion’, ‘the cancer of erosion’.
From this arise phrases such as: ‘the need to combat land degradation’, ‘the
war against erosion’, ‘fighting erosion’ etc. Therefore we have attempted
to stop a supposed cause (erosion) by holding back soil and water, with a nod
to the importance of organic matter.
It is not the case. It doesn’t fit the
observable facts; nor is there any constant, clear and direct cause/effect
linkage between soil loss and yield decline.
In reality, runoff and erosion are foreseeable
ecological consequences of prior changes to soil and vegetation, in
particular diminution of (a) porosity from the surface downwards, and of (b)
organic cover to and content within the soil. They result from of
alteration (natural or induced) of interactions climate/weather x slope x
soil x geology x organisms x
hydrology x management, and represent the
consequent processes following such alteration.
Differences between soil conditions before and after
erosion provide a better explanation of yield differences than do the
quantities of soil lost.
Surveys of land degradation that map vegetation changes
and visible occurrence of erosion, salinisation etc. are mapping symptoms
of prior plant and soil damage (or of a drier climate) rather than mapping
causes of land degradation.
All soil and vegetation can be degraded; therefore
all is ‘at risk’. Assessed differences between areas of their ‘risk
of land degradation’ in fact reflect the differing relative rates of
degradation down towards zero-productivity, if each area is exposed to the same
climatic conditions. Actual rates of decline towards zero
productivity are governed by the conditions of climate, landscape, soil, cover
and management.
‘Vulnerability’ evidently relates to a soil’s fragility –
or ease with which its soil architecture can be disrupted by rain impact,
runoff, tillage, trampling etc. – and the possibility of losing some or
all of its porosity.
‘Resilience’ of soils and vegetation refers to their
inherent abilities to recover after being damaged. Resilience is
lost if those abilities are obliterated.
KEY FACTORS IN REVERSING LAND DEGRADATION
Soil porosity
Groundwater and regular streamflow depends on rainwater
being able to enter through the soil surface. This infiltration
capacity is determined by the soil’s porosity from the top millimeter
downwards. Once that is even temporarily saturated, runoff can occur.
A porous cover which protects the surface against rainfall impact significantly
affects whether, or how soon, such saturation will occur. The
porosity of the uppermost soil layers determines the partition of rainfall
between runoff and infiltration.
That which infiltrates the surface is further divisible
between that which is taken up through transpiration of plants, that which is
retained within the root-zone at tensions which make it unavailable to plants,
that which is retained below the root zone at any tension, and that which
percolates further down towards groundwater.
Physical features of soil porosity which are good for
rainwater penetration are simultaneously good for plants’ root-activity.
Soil fertility
In addition to sunlight and freedom from pests and
diseases above-ground, plants’ functioning depends on the quality of the soil
as an environment for roots.
Soil fertility derives from the interactions between its
biological x chemical x physical x hydric
components, not just from the chemistry of the soil-solution alone.
Continuous large pores and tunnels facilitate both
rainwater movement, gas-exchange of O2 and CO2 ,and the
spread and expansion of roots. The capacity to retain plant-available soil
moisture depends on the spaces within the soil architecture which form the soil
pores of a range of sizes, their volume and size-distribution. In
this context the spaces within soil architecture are of greater significance
than the surrounding framework of physical particles.
Biotic activity in the soil causes transformation of
organic materials into humic materials which are important for both capture and
release of plant nutrients, and for the gumming-together of physical
particles to form porous soil aggregates, the components of soil architecture.
This biotic activity is dependent on there being a sufficient and recurrent
supply of organic materials available as a substrate for meso- and
micro-organisms -- plants, animals and fungi including mycorrhizae.
After porosity is lost for whatever reason, in most situations it can only be
regained through such biotic activity, contributing to the characteristic
of soils’ resilience.
The chemical conditions in the soil, in particular pH and
the quantity, proportions and availability of nutrients, are moderated to
greater or lesser extent by the organic materials and processes in the soil.
It is probable that organic acids from biotic
transformations of organic matter have the effect of liberating nutrient ions
from mineral particles in the upper soil layers.
Experience with zero-tillage systems in Brazil suggests
that minimal disturbance of soil architecture, once brought to good physical
condition, is beneficial to root- functioning and thus to crop yields.
Water-stress in plants causes quicker and more frequent
growth-inhibition than loss of soil. What proportion of yield-loss which
is attributed to loss of soil is in fact due to insufficient soil moisture to avoid
growth-inhibiting water-stress within plants?
Soil degradation makes cropping more risky in face of
drought by diminishing the proportion of rainwater which actually becomes
soil moisture for the active functioning of roots and of other soil-inhabiting
organisms.
Soil fertility (as indicated by its capacity to produce
biomass) is diminished by factors which singly or jointly inhibit the activity
in the soil of soil-inhabiting organisms, including plants’ roots.
Degradation of soil is a consequence of rate of damage to
the soil
ecosystem exceeding its rate of self-recuperation.
Soil improvement is achieved when the rate of self-recuperation exceeds the
rate of any degradation.
Similarly, degradation of vegetation occurs when the rate
of above-ground damage, usually by excessive removal, exceeds the plant’s rate
of self-recuperation by using its accumulated stored reserves.
Even the most-improved plant genetic potentials are not
expressible if soil conditions are unsuitable for exuberant root-growth.
Consider ‘soil’
before ‘land’ for rehabilitation
‘Deforestation’, overgrazing’ and ‘over-cultivation’ are
frequently cited as reasons for runoff and erosion, but three features they
each have in common are the keys to understanding:
|
>Frequently cited> v
Features in common v |
‘Deforestation’ |
‘Overgrazing’ |
‘Over-cultivation’ |
|
Loss
of organic cover on the soil |
ß |
ß |
ß |
|
Loss
of organic matter substrate for bugs |
ß |
ß |
ß |
|
Loss
of soil architecture/porosity |
ß |
ß |
ß |
The above are features of ‘soil’ rather than of ‘land‘,
(though soil is one component of land), and occur at microscopic scale -
root-hairs, micro-pores, soil organisms.
BASES FOR EFFECTIVE AND LASTING IMPROVEMENT
Objectives
The objectives must be to provide better environments for
bio-diversity and biotic activity in the soil in order to achieve:
-
reversal of soil degradation ;
-
recuperation of damaged areas;
-
increase of soils’ resilience to future damages;
-
sustainability of these improvements.
To reach these all together requires rapid simulation and
improvement of the beneficial effects of rotational ‘fallow’ periods for soil
self-recuperation. Key features are (a) biotic rebuilding of porous
soil architecture and (b) bringing back nutrients sufficient to satisfy plants’
requirements over time. The quickest results will be achieved when
reduction in the prior severity of damage is paralleled by large improvements
in the soil’s biological capacity for self-regeneration of the two above
features. This leads to restoration of the complexities of soil
fertility and a return to a simulated ‘forest-floor’ condition of the soil,
together with improvements in water relations, expressed via plant growth and
improvements in groundwater and streamflow.
This implies the need to:
-
Determine, in each situation, which factor(s) of the soil degrade water
relations and inhibit proliferation/activity of organisms in the soil,
including roots;
-
Define and enable the carrying-out of appropriate actions to rectify them, so
as to encourage biotic self-recuperating processes, leading to increased
fertility, soil health, resilience, sustainability.
Soil is a
self-renewable resource
It is useful to ask why the situation is not even worse,
as land degradation has been going on for so long. The answer
provides the seed-idea for its improvement.
Life itself – irrespective of its form of expression –
shows an active propensity, or vitality, to colonise, recolonise,
recuperate and modify environments (on land, in water) to suit itself, e.g.
recovery of rangeland vegetation when given respite from hard grazing;
regeneration of soil architecture by Weeping Lovegrass after tobacco.
“Schrödinger concluded that, metaphorically, the most
amazing property and capacity of life is its ability to move upstream against
the flow of time”. Lovelock.
This life-principle, in the forms of living plants,
animals, fungi, bacteria, etc., provides the common thread running through
‘ecosystem’ + ‘soil health’ + ‘self-recuperation capacity’ + ‘resilience’ + ‘sustainability’.
Thus, in the presence of enough water, soil can be
self-sustaining, and self-renewing after degradation, via organic matter
transformation by biological dynamics, including the formation of humic gums re
soil architecture, porosity, and via organic acids probably de-composing
mineral fragments in the upper soil profile, releasing nutrient ions from the
top downwards.
In the absence of organic matter soil organisms cannot
function; conversely, raw organic matter in the soil is of no value in
the absence of biotic activity.
Validation of
the biotic principle in soil improvement
Consider the Brazil zero-tillage situation (rotations +
cover-crops/green-manures + least soil disturbance): positive changes in
organic matter levels, organic activity, rainwater absorption, soil and crop
resilience, soil health, streamflow hydrology , sustainability, profitability,
livelihood improvement etc., resulting from encouragement of soil-biotic
improvement. There has been farmer-led exponential spread from 0 to
13 million ha in 30 years, spreading across a wide range of climatic and
landscape conditions.
Management is the factor most-adaptable to get optimum
match between soil condition and characteristics of preferred/required type of
land use, more than rigid adherence to formal land-classes.
IMPLICATIONS
For LADA work
(from satellite imagery to ground-truthing)
The fourfold characteristics of soil fertility suggest
what to survey and monitor under LADA, from which key thrusts of rehabilitation
could be identified for each soil unit, within the wider context of land:
-
Chemical: status and plant-availability of nutrients; pH;
-
Physical: soil architecture – water-stability of aggregates,
porosity, pore-size distribution /water-holding capacity, presence of limiting
layers;
-
Biological: biological activity (as respiration), organic matter
content and transformation products, species-composition of communities of organisms,
etc.,
-
Hydric: volumetric capacity for plant-available water (x pore-size distribution above); duration of
plant-available water through the year.
There is a case for taking a strongly pro-biotic approach
to restoring soil productivity now and to sustaining land uses in the future.
For the
definition of soil
Should we properly call the shallow zone at the interface
between rock and the atmosphere ‘soil’ if it has no biotic component ?
Soil should be valued more for the dynamics of its living
components than for pedological characteristics of arrangement of horizons.
In order to focus on appropriate actions we may invert
the emphases in definition of ‘soil’ in any place:
-
not primarily as an inorganic, physical unit of
mineral particles, air, water and nutrient ions which contains and is
interpenetrated by organic matter and organisms in three spatial dimensions;
-
but primarily as a complex and dynamic subsurface ecosystem
of diverse living organisms (including plant roots) and their transformed
organic/humic
products, which inhabits and
interpenetrates an inorganic matrix of mineral particles, air, water and
nutrient ions, and which changes over the fourth dimension of time.
For scale of actions
While survey work may be undertaken at macro- scale from
satellites etc. downwards, soil-recuperation work must have its effects
from micro-scale upwards.
For thinking
In order to envisage what needs attention to reverse land
degradation and to encompass sustainability, think like a river, think like a
root, think like a soil organism.
.oOo.
CONSERVATION CALYPSO
(Handle’s Water Conservation
Music)
(Sung to the tune of a good calypso, changing key
up one semitone periodically
to relieve monotony).
(from: A Land Husbandry Manual (
John Linsley
For fast relief from loss of earth,
For fast relief from famine and dearth,
Take conservation and good husbandry
For maximum sustained productivity.
Erosion proceeds geologically
But accelerated by man’s activity;
By chopping trees and removing crop trash
The soil is exposed to raindrop splash.
Take a look for yourself, you are advised,
Go out when it’s raining, you’ll be surprised:
The power of rainfall you’ll never forget,
Especially as you are soaking wet!
Biological conservation has the greatest effect
For the minimum degradation you can expect:
You’ll be amazed how maize can look sweet and pure
Up to its tassels in cow manure.
Mechanical measures are a final resort:
With many dangers they are fraught.
Their aim: to cause runoff concentration,
So stay home rather than do bad conservation.
One officer thought, just for a change,
Mechanical works in reverse he’d arrange:
Waterways last, he began with a bund;
Now he collects for the disaster fund!
The planning process is the key
For developing good land husbandry.
Take facts about the land, and these assess
In systematic fashion, to avoid a mess.
At aerial photographs take a look
At the land lying open like a book.
Mark crests and waterways line by line
And finish up with square eyes nine by nine.
The object is to lay out lands
In harmony with topography on which it stands,
So that farmers can plant when the rains begin
And celebrate big harvest with local gin.
So in painstaking fashion do a soil survey
So that poorer lands can be kept for hay;
Show land capability; then forget the lot:
Every square inch must bear a plot!
On a contour map draw a layout design;
Sitting in the office this sure looks fine,
But now on you is the final laugh:
You’ve got to lay it out with level and staff.
For fast relief from loss of earth
For fast relief from famine and dearth
Take conservation and good husbandry
For maximum sustained productivity!
.oOo.
LAND
“A
land ethic … reflects the existence of an ecological conscience, and this in
turn reflects a conviction of personal responsibility for the health of the
land. Health is the capacity of the land for self-renewal.
Conservation is our effort to understand and preserve this capacity”.
Aldo Leopold
‘A Sand
.oOo.
BOOKSHELF
CONSERVATION AGRICULTURE : A
WORLDWIDE CHALLENGE
Vol. 1 : Keynote
contributions;
Vol. 2 : Offered
contributions.
L.Garcia-Torres,
J.Benites, A. Martinez-Vilela (eds.)
Papers of 1st World
Congress on Conservation Agriculture
jointly organized by the Food and
Agriculture Organisation of the United Nations (FAO) and the European
Conservation Agriculture Federation (ECAF).
Córdoba (
ISBN 84-932237-0-0 (set).
Note by Francis Shaxson.
Worldwide there is, at last, a burgeoning interest in the
better husbandry of land – as exemplified by what is now commonly called ‘Conservation
Agriculture’ - especially since the astonishingly-rapid and farmer-led
expansion of direct-drilling technologies in Latin America since the 1970s.
‘‘Conservation Agriculture’ ... implies conformity with all three of the
following general principles: no mechanical soil disturbance and direct
seeding or planting; permanent soil cover, making particular use of crop
residues and cover crops; judicious use of crop rotations’ (from the
Preface in Vol.1). Not only has rainwater infiltration vastly improved,
costs of production fallen, yields stabilized and often increased, resilience
in the face of drought risen, and both social and economic conditions of people’s
livelihoods improved, but also erosion of soil, volumes of runoff, flood peaks
and damage to infrastructure downstream have greatly diminished.
Until now, much of the literature on the subject which is
relevant to the tropics and subtropics has been written in Portuguese or
Spanish, but with the publication of these two volumes a wide range of reports
are brought together and presented in English. The writings provide
an encouraging picture of experiences and results which suggest that better
management of the life in the soil provides a positive and dynamic basis for
halting, regenerating, stabilizing and improving the health and productivity of
root-zones, while simultaneously improving the hydrology of soils themselves
and of the catchments whose surfaces they clothe.
The section headings in Volume 1 are: Conservation
Agriculture : Global Improvements; Farmer Experiences with Conservation
Agriculture; International Networks for Conservation Agriculture;
Recent Innovations in Conservation Agriculture; Adaptation of the
Agricultural Industry to Conservation Agriculture; Influence of
Conservation Agriculture on the Environment; Socio-economic Perspectives
and Policy Implications for Development. In Volume 2 the contents
are arranged under the section headings: Farmers’ Experiences and
Network[s] on Conservation Agriculture; Environmental Aspects of
Conservation Agriculture; Soil Quality and Conservation Agriculture;
Nutrient Status and Fertilisation; Cover Crops; Weeds and
Herbicides; Enhancement of Biological Activity in Conservation
Agriculture; Agronomic Studies; Other Technical Studies;
Socio-economic and Policy Perspectives. From the Table of Contents
alone one can see the geographical spread of interest and experience:
Argentina; New Zealand; Western Europe; USA; Brazil;
South Asia; Kazakhstan; Australia; Germany; Uzbekistan;
Mexico; Romania; Vietnam; Madagascar; Italy;
Zambia; Ghana; Kenya; Tanzania; Uganda; Cameroon;
Mongolia; Ukraine; Spain; Canada; Panama;
Morocco; Mozambique; Bolivia; Siberia; Egypt;
Cuba; France; Côte d’Ivoire; Scandinavia; Zimbabwe;
Venezuela; Chile; South Africa; Ethiopia; Costa Rica.
These volumes give an overview of what is going on at
present across the world, and provide addresses of contributors whom one may
contact in order to gain more detail (also watch out for Soils Bulletins and
Land and Water Bulletins relating to the subject which are published by
FAO, some of which have been mentioned in earlier issues of ENABLE).
This FAO/ECAF collection of papers provides strong support
for the principles and concepts which have been espoused by ABLH since its
inception nine years ago. It is encouraging to find so many other
people are fired-up with the subject. Their enthusiasm and
experiences – especially those of farmers large and small – indicate
practical and positive hope for the future.
.o0o.
SUBSOIL COMPACTION:
DISTRIBUTION, PROCESSES AND
CONSEQUENCES.
Horn, R., van der Akker, J.J.H.,
and Arvidsson, editors, 2000: Reiskirchen, Catena Verlag: Advances in
Geoecology 32. xi + 462pp. ISBN 3-923381-44-1.
Review by Martin Haigh
Subsoil compaction is an increasingly serious problem for agriculture.
Worldwide, the productivity of >80 Mha of land may be affected by soil
(possibly including subsoil) compaction, including perhaps 30 Mha in
Several reports bring new insights on ways in which
mechanised agriculture interacts with soil qualities. In
Northern Germany, traffic ruts may carry up to 50% of
runoff and plough pan compaction increases interflow by a factor of 4 in plot
studies of moderate to steep slopes. Arvidsson et al. confirm that larger
machines create greater subsoil compaction, even when surface compaction rates
may be similar. By contrast, Kulli et al. suggest that the main impact of heavy
machinery compaction is in the topsoil rather than subsoil. Here, infiltration
is reduced and water flow forced into macro-pores, mainly wormholes, thus
bypassing the main root zone, which undergoes less wetting. However, Weisskopf
et al show that, at 35-cm depth under conventional tillage, lateral
displacements lead to upward movements in the soil and increased porosity,
mainly as discontinuous macro-pores. Horn and Rostek's literature review
confirms that pore-continuity measurements, based on air-permeability, are more
useful predictors of soil compaction than bulk density. Hallet finds a log-log
relationship between soil aggregate size and strength and links soil fracture
propagation to pore structural characteristics.
Warkentin discusses the role of clay. He points out that
soil shearing can remove organic coatings from clay surfaces, exposing new
surfaces that can help bond compacted soil structures. The reorientation
of clays caused by compaction allows greater inter-particle repulsion, greater
swelling pressure hence reduced aggregate stability. In addition, the enforced
proximity of the clays decreases the scope of organic-inorganic bonding and
alters the habitat towards anaerobic conditions. Ion layers about the clays are
compressed leading to greater acidity, enhanced hydrolysis and greater chemical
weathering of minerals.
Inevitably, much effort is devoted to modelling the interactions
between agricultural practices, soil hydrologic regime and subsoil compaction.
Koolen et al. suggest that it is possible to estimate soil parameters needed
for an advanced finite element modelling (FEM) code from existing soil data.
Mouaxem and Nemenyi also support the use of FEM with its ability to diagnose
the impacts of different tillage tools. Among the plethora of models on parade,
SIBL, which calculates the effect of soil water balance and mechanical
resistance on crop yield, offers strong claims to predict the impacts of
subsoil compaction, albeit through the medium of bulk density changes. As
for the soil-tyre /soil compaction interaction models, comparative testing by
Fedo et al. suggests that the super-ellipse method gives better results than
either the pure ellipse or Upadhyaya and Wulfsohn methods. Alternatively, Berli
et al find from empirical work that preconsolidation load is a useful parameter
for assessing the initial compaction sensitivity of field soils. Voorhees
confirms that the impact of heavy traffic is greatest in the first year
following trafficking, while Davidowski et al describe methods for determining
precompaction stress.
It is widely agreed that the main reason that subsoil
compaction deserves attention is its impact on crop yields. Voorhees confirms,
from a range of
The traditional solution is deep tillage and subsoiling.
In
Preventing soil compaction may be easier than cure.
Arvidsson et al. hope to generate guidelines for use by regulators. These will
assess the risk and describe the economic impacts of subsoil compaction in
In sum, a more gentle approach to land management,
involving smaller machines, less trafficking, more organic additions to the
soil, and greater sensitivity to soil vulnerability, may remain the best way of
avoiding the problems of subsoil compaction. However, perhaps, the main
conclusion to be drawn from this book is that there is now a great need for a
systematic overview of this whole subject area.
.oOo.
‘SHIFTING GROUND : THE CHANGING
SOILS
OF
Peter H. Lindert
Review by D. Gale Johnson.
“The generally accepted opinion is that a large percentage
of the world’s agricultural land is degraded and is being further degraded year
by year. The World Map of the Status of Human-Induced Soil
Degradation produced by the United Nations Environment Program in the late
1980s is a major source of such an opinion. Peter Lindert argues,
persuasively in my opinion, that the basis for the conclusion that a
large percentage of the world’s agricultural land is degraded as a result of
human action is wholly inadequate. The evidence used to reach this
conclusion is not derived from historical comparisons of the status of
agricultural lands but on a description of lands at a particular moment in
time. As Lindert writes, “It tries to measure changes over time in
the absence of data over time” (p.21).
“Lindert (Professor of Economics and Director of the
“Such surveys exist in other countries, including the
“That erosion exists cannot be questioned.
After all, the
“Based on the comparisons of the soil surveys in
The decline in nitrogen content of the soil seems to have
little or no negative effects on yield, however, since nitrogen can be and is
added as fertilizer.
“Perhaps the most striking conclusion is that the depth of
the topsoil has not diminished – erosion has not taken a toll on
“In addition, Lindert finds no evidence that the erosion
of agricultural land in
“To summarize the results presented in this very important
book, Lindert shows that for two of the most populous countries in the world farm
people have taken very good care of their land. Yes, erosion exists but
careful analysis is required to determine whether it is human induced and
whether it affects agricultural land. Lindert’s careful analysis
supports two important conclusions, though these conclusions are not stated
explicitly by him. His work confirms that “Farmers are as smart as
the rest of us” and that “Farm people of
“D.Gale Johnson is the Eliakim Hastings Moore
Distinguished Service Professor of Economics Emeritus at the
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Comment: Prof. John F. Timmons of Iowa State University
wrote that, in a situation e.g. where a soil is eroding, the task of
dealing with the problem includes the identification of the ‘problem gap’ – the
difference between, say, the goal-value of soil loss rate (‘T value’) and the
actual value of soil loss. For instance if T is set at 5 t/a/yr and
the actual rate is 18 t/a/yr., the ‘problem gap’ is 18-5=13 t/a/yr.,
representing the reduction in soil-loss rate which needs to be achieved.
In the phase of identifying the nature and scale of the problem one task is to
identify the ‘failure elements’ that are responsible for the problem gap, and
another is to identify the ‘success elements’ that prevented the gap being
larger than it is, or: ‘Why is the situation not worse than it is already?’.
He pointed out that the identification of the success
elements may bring into focus the basis of an appropriate strategy for reversing
the situation.
I suggest that the apparent paradox which Lindert’s book
indicates can be resolved by invoking the positive soil-building effects
of soil organisms acting on organic matter and on mineral particles, as
suggested by Shaxson (as above, and earlier ). While erosion may have occurred in China and Indonesia,
as studied by Lindert, , the self-regenerating capacity of the soils may have
been continually acting to renew the topsoil, thus resulting in little or no
net loss – or even a gain -- of soil depth or of nutrients over time.
T.F.S.
References:
1. TIMMONS J.F., 1983. ‘Economics of natural resource
management applied to soil and water use in agriculture’. Consultant’s
report to FAOP Project BRA/82/011,
2. SHAXSON T.F. 1981. ‘Developing
concepts of land husbandry for the tropics’. In: R.P.C.Morgan (ed.) ‘Soil
Conservation: Problems and Prospects’.
.oOo.
SOIL AND WATER CONSERVATION
ENGINEERING
by
R.Suresh
3rd Edition, 2000; (1st
edition 1993).
ISBN 81-86308-42-3.
Review by Martin J. Haigh