[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
http://www.certifiedorganic.bc.ca/rcbtoa/training/soil-article.html
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, 
U.S.A.


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 
chemical processes.

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.
Fraction incubated
	
Field Treatments
	
Cultivated
	
Native
Macroaggregates 	
17±2 *
	
68±2
	
(1.15)
	
(2.50)
Crushed macroaggregates 	
38±12
	
94±7
	
(1.62)
	
(3.46)
Microaggregates 	
16±1
	
43±2
	
(1.02)
	
(2.29)
± SD of N mineralized in 20 d (mg kg-1) with mineralized % of total N in 
parentheses.


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 
chemically recalcitrant.
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.

soil structure

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 
matter.

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 
the water.

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 
Sidney, Nebraska.

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 
quantities.

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 
McGrath 1984).

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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