[permaculture] Cyber-Help for Organic Farmers: Let the soil work for us
Lawrence F. London, Jr.
venaurafarm at bellsouth.net
Wed Dec 23 03:34:13 EST 2009
Cyber-Help for Organic Farmers: Let the soil work for us
Let the soil work for us
Elliott, E. T. and Coleman, D. C. 1988. Let the soil work for us. -
Ecol. Bull. 39: 23-32. Reprinted with Ecological Bulletin permission.
Appropriate management of microbial populations in soil can reduce
leakage of excess nutrients from the rooting zone and enhance the
fertilizer use efficiency and agroecosystem production. Manipulation of
the microbial habitat by varying residue and tillage management is an
effective and practicable way to manage soil microorganisms.
Aggregation, pore space and preferential flow are strongly influenced by
cultivation. The architecture of the soil structure can determine the
habitability for soil microorganisms and nutrient fluxes through
agroecosystems. Soil organic matter availability to microorganisms is
related to its position within the soil matrix. A simple hierarchical
model for soil aggregation can explain many aspects of changes in soil
organic matter aggradation and degradation. Likewise, four hierarchical
pore categories are presented which relate to the aggregate structure of
the soil and provide a basis for predicting how soil pore networks
influence ecological relationships among organisms in soil detrital food
webs. Macroporosity is sensitive to variations in cultivation practices
and can increase under no-till management. Less leaching of nitrate was
observed in no-till experimental plots. This was related to increased
infiltration rates and preferential flow of incoming nitrate free rain
water down large pores; this effectively bypasses or short circuits the
nitrate in the surface soil layers. Where soils were tilled, the water
moved down the profile more slowly and subsequently transported more
nitrate deeper. Greater macroporosity and a responsive microbial
community can be used to provide more efficient management of
agroecosystems. Establishment of a new steady state for soils put under
no-till cultivation may take as long as a decade in temperate climates.
E. T. Elliott, Natural Resource Ecology Laboratory, Colorado State
University, Fort Collins, CO 80523, U.S.A. D. C. Coleman, Dept of
Entomology and Institute of Ecology, Univ. of Georgia, Athens, GA 30602,
Management of microbial populations
This paper will concentrate on the general theme of management of
microbial populations through modification of their habitat in the soil.
We submit that it is through the management of the soil detrital foodweb
that we may be able to reduce the intensity of agriculture, but still
maintain the necessary profit margins for successful farming. Through
such management we may be able to increase soil fertility by increasing
components of soil organic matter (SOM), use the microbial biomass to
increase fertilizer use efficiency and enhance soil structure. These
changes can simultaneously result in better infiltration, yet less leaching.
It is probably very difficult to directly control the populations of
organisms in the soil. Perhaps the best way to manipulate the soil biota
is through management of the environment in which they live.
Let the soil work for us
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aggregates in soils graph 1
aggregates in soils graph 2
Fig. 1. The size frequency distribution of soil aggregates in soils that
were vapor wetted (misted) or left dry (slaked) before wet sieving for
native sod soil or cultivated soil. More microaggregates (0.05 - 0.250
mm) result when the soil is slaked than misted because disruptive forces
of escaping air from macroaggregates when dry soils are wet sieved cause
macroaggregates to break down into the smaller microaggregates. There is
a higher proportion of water stable macroaggregates in the native sod
slaked treatment than cultivated soil (Elliott 1986).
Concentration of organic C in soil - graph 3
Concentration of organic C in soil - graph 4
Fig. 2. The concentration of organic C in the different size classes of
aggregates. When misted, the macroaggregates from the native sod soil
have a higher C concentration than those from the cultivated soil. When
slaked, the concentration of C varies much more among size classes but
there are no differences in C concentration between tillage management
treatments (Elliott 1986).
The two key factors are the control of the incoming substrates and
control of the soil structure. We will concentrate on the latter factor.
Bioengineering of microorganisms, selective choice of root symbionts and
introduction of biocontrol agents are other ways in which we may be able
to use soil organisms to our advantage. However, their survival in the
soil can determine the long-term benefits these organisms may have. The
problem of use of introduced organisms reduces, at least partly, to
obtaining an understanding of their survival in the soil environment. We
want to convey that the soil structure plays an integrating role in
bringing soil biological processes together with soil physical and
There are strong feedbacks between the soil organisms and soil
structure. Soil organisms are partly responsible for the formation of
soil structure yet are restricted by it as well. The feedback can be
either positive or negative depending upon the prevailing conditions.
These conditions can be, at least partially, under our control.
Structural controls of soil processes
Soil function is a result of a complex combination of physical, chemical
and biological processes which are played out in a structurally
heterogeneous and materially complex environment (Coleman et al. 1983,
Coleman and Elliott 1987). Ecological interactions among functional
groups of soil organisms, such as competition and predation, can
influence the flow of major elements in ecosystems (Coleman 1985, Hunt
et al. 1987). Control on the lowest level producer (saprotrophic
bacteria and fungi) may be indirectly altered, moving between nutrient
limitation and consumer limitation as the prominence of the top predator
changes. Where ecosystems are dominated by pulsed events such as
rainfall, the transfer of production up the food chain has been observed
(Elliott et al. 1987). To a large extent, the controls on these
processes and the consequences of them are manifest in the soil structure.
In the following section three aspects of soil structure are discussed,
each of which can exert considerable control on how the soil contributes
to ecosystem function. Soil structure controls: (1) the formation and
destruction of soil organic matter, (2) soil porosity and therefore the
activity of soil organisms, and (3) the movement of water and solutes
through the soil profile and therefore the net flux of some elements
through the ecosystem. These structural attributes are not independent
and can be combined to form the framework for an integrated general
description of the functioning of the soil portion of the ecosystem.
Table 1. N mineralization of different aggregate fractions from
cultivated (14 yr) and native sod soil from Sidney, Nebraska. There was
more total mineralization and higher specific rates of mineralization in
the native sod treatment and in macroaggregates.
± SD of N mineralized in 20 d (mg kg-1) with mineralized % of total N in
Soil aggregation and organic matter turnover
Tisdall and Oades (1982) present a conceptual model describing the role
of soil aggregation on organic matter dynamics. There are four spatial
hierarchical levels to this model. At the first level, amorphous
inorganic and organic materials are attached to clay surfaces. These
then attach to microbial debris, encrusting them so that they then bind
together with each other and with primary soil particles forming
microaggregates. The last level is where roots and mycorrhizae bind
microaggregates together, thus forming macroaggregates. The materials
responsible for the high stability of microaggregates are described as
persistent (Tisdall and Oades 1982). At this level, polyvalent cations,
such as iron, aluminum and calcium serve as bridges between the
predominantly negatively charged clays and organic matter (Edwards and
Bremner 1967). The transient agents that bind microaggregates into
macroaggregates may contain organic matter other than just roots and
mycorrhizae (Elliott 1986) and may constitute the important slow organic
matter pool that is conceptually described by Parton et al. (1983, 1987).
Elliott (1986) suggests that it is the intermicroaggregate organic
matter which is responsible for the long term fertility of grassland
soils that are cultivated. Soils were taken from a paired cultivated and
native grassland site (Sidney, Nebraska) and wet sieved. The soils were
either gently vapor wetted before sieving or were left air dried
(slaked). With slaking there is a greater breakdown of soil structure,
hence more microaggregates and less macroaggregates relative to the
vaporwetted samples (Fig. 1). The macroaggregates in slaked soil were
considerably enriched in organic C compared with microaggregates (Fig.
2), even when corrected to a sand free basis (microaggregates and
macroaggregates have different proportions of sand). When soil was vapor
wetted, the aggregate size distribution was similar for the cultivated
and native soil but there was a higher concentration of organic C in the
macroaggregates from the native than cultivated soil. When the soil was
slaked, there were relatively more macroaggregates in the native than
cultivated soil (Fig. 1) and fewer microaggregates. However, the
concentration of organic C was the same in the different aggregate size
classes (Fig. 2).
The higher C concentration in the vapor wetted native sod
macroaggregates resulted in greater stability when slaked than the lower
C concentration macroagregates in the vapor wetted cultivated soils.
This suggests that it is the intermicroaggregate organic matter which
gives the macroaggregates higher organic C concentrations and
stabilities than the microaggregates. There are not enough roots and
mycorrhizae to account for the amount of intermicroaggregate organic C.
When macroaggregates, crushed macroaggregates (the size of
microaggregates), or microaggregates were incubated, more N was
mineralized from the macroaggregates than microaggregates (crushed or
not, Tab. 1). Also, the C/N was lower in micro than macroaggregates,
indicating the more highly processed nature of the SOM in microaggregates.
We suggest an explanation for the difference in the views of Tisdall and
Oades (1982) and Elliott (1986) concerning the agent that binds
microaggregates into macroaggregates. When a native grassland is
cultivated, as that studied by Elliott (1986), there is much
intermicroaggregate organic matter. This declines with cultivation, and,
as it is mineralized, nutrients are released and the macroaggregates
disintegrate into microaggregates. Conversely, when cultivated land is
put into a grass ley, as studied by Tisdall and Oades (1979), the roots
and mycorrhizae may be the initial binding agents. However, plant and
microorganism polysaccharides accumulate between the microaggregates,
and the total soil organic matter levels increase after many years. This
suggests that there is a hysteresis of soil organic matter. The
trajectory of soil organic matter degradation and aggradation may not be
the same. This may be especially true when considering relative changes
in intermediate (~ 50 yr. turnover time) versus old (~ 2000 yr. turnover
time) soil organic matter. The possible mechanism by which soil organic
matter may increase and its relationship to the production of both
micro- and macroaggregation is suggested below.
Oades (1984) postulated that microaggregates are formed at the center of
macroaggregates. Initially, fragments of decomposing organic matter may
be at the center of water stable macroaggregates. In many cases this may
be a root that has deposited considerable amounts of mucigels and other
exuviae in the region before its death (Foster 1981, 1985). As
decomposition of this fragment proceeds, clay and microbially produced
mucilages encrust the organic matter fragment, which eventually retards
decomposition. The centers of the macroaggregates can be anaerobic
(Tiedje et al. 1984) resulting in more reducing conditions. This has
three possible consequences: (1) The solubility of some polyvalent metal
cations such as Fe and Mn may increase, thereby contributing to the
stability of the SOM being formed at this site through the bridging
mechanisms mentioned in the previous section; (2) the end products of
decomposition may be more humified than with aerobic decomposition; and
(3) the rate of weathering can increase, possibly resulting in the
formation of amorphous aluminosilicates and eventually secondary fine
clay particles. This material intimately associates with the organic
matter in the decomposing fragment. The end result of the above
described processes is the formation of a new and quite stable
microaggregate. The organic matter is not only physically stabilized
with clays and physically occluded inside the microaggregate but also
highly structured soil
Fig. 3. A vertical cross sectional view of a highly structured soil. The
largest soil units shown are macroaggregates (~ 2 mm diameter). They are
composed of microaggregates (~ 0.1 mm) and sand grains, as shown in the
center left of the macropore. Four hierarchical classes of soil pore
space are illustrated: (1) macropore, (2) intermacroaggregate, (3)
intermicroaggregate (includes intramacroaggregate space, see arrow) and
(4) tramicroaggregate space. Illustration by S. L. Rose.
As a result of the intense microbial activity at a microsite at the
center of a macroaggregate, considerable amounts of microbial
polysaccharides are produced which coat the surrounding, previously
formed microaggregates, adding to the stability of the macroaggregate.
This coating material will not be as resistant to decomposition as that
at the center of a microaggregate but may be protected from
decomposition because the microbes do not have easy access to it until
there is some physical disturbance. As a result of the above processes,
the turnover rate of microaggregate SOM may be considerably slower than
that of SOM between microaggregates especially when the soil is
disturbed as with cultivation.
The above is a mechanistic explanation of how the slow and passive pools
of SOM are formed and why the turnover rates are so different. However,
we must re member that it is the soil microorganisms that are directly
responsible for the changes in SOM that we observe. They live in the
pore space of the soil.
If we assume that the surfaces of the aggregates and soil particles are
the walls of the pore space, then we can integrate ideas concerning the
two main ways of conceptualizing soil structure: aggregates and pores.
Soil pore space and organism activity
Soil organisms are controlled in a number of ways by the soil pore
space. Another way that pore space can control microorganism activity is
by restricting movement of organisms among different size categories of
pores. It is not just the size of the pores that is important. The size
of the pore necks that lead to the pores and their continuity may be
even more important. By analogy, it is not the size of the rooms that
controls the accessibility but rather the size of the doors and length
of hallways leading to the rooms. The size of soil organisms that are
restricted by pores (i.e., those which cannot move the soil itself) can
range from less than 1 mm for bacteria to over 1000 mm for some
nematodes and mites. This large range of pore sizes can be effective in
governing organism movement and activity in the soil. Since the pore
space controls the distribution of water, water availability is a
secondary effect that pore space has on organisms.
For highly structured soils that fit the hierarchical description given
by Tisdall and Oades (1982), four basic categories of pore space can be
defined (Fig. 3). The largest category of pore space is macropores,
usually created by roots or earthworms (Lee 1985) but may also be the
result of cracking in shrink/swell soils. These pores are drained of
water when the soil is at field capacity and are important for quick
drainage and deep penetration of water, as will be discussed in the next
section. These pores may provide a relatively continuous path for
movement of microarthropods, especially those pores formed by roots or
worms. This size class of pores is most easily destroyed by cultivation
but may develop with time in agricultural soil under no-till cultivation
in structurally stable soil. The next smaller size of pore space is that
between macroaggregates. Water is retained in many of these pores when
the soil is at field capacity and pore space is large enough to be
inhabited by nematodes. The pores between microaggregates but within
macroaggregates are large enough to accommodate small nematodes and
protozoa and may be the chief habitat of fungi. The smallest class of
pores, those within microaggregates, may be only about 1 mm, maximally,
and may be inhabited mostly by bacteria (Kilbertus 1980). A more
aggregated interpretation of the relationships described above is shown
in Fig. 4.
Some soil organisms require free water in pools or films to remain
active (e.g., bacteria, protozoa and nematodes), while other organisms
can remain active without free water surrounding them (e.g., fungi and
micro arthropods). Therefore, the pore size distribution and the amount
and subsequent distribution of water in soil pores can differentially
control the activities of various groups of soil organisms. Therefore,
interaction of the timing and amount of rainfall with the pore size
distribution and the rate of evapotranspiration can likewise affect the
relative activity of inhabitants of air-filled versus water-filled
pores. For example, in soils that are exposed to frequent rainfall,
macropores may be especially important for inhabitants of air-filled
pores because few of the pores are drained at field capacity.
In soils under drier moisture regimes, macroporosity may be relatively
less important because there may be adequate air-filled pores. However,
the number of macropores may limit habitat availability where there is a
high degree of compaction. Of course, under xeric conditions the amount
of water-filled pore space is often a limitation, especially for the
inhabitants of water-filled pores. This simple scenario becomes
considerably more complex when considering the series of events that
occurs during a wet-up and dry-down event (Elliott et al., in press).
Oxygen concentration will also control the kinds and distribution of
organisms in soil and is affected by pore space. Zones of lower oxygen
content are likely to be in smaller pores, away from macropores and
channels and at the centers of macroaggegates near decomposing organic
Pore size distribution also controls predator-prey relationships. For
water film inhabiting organisms it is the size of the pores and whether
or not they are filled with water that controls their activity, as
demonstrated by Darbyshire et al. (1976) for ciliated protozoa. They
found that it took longer for ciliate populations to develop with lower
water potentials and hypothesized that at lower water potentials part of
the bacterial population was inaccessible to the ciliate predators.
Soil pore diameter size
Fig . 5. Soil pore diameter size class distribution (water absorption)
for cores either sieved and repacked to three bulk densities (1.0, 1.2,
and 1.4 g cm-3) or taken intact from different tillage practices; bare
fallow, stubble mulch, and no-till (bulk densities of 1.24, 1.12, and
1.07 g cm-3 respectively), taken from the fallow portion of winter
wheat. The largest treatment differences are associated with the
macropores in both sets of cores (from Heil, unpubl.).
Vargas and Hattori (1986), using a similar bacterial-ciliate system,
investigated two domains of soil pore space, the so called inner and
outer zones of macro aggregates. They convincingly demonstrated that the
small pores of the inner zone served as a refuge for the bacteria and
that exploitation of the outer zone by ciliates was dependent upon the
colonization of particular "compartments" separated by "walls". We
suggest that their inner zone is the same as the intermicroaggregate
pore space described above and that their outer zone is what we have
described as the intermacroaggregate pore space. Their "compartments"
are probably water-filled pores separated by air-filled pores ("walls").
Elliott et al. (1980) showed that the texture of soil can influence the
interactions between predators and prey, nematodes and amoebae in this
case, presumably as a result of differences in pore size distribution
among different textures of soil. Holt (1981) studied the relationship
between cryptostigmatid mites and macroporosity in rain forest soils and
found that for the larger-bodied mites there was no relationship between
these factors, but for smaller-bodied forms (50-125 mm in width) there
was a strong positive correlation between the percentage of mites in
this size category and number of pores of the same size. He suggested
that, "Very small cryptostigmatids are easy prey for many soil animals
and would be able to use small pores in the soil as refuge". To obtain a
better understanding of the relationship between soil organism body size
and pore size, it is not only important to be able to determine the
organismal component but equally important to be able to characterize
the pore space in the soil; this is not usually an easy task.
There are two basic approaches for determination of soil pore space;
displacement of the air in the pores with a liquid, and direct
observation. Water is most commonly used for the liquid and works well
for the larger pores, but mercury intrusion porosimetry is also
frequently used and is particularly useful for the smaller pore sizes
(Lawrence 1977). Comparison of water and mercury methods has been made
(Olson 1985). Non-polar liquids have also been used (Lenhard and Brooks
1985). While the liquid displacement methods are useful for
characterization of the average pore distribution for the entire sample
it does not give information on the geometry, orientation, or continuity
of the pores as the direct observation methods can. These latter methods
are becoming quite sophisticated with the use of automated image
analysis (Ringrose-Voase and Bullock 1984). Darbyshire et al. (1985)
used direct observation methods to determine the pore network available
to protozoa. They found it particularly difficult to determine the
continuity of the pores.
Determination of pore size distributions by use of water release curves
(Klute 1986) may be the most practical for most soil microbiologists
since the equipment (pressure plate apparatus) can be found in most
soils laboratories. When water is incrementally removed to known matric
potentials from a saturated soil sample and the results plotted against
the volumetric water content (desorption curve) the distribution of the
amount of water (or equivalently the pore space) held in pores with
calculable pore neck sizes can be determined (Danielson and Sutherland
1986). When water is incrementally added to a soil sample (absorption
curve) the distribution of the amount of water held in pores with
calculable mean diameter pore widths can be determined. Desorption curve
measurements may be the most useful for organism studies because it is
probable that pore neck size rather than the diameter of the pores
controls the movement of organisms.
pore space in cores of soil
It is theoretically possible to determine the distribution of sizes of
pores within categories of pore necks of known diameters based on
absorbtion/desorption curves if the scanning curves (trajectories
between the absorption and desorption curves at points along each curve)
are also known. This is probably impractical because of the amount of
effort necessary and the need for high precision for this type of
measurement (J. Heil et al., unpubl.).
We have made determinations of pore space in cores of soil repacked to
different bulk densities (Heil et al., unpubl.) and in intact cores
taken from different tillage practices in the field (J. Heil, unpubl.).
In repacked cores there is a decrease in pore space as bulk density
increased. This occurred mostly with the larger size pores (Fig. 5). The
amount of pore space in pores >100 mm decreased from 0.18 cm³ g-1 to
0.02 cm³ g-1 as the bulk density increased from 1.0 to 1.4 g cm-3.
Although the disparity in bulk density was not as great for the intact
cores from the different treatments (1.07, 1.12 and 1.24 g cm-3 for the
no-till, stubble mulch, and bare fallow treatments, respectively) as for
the repacked cores (1.0, 1.2 and 1.4 g cm-3), the differences in pore
size distribution were also mostly in the largest pore size category
(Fig. 5). The <0.2 mm category is not really pores, as such, because the
water within this soil space does not behave according to capillary
forces but, rather, is affected much more strongly by adsorptive forces.
Therefore, the smaller amount of soil space in the <0.2 mm class for the
bare fallow treatment is probably due to differences in other soil
factors such as soil organic matter content.
Macroporosity and preferential flow
A third important consequence of soil structure is the preferential flow
of water, and the solutes contained therein, down macropores (Thomas and
Phillips 1979, Beven and Germann 1982). In structurally stable soil,
macroporosity created by roots or earthworms may persist in the absence
of cultivation. When the soil is cultivated, the continuity of the
macropores is destroyed and infiltration rate is reduced. The classic
piston flow (Darcean flow) concept of water movement in soil works well
for conventionally cultivated soils. With increasing adoption of no-till
management and the development of stable macroporosity in these
management systems, preferential flow must be considered in order to
give a good description of water movement and the solutes contained in
Water moves preferentially down macropores and bypasses, or short
circuits, the water in the soil matrix. Preferential flow is a much
greater proportion of the total flow during saturated than unsaturated
flow. Scotter and Kanchanasut (1981) reported that when the volumetric
water content of their soil decreased from 0.56 g cm-3 (saturation) to
0.53 g cm-3 the hydraulic conductivity decreased by two orders of magnitude.
In most soils both piston and preferential flow occur simultaneously and
it is the relative proportion of each of these kinds of flow that
determines the kinds of leaching that may occur. If the incoming water
is devoid of solutes, then the rate of mixing of the solutes in the
matrix solution with the macropore water is an important determinant of
the rate of solute movement through the profile. If the rate of water
input is high, more water will move down the macropores resulting in
less mixing of the matrix and macropore water than if the water input
rate is low. Thus, rainfall intensity is an important factor in
leaching. If the incoming water has high concentrations of solutes and
the water flow is high, then preferential flow will result in deeper
movement of solutes than when piston flow dominates.
There are a number of key factors which determine the movement of water
and solutes in soils; they are the degree of macroporosity, the relative
distribution of solutes in the macropores and the soil matrix, and the
rate at which the water is coming into the system. The structural
stability of the soil, which is a function of the texture and organic
matter content, will determine the potential that a particular soil has
for developing macrostructure. As a function of this potential, the type
of management practice and period of time under that management will
determine the actual macroporosity of a soil.
Another consequence of greater macroporosity is higher infiltration
rates, hence less runoff and erosion. Mielke et al. (1984) observed that
there was much higher water infiltration rates in no-till (herbicide
weed control) compared with conventional bare fallow (moldboard plowed)
or stubble mulch management (soil is disked to control weeds) of winter
wheat. On the same experimental plots, we found that there was less
leaching of mineralized N (as N03-) under no-till than bare fallow or
stubble mulch treatments (Elliott et al., unpubl.; Fig. 6). The
distribution of nitrate within the profile was a function of the timing
of mineralization (no fertilizer was added), rainfall, and the physical
characteristics of the soil. We sampled seven times during the summer of
1984 in the fallow rotation (no crop present) of each treatment at
There was no evidence that there were differences in mineralization
rates among treatments. Effects of the higher moisture contents in the
no-till treatment were probably offset by lower temperatures; the net
result being similar amounts and timing of mineralization. During wet
periods of the experiment, specific leaching events were observed (i.e.,
between 17 July and 14 August and 11 September and 1 November). During
these periods there was less downward movement of nitrate in the no-till
treatment (Fig. 5).
Kanwar et al. (1985) obtained similar results when fertilizer N was
initially sprayed onto the surface of their experimental plots with a
small amount of water. They found that in the no-till plots 40% of the
added N remained in the top 30 cm after 127 mm of simulated rain and 33%
remained after an additional 635 mm was added. The conventionally plowed
plots had only 19% and 9% remaining in the top 30 cm after similar
additions of water. Germann et al. (1984) found an opposing trend to the
two above described studies with more solute moving deeper in the
no-till than in conventionally tilled soil, especially at higher water
application rates. The solute was in the incoming water and not in the
soil matrix as noted in the previous two experiments. This contrast of
results emphasizes the importance of knowing the placement of the solute
for accurate prediction of leaching in structured soils.
Two phase flow in no-till systems presents some interesting
possibilities for management of fertilizers that may be useful for
enhancing fertilizer use efficiency and reducing ground water
contamination by keeping the solutes in the rooting zone for longer
periods of time. A key aspect is maintenance of macroporosity. This can
be accomplished by reducing or eliminating tillage. In soils with poor
structural stability, organic matter levels may need to be increased
before such management is effective in reducing leaching. Sandy soils
may not be responsive to management changes by showing reductions in
leaching potentials. Another key aspect for management of fertilizers in
no-till systems is facilitation of movement of solutes into the soil
matrix before major water inputs occur. This could be done by injecting
ammonia or surface applying fertilizer during a period when small,
rather than large rain storm events are predicted.
The interacting effects of soil macrofauna and macroporosity in
conventional (CT) versus no-tillage (NT) systems are of interest to
agronomists and ecologists. Barnes and Ellis (1979) showed that
earthworm populations increased in NT versus CT wheat and barley fields.
There were no differences in crop yields, but a significant increase in
macropores (>1.5 mm diameter) at the 20-30 cm depth. This may affect
crop growth markedly only where rainfall is sporadic, and macropores
enhance infiltration during heavy showers (Lee 1985). The
species-specific nature of earthworm responses must be noted. Thus,
Edwards and Lofty (1982) found as much as 17.5 times more deep-burrowing
species (Lumbricus terrestris and Allolobophora longa) in NT plots,
whereas shallow-dwelling forms such as A. caliginosa and A. chlorotica
were only 3.4 times as numerous.
Another facet remains to be considered: Earthworm activity may increase
cation exchange capacity, exchangeable Ca and Mg, nitrate-N and
available P, as well as infiltration rates, yet have no significant
effect on crop production (Lal 1974). Numerous questions remain to be
answered, including impacts of castings on subsequent microbial activity
and plant growth.
An approach for agroecosystem management
If we are to manage agroecosystems effectively, we need to consider the
long-term investments versus the short-term gains. Recent information
suggests that it may take as long as 10 years to start benefiting from
the changes which occur when converting from conventional tillage to
no-tillage (Phillips and Phillips 1984, Rice et al. 1986). It may take
this long to develop significantly better soil structure and for the
immobilized fertilizer to begin to be remineralized in significant
However, after this time we should be able to use lower inputs and
reduce the intensity of agriculture because the use efficiency is
higher, i.e., there is better internal recycling of the nutrients. The
aim is to keep the profit margin at about the same level, and perhaps
most importantly, reduce the leakage of excess nutrients from the
ecosystems. We should try to obtain an optimum agriculture, not
necessarily a maximum.
We suggest two basic considerations for the design of lower intensity
but more efficient agroecosystems. First, reduce tillage as much as
possible in order to benefit from better soil structure. This will
hopefully lead to (1) more and better quality soil organic matter; (2)
higher soil microbial biomass, which acts as a large and dynamic source
and sink of nutrients; and (3) increase macroporosity which allows
fertilizer management options which can lead to less leaching and better
fertilizer-use efficiency. Second, minimize the use of biocides and
reduce toxic material inputs such as heavy metals, which are harmful to
the soil biota and which could result in a reduction in activity of the
detrital food web. With respect to this latter suggestion, it may take
some years until the system recovers from such perturbation (Brookes and
An important question in a reduced tillage system is whether
pathological organisms or their biological control agents will become
dominant. The fact that many fields plagued by take-all disease of wheat
eventually become suppressive to this disease (Hornby 1979, Chakraborty
and Warcup 1983) gives us some hope that natural biological controls can
be nurtured in soil and used to our advantage. We will need this if we
are to circumvent problems of residue accumulation and pest outbreaks in
reduced tillage systems.
We are not advocating that agroecosystems will move to some ethereal
"natural state". We should manipulate the agroecosystems to obtain
better yields. For example, introduction of biological control agents,
bioengineered or otherwise, should be used while trying to reduce heavy
pesticide use. What we are suggesting cannot be accomplished quickly but
could result in long term benefits to agriculture. We clearly need a
tremendous amount of good, basic research and the integration of current
information to the ecosystem level.
We should extend our definition of damaged soils (Tinker, this volume)
to include the soil biota. Criteria for this have yet to be determined.
Do we care for the living soil, or are we treating it like dirt? We
should allow the soil to work for us and not work against it.
Acknowledgements - Rusty Scott, Doug Hansen and Cindy Cambardella
provided the technical assistance in the field and laboratory. Sharon
Rose is thanked for the use of the illustrations. Vern Cole and Dave
Walter commented on earlier drafts. Justin Heil and Arnold Klute
provided unpublished data and ideas for the pores size section.
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