Chapter 2. Essentials of Soil Fertility

Organic Matter and Biological Activity

Summary

Owing to the energy which it contains, organic matter serves many purposes, on its own as well as indirectly through the soil organisms which it nourishes.

In most cases, a goal for an optimum organic content in the soil should take into account the expected content in an undisturbed state.

Through several cycles in which some varieties of organisms feed on the remains of others, and by means of purely chemical reactions, organic matter passes through several stages, each of which has a unique effect on the nutrient supply and plant growth.

Consequently fertilizers containing organic matter should get credit for the value of its energy in comparing them with inorganic fertilizers.

In most soils, biological activity is limited by the energy available from carbonaceous organic residues. For this reason, biological activity is rarely stimulated by fertilizers; their use should be timed to feed the crops rather than to stimulate biological activity.

The accumulation of humus, however, depends upon the nitrogen available. If it is supplied, though slowly enough to minimize losses, a greater amount of humus can result from the decaying organic matter. With the help of simplifying assumptions, it is possible to estimate the fraction of organic residues transformed into humus.

Fresh organic residues are a good source of plant nutrients except, in most cases, calcium and magnesium. After decomposition the result is rich in nitrogen, phosphorus and sulfur but low in potassium.

The Value of Organic Matter

Organic matter is the unifying element in the soil, having a prominent influence on soil organisms, plant growth and on the physical properties of the soil. We might regard the soil as the furnace of life, wherein organic matter is the fuel, soil organisms are the fire consuming the fuel, and the plant nutrients are the ashes of the combustion. The fire needs no matches, only fuel and a modest amount of air and water; it is vigorous at the first addition of residues but slows to a smouldering oxidation that can last for centuries.

Although this metaphor may be as good as any, it doesn't do justice to the value of organic matter. Organic matter can, for example:

In turn, soil organisms can:

Many of these points will be discussed later.

The Energy Index

All of the advantages that organic matter and soil organisms offer to the soil and plant growth are due to the fuel in the organic matter and the fire from the organisms. The fire is necessary to break down organic residues and make their nutrients available to plants.

Organic matter contains more energy than anything of value to plants, and yet few people credit it as the fuel for the soil furnace.

In fact this energy reduces the need for fertilizers by facilitating the storage of water, the fixation of nitrogen, the dissolution and accumulation of minerals, the effortless movement of roots through a superior soil structure, and the production of growth hormones and vitamins. In providing a suitable environment for predators, it reduces the need for pesticides. In encouraging a diversity of soil bacteria that feed on weed seeds, it inhibits the abundance of weeds [4].

Energy has a value, and the organic matter should receive credit for this value. Due credit justifies the cost of producing and recycling residues. It permits an estimate of the loss of depleted soil and of the value of organic fertilizers1.

One way of quantifying the value of the energy in organic residues or fertilizers is to create an energy index: determine the amount of fuel oil with the same quantity of energy. Appendix B. The Energy Index contains a derivation of a proposed index applied in this book.

How would such a determination work? The potential energy of #2 fuel oil is about 140,000 BTU/gallon. We can compare this value to the energy content of an organic substance. The energy is released upon the oxidation of carbon to carbon dioxide. For example, a ton of fresh cow manure with 20% organic matter has the same amount of energy as 20 gallons of #2 fuel oil. If fuel oil costs $1/gallon2, a ton of this manure is worth $20 for its energy in addition to the value of its nutrients.

Other examples:

The proposal does have defects. Energy is more easily extracted from green manures than from wood chips. Some residues, such as manure and compost, are already partly decomposed, and the lost energy has been exchanged for beneficial organic byproducts.

Some people would dispute the claim that the energy in organic residues has the same value to agriculture as the energy in fuel oil. It would be difficult to prove or disprove it, since the fairness of the price of oil, the value of agricultural land, and a farmer's wages are based on economic and political confrontations rather than moral or long-term considerations.

We are, however, not only beginning to recognize the benefits of organic matter but also the cost incurred when the benefits are ignored in favor of short-term profits. We are familiar with the loss of quality and yield from eroded soils in the Midwest and the massive machinery necessary to manage problem soils in California. We know that the soils in southeastern states and in some areas of New York are quarantined because of the presence of pests in epidemic proportions.

When fertilizers are added without also adding energy to compensate for the loss of organic matter that inevitably results from cultivation and the removal of crops, the soil deteriorates, and the cost of preventing deterioration is the cost of the energy required to maintain the organic content.

There may be a better measure of the cost of this energy, but at the present time fuel oil is a convenient basis for calculating energy, so why not compare it to the energy in the soil? In any event, nothing is perfect, and here is one way of giving credit to organic materials for their inherent contribution to the soil.

So, the definition of an energy index as used in this book is the number of gallons of #2 fuel oil which contains the same amount of energy as a ton of organic fertilizer. It is tabulated in parts II and III for the purpose of comparing the value of fertilizers.

Optimizing Organic Matter

What is the optimum organic content of a soil? Can a soil have too much organic matter? These are questions that are difficult to answer, partly because they are difficult to define. What is meant by optimum and organic content? The word optimum may imply optimum biological activity, optimum quantity or balance of nutrients, minimum labor requirement, or an optimum profit and loss statement. In principal it should have a meaning that integrates all of these meanings, but some may have more importance than others. The term organic content is also not clear. It includes a variety of residues ranging from raw litter to highly stabilized humus.

One measure of an optimum organic content is the humus level in an undisturbed soil. This varies with the environmental conditions; the humus level of some acid New England soils, for example, is often about 10%, and in desert areas it is less than 1%. Within a few years of cultivation losses caused by exposure of organic matter to air result in a new equilibrium.

Most likely, maintaining the organic content in a cultivated soil at about half its natural level is a reasonable goal. A higher organic content may be better, but whether it is worth the additional effort to fight natural processes is questionable, except in an intensively managed garden.

Even this guide has limitations. A gravelly soil, for example, is likely to require much more organic matter than it might accumulate naturally within a reasonable geological period of time.

Furthermore, the optimum level of organic matter may depend upon its state of decay. A sandy soil needs organic matter for water and nutrient retention. This requires organic matter that is reasonably well decayed; fresh plant residues tend to repel water rather than absorb it 3, and fresh residues have a low capacity to retain nutrients.

In contrast, a clayey soil already has a high capacity for retaining water and nutrients; its most important requirement is something to open up the soil. Either fresh residues or humus might do this by different mechanisms, but a greater short-term benefit should come from fresh residues.

In warm climates, soil decomposition is rapid, and organic matter tends to stabilize in a relatively short time, so fresh residues tend to be more important. In cold climates, organic matter has less chance to stabilize, and well decomposed organic matter is at a premium.

The question of whether the organic matter is excessive implies the possibility of an adverse effect on the soil. As with the question of an optimum organic content, the answer may depend more on the state of decay of the organic matter rather than the quantity. No practical evidence seems to exist that an excess of humus is harmful. Nor is there a universal indication that an excess of fresh residues is harmful - many people garden successfully with massive hay mulches.

However, adverse effects of excessive unstable residues are possible:

  1. the balance of nutrients may be poor: a high C/N ratio or a high potassium/magnesium ratio
  2. some of the initial byproducts of fresh residues may be toxic to seedlings
  3. insect pests may be attracted to a soil with a high content of fresh, moist residues
  4. fresh residues together with soluble nitrogen fertilizer encourage nitrogen loss by denitrification.

Fertilizer Value of Organic Matter

Organic matter is principally a source of nitrogen, phosphorus, and sulfur - nutrients which soil organisms require and retain. These nutrients slowly become available as the organic matter continues to decompose. Most of the calcium, magnesium and potassium in the decaying organic residues are discarded by the soil organisms.

The Nature of Organic Matter

Many statements here and later in the book will depend upon an understanding of organic matter. One problem, however, is that current knowledge is not entirely clear on what organic matter is, nor are different viewpoints easy to integrate. Discussions in this chapter and indeed in the entire book concerning organic matter are an attempt to integrate different points of view, insofar as it is possible. The principle references are [15] and [83], although [17] is sometimes helpful.

Organic matter in the soil is litter or residues in various stages of decomposition and transformation. These generally fall into two contrasting groups: succulent material often rich in nitrogen and associated with rapidly growing plants; and tough, fibrous carbonaceous material associated with mature plants. The latter has a high content of lignins; both are high in carbohydrates. Examples: green manures, hay and grass clippings are succulent, non-lignaceous residues; straw, wood and bark have a high-lignin content.

Most likely it was the slow rate of decomposition that led to the lignin theory of humus, which states that humus is essentially lignins that have not decomposed. The actual evidence, however, was weak, and the theory could not explain the fact that humus has the same carbon/nitrogen ratio and similar mineral levels as soil micro-organisms.

This objection resulted in the microbial synthesis theory. It postulates that humus is the remains of soil organisms [15]. Accordingly, equal quantities of organic residues, no matter what their lignin content, will eventually result in equal quantities of humus, although lignaceous residues will require more time to break down.

More recently, however, the lignin theory was revived by new evidence using radio-active carbon techniques. It indicates that microbial tissue comes from approximately the same amounts of lignaceous and non-lignaceous residues. But the greater portion of lignaceous material is converted to humus; while the greater portion of non-lignaceous material is oxidized to carbon dioxide[83] and is the principal source of energy for metabolic activity.

With some compromise on both sides, the two theories are compatible, because the lignin theory hypothesizes what humus is, and the microbial synthesis theory tries to expain how it is produced. Compromise is necessary. Although humus may be predominantly derived from lignins, a significant amount of humus nevertheless must come from non-lignaceous material; after all, humus does contain a large amount of nitrogen and minerals not present in lignins.

On the other hand the production of humus is not entirely microbial. Humus materials produced microbially can bond to one another chemically, increasing in size and stability. Furthermore, a non-lignaceous material can also bond to humus chemically, the result of which increases the stability of the non-lignin.

So it is plausible to adopt the following model of humus formation: humus develops from the decomposition of dead microbes, whose tissue consists of both lignaceous and non-lignaceous materials. Living microbes attacking the dead tissue use both portions, but more of the latter for energy. Left behind are also portions of both, but primarily modified lignaceous materials which contribute to the pool of humic substances. In time, these humic substances bond to one another, growing and stabilizing further.

With this model we can postulate some of the properties of organic matter, which may help in choosing the best agricultural management of the soil.

For example, we can visualize two stages in the formation of humus: the initial decomposition of organic residues to a point where they are unrecognizable, and the buildup of humus4.

In the first stage, fungi are prominent in the decay of the residues, using some of the residues for energy and some for building cell tissue for growth and reproduction. Their initial domination is at least partly because not only do they require less nitrogen for cell growth than other micro-organisms, but they are alone in their ability to attack woody tissue.

In the second stage, the early feeders are preyed upon by other, different organisms, or they die when the food supply runs out, and their remains are subject to attack by others. In like manner the second generation of decay leads to a third and still different generation. With each successive cycle of consumption and decay, some energy is removed, different types of organisms flourish, and the cellular tissues of those organisms become increasingly stable and more resistant to further attack and decomposition.

Throughout this process of consumption and transformation, the properties of the residues change. The initial instability makes them chemically and biologically active. They are easily broken down, and so their nutrients become readily available; they attract heavy metals, so they can chelate trace elements5 and make them available. At the same time they can lock up excess amounts of toxic metals.

Such instability, however, is also a disadvantage. Many of these active organic substances are water soluble and leach easily, hastening the depletion of soils, especially in humid climates.

As organic matter continues to decompose, it becomes less active. It is not as influential in chelating trace elements, and it has little influence on the soil structure6. Its value lies in the nitrogen and minerals it contains, which eventually become available, and in its buffering capability7.

Requirements For Biological Activity

The conditions needed for a diverse mixture of soil life are a warm soil, adequate moisture and drainage, and a soil pH above 6. These conditions also assure a high biological activity. Both diversity and high activity are important to achieve the benefits of organic matter.

The same conditions that enhance soil life usually produce maximum plant growth, although exceptions do occur. The most common exception is the pH requirement. Blueberries, azaleas, rhododendron and other plants evolved in very acid soils and do not grow well at a pH above 6. Many varieties of potatoes are subject to a scab which is most prominent at a pH above 6. Sandy soils along the Atlantic coast and in the south have difficulty in supplying manganese when the pH is much higher than 6.

Some of these adverse effects of a higher pH, however, can be controlled. With potatoes for example, there is evidence that a supply of fresh organic residues without an excess of nitrogen will reduce the incidence of potato scab, by encouraging competition from other soil organisms. Moreover, as discussed in chapter 16. Micronutrients , the nutritional value of potatoes is likely to be better in a less acid soil owing to higher availability of molybdenum.

In sandy soils, an organic mulch will supply manganese and other trace elements and also reduce leaching losses.

In practice, carbon is always the element limiting biological activity in agricultural soils, even though it may be in excess. It is not just the amount, but also the nature of the organic matter which determines the limiting factor. Lignaceous material is so resistant to decomposition that the rate of decay in soil is low enough to allow existing reserves to supply whatever nitrogen is necessary to meet demand.

For most soils the C/N ratio lies in the range from about 8 (8:1) to 14 (14:1). Consequently the soil is rich in nitrogen compared to carbon. Although much of it is in relatively stable humus, enough does become available to supplement whatever the residues offer. Lignaceous residues offer little nitrogen, but the decay rate is slow enough that nitrogen reserves are available when needed.

Sawdust is an example. If 1% softwood sawdust is added to a soil containing 1% organic matter, nitrogen fertilizer does not increase the rate of decomposition. This is because the resins in softwood slow down the decomposition process to the extent that organisms can easily meet their nitrogen requirement.

However, if 1% hardwood sawdust is added, nitrogen fertilizer can increase the rate of decomposition, but enough fertilizer to reduce the carbon/nitrogen ratio of the sawdust from about 500 to 35 only increases the rate of decomposition by about 50%8.

In the field, 1% sawdust amounts to a layer about 1/2 inch thick turned into the soil. Normally, raw, uncomposted sawdust is not tilled into a soil but rather spread as a mulch; in which case the soil is exposed to a much smaller amount at one time.

Consequently, adding nitrogen fertilizer to increase biological activity and hasten the decomposition of carbonaceous residues is not worthwhile. In fact, doing so is wasteful if the nitrogen is soluble or readily available, because it is likely to be lost by denitrification9.

The issue is not food for the soil organisms but for the plants. If carbonaceous residues are worked into the soil, most of the nitrogen which might otherwise be available to plants will instead be immobilized within the cell tissue of soil organisms. Eventually biological activity will burn up enough carbon to release nitrogen; but this delays availability to the plants.

Consequently nitrogen fertilizer is necessary at the proper time for the plants rather than for soil organisms. If residues are turned into the soil in the fall, nitrogen need be spread only in the spring for the sake of the crop.

The other nutrients in humus - phosphorus, sulfur - are even less likely to be a limitation to biological activity.

Humus Renewal and Conservation

Even though fertilizer nitrogen is unnecessary for biological activity, it may increase the amount of humus produced. The efficiency with which an increase occurs depends on two factors: the rate at which nitrogen becomes available for consumption and the nature of the residues.

Nitrogen constitutes an anchor for cell tissue. Additional nitrogen produces a corresponding addition in the growth of living organisms; having died, the organisms contribute to the pool of humus. For efficient use of the nitrogen, however, its availability must not occur at a rate in excess of the need. This rules out soluble fertilizers for increasing humus efficiently[45].

However, the flow of organic matter is a dynamic process; there is no such thing as an absolutely stable state. A steady loss of humus through biological activity is always occurring and must be replaced by fresh sources.

Consequently, there are two objectives, to increase biological activity and to build humus. To maximize biological activity, we need adequate warmth, aeration, moisture, a near-neutral pH, growing plants, and organic residues. To increase humus we need organic residues as a source of food for the organisms and a slow but steady increase in nitrogen. These are most likely to be effective with minimum losses of humus by cultivation.

Trying to predict how much humus will remain from the application of organic residues is a hazardous undertaking; there is no clear distinction between residues and humus, and even humus continues to degrade. The main problem is choosing the C/N ratio at which residues become stable enough that further decomposition can be measured in months or maybe years rather than days or weeks.

Nevertheless, it may be worthwhile to try, if only to get an idea of what is happening. The minimum C/N ratio in soils is about 8. Since 10 is a round number and convenient to use, we'll choose that as the criteria for the definition of stable humus.

If we start with cow manure having a C/N ratio of 20 and a moisture content of 80%, and if 25% of the original nitrogen remains for humus buildup, then the final amount of humus will be about 1/40 of the fresh weight of the original manure10.

With these assumptions, an application of 10 tons of cow manure/acre should result in a humus addition of about a quarter of a ton/acre. The weight of an average plow-depth layer of soil is about 1000 tons/acre. So 10 tons of manure should increase the humus content by about 1/4000. At this annual application rate it would take 40 years to increase the humus content by 1%11.

A long time is required to build humus on a depleted farm. Manure can be spread at a higher rate, and this may result in a faster buildup of humus if nitrogen losses do not become too high; rates up to 25 tons/acre for growing corn are sometimes recommended, and gardeners may spread more than that.

Organic matter could be increased more rapidly in a garden with the use of a hay mulch. If we begin with air-dried hay at 16% moisture and having a carbon/nitrogen ratio of about 40, and if we assume, as we did with manure, that 25% of the nitrogen is available for humus formation, we shall find that about 1/20 of the mass of the original mulch will remain by the time the carbon/nitrogen ratio drops to 1012.

If one were to spread baled hay to a depth of about 3 inches, the total amount of hay spread would be equivalent to approximately 40 tons/acre. If the above estimate is correct, this should add about 2 tons of humus/acre, or about 0.2% with a one-year application. It would then require only 5 years to raise the organic content by 1%, a much more optimistic result than we got with the manure calculation.

On the other hand, we are comparing a moderate application of manure with a massive application of hay; we would have to grow a 5-ton crop of hay on 8 acres of land to supply 40 tons for a one-acre garden. Furthermore it would still take several years for the mulch to become incorporated as humus in the soil.

Similarly, green manures have little chance of increasing humus. The fact that the tops are not lignaceous may be partly compensated by the roots. Even so, to expect to grow within a few weeks enough organic bulk on one acre of land to be equivalent to a hay crop grown during an entire season on 4 - 8 acres should seem like an unfounded hope.

Compost is a third alternative for increasing the humus content, but the nitrogen content of the compost and the losses are much more difficult to estimate. However, with practically no limitation on the application rate, humus can be increased faster and probably more safely with compost than with fresh manure, and humus stabilization could be faster with compost than with a hay mulch.

Nevertheless we are not getting something for nothing. To do as well as we might with our 40 tons of hay, we would have to compost those 40 tons with no loss of nitrogen; so we would still need a full crop of hay from at least 8 acres of land in order to increase the humus level of 1 acre by 0.2%. One advantage of compost, however, is that we do not need hay; any waste product will do.

Whatever the merit of these calculations may be, they should be good enough to indicate that humus development is a slow process. If we also take into account losses from tillage, the result is that we often have to work hard just to stay even.

The point, however, is not to discourage people from working to increase humus but to illustrate in one additional way how fragile our environment is and the loss to the future of using resources developed over centuries of time for short-term gain.

No matter how long it takes to rebuild soil, someone should start sometime or civilization as we know it will disappear.

Air and Water

Summary

Air and water are necessary in photosynthesis, metabolism, growth and plant structure and temperature regulation.

A good soil structure permits movement of air and optimum use of available water. In a well-structured soil, the soil particles are bound together in small aggregates of variable size. Soil structure is determined by various physical and biological processes which influence the formation and stabilization of the aggregates.

The Importance of Oxygen and Water

Air and water are essential to the storage and release of energy in the plant. During photosynthesis, energy from the sun is trapped by the plant leaves. The energy provides the means to break up carbon dioxide from the atmosphere into carbon and oxygen. The plant manufacturers sugars from carbon, oxygen and water for temporary storage of the energy. Eventually the plant grows by the construction of carbohydrates from sugars.

During respiration, various plant mechanisms expose oxygen to the sugars and carbohydrates. This oxidizes some back to carbon dioxide, during which time the stored energy is released and utilized in metabolic reactions. The release of energy from the oxidation of carbon is the source of energy for plants and soil organisms; it is the same energy that we gain from burning fuel oil, peat and wood.

In addition to its contribution as a constituent of sugars, water has physical effects: by filling plant cells, it supplies a structure that keeps the plant erect; without it, the plant wilts. Water evaporating from leaves cools them, in the same way that we are cooled by perspiration. Water also carries dissolved nutrients across the root surface.

Water is usually a limiting element to plant growth at some time during a growing season. Shallow-rooted crops require special attention to water because of the limited ability of the roots to scavenge for phosphorus.

Plants do not have a mechanism to transport gases. So roots need an independent source of oxygen in order to utilize the sugars produced by the leaves. They obtain it from air present in soils.

The higher the soil fertility, the greater the demand for oxygen. A high fertility results in greater plant growth and biological activity. This results in a higher rate of respiration. As respiration increases, so does the need for oxygen. Soil organisms compete with plant roots for available oxygen, and in a marginally aerated soil the roots may not get enough. Roots lacking oxygen are stunted.

As it turns out, a lack of oxygen in the soil is not all bad. Iron, manganese and copper are more available to plants in their oxygen-deficient state. Ferrous phosphate is more soluble than ferric phosphate and can be a significant source of phosphorus.

Almost all soils, however, are anaerobic at some time to a sufficient extent to produce the necessary benefits without our having to deliberately induce such a state. Usually the greater concern is to satisfy the conflicting requirements of adequate water and oxygen.

Soil Structure

An important function of a soil derives from its physical characteristics. Soil supports a plant while permitting movement of the growing roots, and it provides air and water. Fresh rainwater carries dissolved oxygen needed by the soil, so the soil must be porous enough to permit good drainage and to prevent the water from standing and becoming stale. If drainage is too rapid, however, the soil is droughty. Ideal drainage occurs in a soil which contains open spaces of various sizes; wide spaces permit drainage and access to oxygen; small spaces trap water and allow its movement by capillary action.

The texture and structure of the soil influence the dimensions of the open spaces. The texture refers to the proportions of sand, silt and clay particles. Structure refers to the extent to which the soil particles are bound together. The two are sometimes correlated; in pure sand, the particles are not bound at all, and in clay they are so strongly bound that the resulting blocky chunks can be broken up only with great difficulty.

A good soil structure is important in two ways:

  1. it permits the movement of water and air through the soil
  2. it facilitates the development of an efficient root system by minimizing the work required by root hairs in their growth.

In turn a good root system can better forage for nutrients and water. A good soil structure can substitute for some of the fertilizer and irrigation that would otherwise be necessary.

The texture of a soil is not easy to change, except on a small scale where sand may improve a clay soil. The structure, however can be altered by encouraging the formation of aggregates of varying size. We often say that the soil has good tilth or good crumb or a granulated structure if it is well-aggregated.

Soil aggregation occurs in two phases, formation and stabilization. Formation occurs by physical forces, such as freezing and thawing cycles, or wetting and drying cycles. These clump the particles together, but without stabilization the same forces will break them up.

Stabilization can occur either physically - by fungi surrounding the aggregates - or chemically - with cements from decaying starches.

Plant roots contribute to both formation and stabilization. Roots pushing through the soil and absorbing water create differential pressures which form the aggregates. And they slough off dead tissue, some of which decays into cements that bind the aggregates. Grasses and grains are particularly effective in promoting good soil structure, owing to the extensive network of their root system.

Regardless of the soil texture, organic residues have a significant effect on soil structure. They provide the raw materials for the cements which bind and stabilize soil aggregates, and they stimulate the growth of micro-organisms and soil animals that contribute to aggregate stability.

Residues high in carbohydrates are best in promoting stable aggregates[15], [83, chapter 11]. Simple sugars also produce cementing agents and at a faster rate than carbohydrates; but their stability is poorer because they are more easily attacked by further biological activity. The higher the molecular weight of a cementing product, the slower but more long-lived its effect. In particular, cellulose produces the most stable aggregates.

An argument could be made for the claim that the most important agricultural value of organic residues, though certainly not the only one, is their effect on soil structure. Alternates exist commercially for other benefits, whatever their merits: fertilizers, pesticides, hormones, etc. There is, however, no practical way to create a stable and effective soil structure without organic residues; people have tried and failed.

Three practices are necessary to encourage a good soil structure:

  1. keep a crop growing as much as possible, to encourage maximum root growth
  2. recycle residues to replace carbohydrates lost through biological activity, carbohydrates which are necessary for the cements binding soil aggregates
  3. minimize disturbance of the soil which would reduce biological diversity and accelerate the destruction of soil structure and organic matter.

Tillage

Tillage can improve soil structure by breaking up clods, and it can contribute to the forces which form the soil aggregates. But tillage can also be damaging; when the soil is too dry, it shatters the aggregates; when too wet it compacts them.

Moisture conditions are especially important with heavy clay soils. These soils should be moist when tilled but not sticky. One criterion for estimating the best time for working a clay soil is that it should be just dry enough that a person can walk onto it without soil sticking to boots. Another is that a handful of soil squeezed into a ball should have no excess water running out and should crumble slightly when released.

Tillage tends to dry out a soil and must be timed carefully. Rewetting, especially after a long period of dryness, causes a flush of biological activity which rapidly oxidizes organic matter. Sandy soils suffer from improper tillage, because they need the organic matter for water and nutrient retention, and clayey soils because they lose soil structure.

Be aware of the degree to which rototillers degrade the soil environment. They dry out the soil; they whip it up and shatter the aggregates; they destroy the capillarity; most compact the subsoil13; they reduce the diversity of micro-organisms to those able to survive a temporarily hostile environment; and they destroy earthworms and other soil animals.

Rototillers are especially inadviseable in dry climates because they increase the frequency needed for irrigation. They should also be avoided where organic matter is low: although the subsequent release of nutrients by increased exposure to oxygen may offer a short-term benefit to plant growth, the result accelerates the loss of organic matter.


1 See, for example, table 18. Comparison Of Nitrogen Fertilizers and the discussion of nitrogen fertilizers in chapter 10. Nitrogen     [return to text]

2 As noted and justified in the preface, quoted costs are representative of those in 1990    [return to text]

3 because of the natural waxes which have not yet broken down    [return to text]

4 This division of decomposition into two stages was noted by E.E. Pfeiffer, in his studies of the preparation of compost by Biodynamic methods, long before our current theory of organic matter became important, but the theory helps to explain his observation.    [return to text]

5 Chapter 16. Micronutrients     [return to text]

6 Chapter 2. Essentials of Soil Fertility     [return to text]

7 For a discussion of buffer capacity and cation exchange, see chapter 14. Calcium And Soil Ph     [return to text]

8 [59]. This article did not state the organic content of the soil used, but it did state a nitrogen content of 0.057%. With the assumption of a C/N ratio of 10 and a carbon content of 50% of the organic matter, the organic content is calculated to be 1.1%.    [return to text]

9 Chapter 10. Nitrogen section 10. Nitrogen - losses     [return to text]

10 As a rule, about half the nitrogen in fresh manure becomes available to plants or is lost in the first season. At least half of the remaining nitrogen will become available during the next several years. So only one-quarter or less remains for humus development. If we assume that one-quarter remains, then the carbon content must also drop to one-quarter in order to maintain a carbon/nitrogen ratio of 20, and it must drop to one-eighth for the carbon/nitrogen ratio to fall to 10, the value that we defined for humus. If the manure contains 80% water and 20% dry matter, then the final amount of humus will be 20% of 1/8, or 1/40 of the fresh weight of the original manure.    [return to text]

11 This may be an overly conservative estimate. Possibly the assumption that the C/N ratio must drop to 10 before the organic residues are stable enough to be considered as humus is too strict, and perhaps less nitrogen is available to plants and more kept for humus development. Nevertheless, it is difficult to see how the assumptions could be loosened up enough to predict a humus increase of 1% in less than 20 years.    [return to text]

12 Following the same reasoning as we did with manure, we find that the carbon content of the decomposing hay will drop to 1/16 of its original value. A moisture content of 16% means a dry matter content of 84%, so that the amount of humus remaining is about 1/20 of the fresh, air-dried weight of the original hay    [return to text]

13 Rototillers with a horizontal axis of rotation compact the subsoil through the action of the tines, which push down on the soil at the low end of their rotation about the shaft. Tillers with a vertical axis of rotation do not compact the subsoil, although they are destructive otherwise.    [return to text]

© 2013 Robert Parnes

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