Chapter 1. Introduction

Two questions confront most people who use fertilizers. These are what specific fertilizer to use, and how much to spread. Unfortunately, there are no universal answers, and this book does not offer any, other than a general one: a decision should take into account the energy in organic fertilizers. A major part of the book is an attempt to assign a value to this energy.

Otherwise, the book's purpose is to guide the reader to a personal answer to these questions.

One problem, however, is the wide diversity of opinions about what is a fertilizer; it varies from the well-known NPK trio to a wide range of elements including all the trace elements one can imagine.

Dictionaries offer little help. The definition of fertilizer in Merriam-Webster is: “Natural or artificial substance containing the chemical elements that improve growth and productiveness of plants;” in American Heritage Dictionary of the English Language: “Any of a large number of natural and synthetic materials ... spread on or worked into soil to increase its capacity to support plant growth;” in Macmillan: “A natural or chemical substance added to soil in order to help plants grow.” They are too inclusive: water, air and organic carbon compounds are natural materials. Plants do not grow at all without water; they grow poorly without air; and they can grow without organic carbon, but only with special attention and at increased expense. An artificial boundary separates a traditional sense of a fertilizer and other elements which are, in some cases, more essential to plant growth.

So the following is a definition which is reaonably consistent with its meaning in this book: a fertilizer is any substance except water, carbon or oxygen which, upon absorption by a plant, assists in its growth and is at least potentially capable of being bought and sold.

The next task then is to determine whether a fertilizer is necessary and, if so, in what quantity. Chapters 4. Nutrient Requirements and 5. Soil Nutrient Supply should help. They discuss the nutrient requirements of plants and the possible ways of determining what the soil might supply. But they are only a rough guide. The nutrient content of a plant is an approximation to its nutrient requirement, which itself is an approximation to the fertilizer requirement. Plants may take up more than they need, and roots may not be able to absorb all the fertilizer which is offered to them.

The data in tables 3. Estimated Fertilizer Requirements - Field Crops and 4. Estimated Fertilizer Requirements - Vegetables And Fruits show the wide variation in nutrient content that can exist, as well as the wide variation in fertilizers used throughout the U.S. The ability of the roots to accumulate nutrients depends upon the supply, but it also depends on other factors, such as the soil structure, the availability of air and water, and the population of soil organisms surrounding the roots. These are noted, but they can be discussed only in a qualitative way.

Soil tests are usually good. But crops in some soils do not respond to potassium or phosphorus fertilizer even though tests show a deficiency. Cold soils in spring may cause a phosphorus deficiency even though tests show an adequate amount. We have seen those seasons with terrible crop results no matter how much nitrogen was used, and other years when nothing could go wrong.

When we add to these variables the variation in the nutrient content of organic residues, the very fertilizers that are encouraged in these pages, we must conclude that the idea of being able to calculate fertilizer applications with any hope of certainty is an illusion. At best we can apply what we think is necessary, but we should be prepared to make appropriate modifications either later in the season or in the following year, taking into account the weather and the growth of the plants.

The second task of choosing a particular fertilizer is, of course, a matter of opinion and personal philosophy, but it is not as beset with difficulty as choosing the quantity to use. The forces controlling our current state of affairs define three kinds of fertilizers: organic residues, naturally occurring inorganic minerals, and synthetic products.

Most organic residues are poor fertilizers. Fresh cow manure, for example, may have a nitrogen content of 1/2% or lower.

Natural inorganic minerals, such as calcitic limestone and other rock powders, have an intermediate to high nutrient content. Some are moderately to highly insoluble (limestone, rock phosphate, granite dust, greensand, sulfur), and some are very soluble (Chilean nitrate, potassium magnesium sulfate, epsom salts, potassium sulfate).

Synthetic fertilizers also have an intermediate to high nutrient content, but mostly high, and they are all very soluble. Common examples are urea, sodium nitrate, ammonium nitrate, liquid ammonia, ammonium phosphate, ammonium sulfate, triple phosphate, and potassium chloride. Superphosphate is also synthetic but is no longer common, its popularity having fallen after the development of more powerful fertilizers such as triple phosphate and ammonium phosphate.

Two exceptions to these categories are bone meal and wood ashes. In one sense they are organic residues, but they contain no organic matter. Neither are they natural minerals or synthetic products. So, to resolve the dilemma, we shall regard them as organic residues but keep in mind that they do not contain the advantages of organic residues noted in the book.

In view of their usually low fertilizer value, the obvious question is what is the advantage of organic fertilizers?

The discussion in chapter 2. Essentials of Soil Fertility argues that organic residues supply energy to organisms which maintain soil structure and fertility. The large amount of energy in the residues compared to the small amount of nutrients is a measure of the importance of that energy. Plants and a soil system containing thousands of different participants have adapted to such an energy-nutrient balance. Any change from this balance must take into account the complexity of interactions within such a system, interactions which are likely to make a sustainable alternative economically impractical.

The abandonment of many eastern soils, quarantine of southeastern soils, erosion of midwestern soils and compaction of western soils are results of our attention only to fertilizers and a refusal to take into account the entire agricultural system.

Moreover, in these days when energy demand is a major cause of the conflicts among nations, it should come as no surprise to learn that energy is a primary factor in the vigor and fertility of soils.

As the natural soil cycle has necessarily evolved, organic residues supplied in sufficient quantity and variety will not only maintain the energy supply but will also furnish an adequate amount of most nutrients for growing crops. In fact, gardens which receive large amounts of residues often have a surplus of nutrients.

Nevertheless, situations occur where organic residues are not available in sufficient quantity to supply the required nutrients. This may happen when the soil is chronically low in a nutrient; when a cash crop is grown on a small parcel of land with a limited availability of residues; or during a period of transition to a system relying principally on residues. At such times an alternative is necessary. What then?

Although this book promotes the use of organic fertilizers, what is irritating is the extremes that some people take to justify claims that synthetic fertilizers are harmful. Some indeed are, but the broad strictures against them all is carried to the point of absurdity.

An obvious example is nitrogen. One argument against soluble nitrogen fertilizers is that they interfere with the natural nitrogen-producing capability of the soil. A second argument is that their production is energy-intensive and is ranked either first or second in the amount of energy consumed on the farm for many crops [68]. Furthermore, their high nitrogen concentration discourages their use in conservative or moderate applications. These arguments favor the use of naturally occurring Chilean nitrate, for example, over synthetic fertilizers.

The problem with the first argument is that organic nitrogen residues also interfere with natural nitrogen-fixing processes; nitrogen-fixing processes require energy and will only occur where they are necessary in a nitrogen-deficient soil. The second argument does not take into account the energy required to mine Chilean nitrate and transport it halfway around the world, where it is unloaded and shipped by truck or rail over, on average, half a continent.

The third argument is reasonable but is equally valid for high-nitrogen organic products such as blood meal.

In choosing a mineral fertilizer, the many claims in favor of natural inorganic fertilizers over synthetics are even stranger. Why, for example, is rock phosphate preferable to superphosphate or triple phosphate?

Following is an accumulated list of claims made at various times in favor of rock phosphate over the soluble phosphates; most of the discussions where superphosphate is specified apply equally well to other phosphates.

1. Claim: As an insoluble product, rock phosphate is consistent with the assertion that one should feed the soil and let the soil feed the plant. Soluble superphosphate feeds the plant directly.

Comment: What does it mean, to feed the soil? Chapter 2. Essentials of Soil Fertility shows that the only food the soil needs, except under extraordinary circumstances, is whatever furnishes energy; it needs nothing else. Mineral fertilizers contain no energy, and rock phosphate does not feed the soil.

Apparently what is meant is not that rock phosphate feeds the soil but that the soil feeds the plant by making rock phosphate available. Why should this be important? Perhaps because non-renewable energy sources are conserved by leaving the work to soil organisms; if so let us continue this direction of thought when discussing claim 3. The only other possible rationale is a philosophical one, in which case we should defer to claim 9.

2. Claim: Despite its high degree of insolubility, rock phosphate releases its nutrients slowly over a long period of time; while superphosphate dissolves quickly but soon reacts with the soil and becomes firmly bound.

Comment: Whether superphosphate is more firmly bound than rock phosphate depends upon the pH. In very acid soils, one might expect that the phosphorus in aluminum phosphate is more firmly bound than the phosphorus in rock phosphate, but in a neutral or alkaline soil, aluminum is tied up and does not combine with superphosphate. Experimental studies comparing the residual fertilizer value are inconclusive, some favoring rock phosphate and others supporting superphosphate [31], [29].

Whatever differences may exist in a particular situation are reduced considerably with the use of organic residues to stimulate biological activity.

Furthermore, phosphorus does not leach from the soil; any phosphorus, no matter in what form, eventually becomes available at a rate dependent primarily upon biological activity.

3. Claim: Rock phosphate is a natural product; while superphosphate is chemically processed and energy-consumptive.

Comment: Why, however, should a natural product be advantageous? Possibly because it contains essential trace elements, and rock phosphate does contain trace elements, very little but perhaps enough to be important in extreme cases. No one has claimed that crops are more nutritious when the soil is fertilized with rock phosphate rather than superphosphate.

The one advantage that a natural fertilizer often offers is its appeal to the consumer in the marketing of produce, especially with the implication that no pesticides were used. It is unfortunate that unscrupulous merchants mislabel food products as natural or organic, with the result that some states now have rigid labelling requirements. It is also unfortunate, however, to encounter those who thoughtlessly connect the dangers of pesticides to alleged dangers of mineral fertilizers.

Acidulated phosphates are indeed more energy-intensive to produce than rock phosphate. Owing to the additional energy required to transport rock phosphate, however, the net energy difference is small compared with other agricultural energies.

4. Claim: Rock phosphate raises the soil pH; while acidulated phosphates lower the pH. According to one source, triple phosphate can lower the soil pH down to pH 2, with a devastating effect on soil life.

Comment: No such acidifying effect has been shown, and the source referred to offers no documentation. However, it is possible. The extent of a drop in pH would depend upon the buffering capacity of the soil1. It should not be severe in a soil with adequate organic matter, and it should be highly localized and temporary, lasting no longer than a few hours. Organic products can also destroy soil life [40].

An estimate of the acidifying tendency of superphosphate and triple phosphate is in appendix C. Acid and Basic Nature of Fertilizers .

5. Claim: The sulfur in superphosphate can attract soil organisms which attack beneficial soil fungi.

Comment: This is untrue and a confusion over the distinction between elemental sulfur and its oxides. The sulfur in superphosphate is in sulfate form, which is oxidized sulfur. Sulfate attracts no organisms unless the soil lacks oxygen, in which case fungi cannot survive anyway. Elemental sulfur does attract certain bacteria, which oxidize it to sulfate; but no phosphate fertilizer contains elemental sulfur. Furthermore, plants absorb sulfate for the nutrient value of the sulfur.

However, an excess of sulfate can lock up molybdenum, so superphosphate may be harmful if used in excess.

6. Claim: Rock phosphate can be spread every four years or so; while superphosphate is customarily spread every year, requiring extra labor.

Comment: This is a valid point, but only if no fertilizer other than superphosphate needs spreading annually.

7. Claim: Rock phosphate encourages the growth of certain root-associated fungi which are capable of breaking down insoluble mineral products and transferring the nutrients to the roots; while soluble phosphorus depresses their growth.

Comment: This is true inasmuch as these fungi only seem to proliferate where their usefulness has an advantage. Reliable experiments with them have been difficult, and whether they would supply enough phosphorus from unavailable reserves to meet the requirements for fast-growing and high-yielding annuals has not been established.

In a typical soil, however, rock phosphate should be no better than soluble phosphorus fertilizers in encouraging these fungi. The presence of the desireable fungi depends more upon the available phosphorus level in the soil rather than upon the availability of phosphorus in the fertilizer. Figure Figure 1. Soil Test Results is the result of a survey of soil test results, mostly of organically managed soils, and shows that most such soils have moderate to high levels of phosphorus. There is a much greater desire to assure a good start to transplants in cold soils than to worry about fungi, and the tendency to hedge by continually adding phosphorus suppresses these fungi no matter which fertilizer is used.

8. Claim: Concentrated phosphates contain significant amounts of cadmium, high enough that one brand was banned by the Canadian government [60].

Comment: This is true, but the cadmium comes from the rock phosphate ore used in production, and the amount present varies with the different ore deposits. Compared with the phosphate content, some synthetic fertilizers contain no more cadmium than the colloidal rock phosphate from Florida, but others have more. When using appreciable amounts of synthetic phosphate, one would have to pick a brand carefully.

On the other hand, since the phosphate ore itself contains cadmium, simply being natural does not mean that a fertilizer is safe; in addition to cadmium, rock phosphate contains fluorides and has a small amount of radioactivity. None of these impurities have been found to be hazardous.

However, breathing dust from colloidal phosphate without a protective mask can be detrimental. One must be careful when working with any agricultural product, whether organic, natural or synthetic.

9. Claim: Rock phosphate contains phosphorus which becomes available over a long period of time; while the acidulated phosphates are highly concentrated phosphorus carriers. We view the soil as one integrated, living organism that should be treated gently. Those of us who favor a preventive health plan can appreciate the unpleasantness of a strong medicine, whose side effects may be worse than the disease. So it is with a concentrate in the soil; even though we may not know its side-effects, we have an uneasy feeling about it. This feeling is reinforced by the realization that we do not need the concentrate. With good management we can grow superior crops without it.

Comment: The force of this argument, though compelling, is diminished by the realization that some organic products are also concentrated. Chicken manure, bloodmeal and cottonseed meal are as strong as some synthetic fertilizer blends.

If anyone has additional reasons for preferring rock phosphate, I should like to know them. In the meantime, my opinion is that the last claim, #9, is the strongest argument in its favor. The others may have merit but are essentially attempts to find a practical rationale for a philosophical point of view. If this is so, it should be so accepted. It is perhaps a manifestation of the notion that we should understand the soil and become sensitive to the effects of what we do to it.

Nevertheless, as the criterion, this philosophy limits the economic value of organic agriculture. Triple phosphate is cheaper than rock phosphate and more easily available. If the claim against synthetic phosphates is based on a philosophical argument, and if the product does not degrade the soil when used judiciously and if it does not produce crops of inferior quality, then this shuts out people who are concerned about soil and crop quality and accept the value of organic residues but cannot afford the premium for rock phosphate.

Furthermore, being more labor-intensive, organic produce is more costly; it doesn't seem fair, especially to low-income people, to add an unjustifiable cost based on irrelevant claims.

The major emphasis for good soil management should be on recycling organic residues; further restrictions based on conjectural comparisons of inorganic fertilizers are not only an unnecessary digression but also a dilution of the major value supporting organic agriculture.

Furthermore, whatever rationale is applied in establishing a prohibition against some fertilizers should apply to the entire agricultural system. No fertilizer has as traumatic an effect on soil structure, moisture and soil life as a rototiller2. If we are sensitive to soil processes, we should ban the rototiller. If we are concerned about non-renewable energy use, we should ban plastic mulches and gas-powered machinery. The most aggravating part of the claims favoring natural over synthetic fertilizers is their selectivity.

One of the purposes of the book is to forget our preconceptions, if only temporarily, and to examine the consequences of all fertilizers.

Two general statements regarding fertilization for gardens may be appropriate. The first is that a garden differs fundamentally from a farm.

A farm is a source of nutrients, and a garden is a sink.

A farm produces hay and straw for mulch, and it produces animal manure, both of which contribute to the fertility of a garden. A garden takes all that fertility for producing a high intensity of valuable crops.

Moreover, the tillage required to maintain a garden tends to destroy fertility, whereas the tillage on a farm, at least under reasonable conditions on small farms, builds fertility.

These distinctions are stereotypes, but they do indicate tendencies. To build and maintain a sustainable agricultural system, people who garden should either integrate with a farm, or they should adopt some of the practices used on a farm to supply the fertility poured down their garden sink. Calculations showing the importance of farmland to garden fertility, both in terms of building humus and supplying nutrients, are presented in chapters 2. Essentials of Soil Fertility and 6. Unprocessed Residues - Animal Manure .

The second statement is that an inverse relation exists between soil fertility and ecological diversity: over a broad range, increasing one decreases the other. A soil high in nitrates decreases the activity of those soil organisms that produce nitrates. Nitrogen fixation by bacteria associated with legumes is suppressed when available soil nitrogen is already high enough to support legumes. Those classes of fungi which supply a plant with phosphorus and other minerals are suppressed if the minerals are already abundant.

Tillage limits ecological diversity by favoring those organisms that can temporarily go into a dormant stage during unfavorable conditions.

Gardens require adequate fertility and tillage management for a satisfactory harvest, but it is not wise to go beyond a prudent course.

Finally, the tone of the book may be summarized in a statement by L. L. Van Slyke [79], who in 1912 listed four advantages of commercial fertilizers (convenience, opportunity for choice, uniformity, and uniform mixing of blends). He also discussed three disadvantages, the third of which follows: “(3) Lack of educational incentive.--The most serious disadvantage in the use of commercial fertilizers, as they are actually used in most cases, is that farmers are not stimulated to acquire needed information in regard to plant-foods and their proper use. Many farmers use commercial fertilizers blindly in somewhat the same way that people use patent medicines. In the hope of increasing yield of crops, without definitely learning why crops are decreasing, commercial fertilizers are tried, some brand being used in accordance with the recommendation of a neighbor or some seller of fertilizers. It is easy to acquire the 'fertilizer habit' and difficult to abandon it. This blind, slavish use of fertilizers deadens the intellectual activity and in many cases has led to actually decreased productivity of soil when sole dependence has been placed on their use for long-continued periods.”

See [15], [21], [50], [86], [94] for further reading. These references are the basis for much of this book.


1 Chapter 14. Calcium And Soil Ph has a discussion of cation exchange and buffering    [return to text]

2 Chapter 2. Essentials of Soil Fertility - Tillage offers an argument against the rototiller    [return to text]

© 2013 Robert Parnes

creative commons icon

This work is licensed under http://creativecommons.org/licenses/by-nd/3.0