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.
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.
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.
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.
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.
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 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 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.
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.
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.
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.
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.
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