Chapter 9. Introduction to Part III


Except for this one, each chapter in this part covers one of the major nutrients or, collectively, the micronutrients (or trace elements).

Three major topics in those chapters are:

This chapter restricts itself to two auxiliary topics: fertilizer blends and effects of the ionic nature of substances taken up by plants.

Fertilizer Blends

This book has no discussion of synthesized blends, although the relevant chapters do cover the individual ingredients. The purpose of mentionning them here is to note the convention in specifying their content.

Blends are mixtures of commercial fertilizers, such as urea, triple phosphate and potassium chloride. They are a convenience prepared to supply specified amounts of nitrogen, phosphate (P2O5), and potash (K2O)1. A 10-10-10 fertilizer contains 10% of each of these components. Potassium chloride alone carries a 0-0-60 analysis, meaning that it contains no nitrogen or phosphate but has 60% potash.

Some blends contain an additional nutrient, such as magnesium. In that case the specification will include its content with a specific identification, such as 10-10-10-5Mg.

Note that the identification specifies phosphorus and potassium in an oxide form. This convention arose in the early years of analysis. At that time, the procedure was to roast a sample in a standardardized process. This oxidized the minerals, which were weighed and reported as such. Procedures have changed but the convention remains, perhaps in order to avoid the reaction from buyers who think they are suddenly receiving less for their purchase. The actual nitrogen-phosphorus-potassium content of a 10-10-10 fertilizer is about 10-5-8.

In order to avoid confusion, the tables in these chapters maintain this same convention: expressing phosphorus in terms of the oxidized form phosphate and potassium as potash.

Some blends such as 10-10-10 are characterized as complete fertilizers, even though they contain no other nutrients. Moreover, the only information usually available on the commercial composts and organic mixtures is their NPK content. So everyone who uses these products should understand the notation.

Cations and Anions

Nutrients absorbed by plants are in ionic form.

Ions are electrically charged chemical elements or compounds. They are the result of salts dissolved in water. Table salt is sodium chloride and dissolves in water, producing sodium ions and chloride ions. The sodium ions have a positive electrical charge, and the chloride ions a negative charge.

If two electrodes are placed in salted water and connected to a battery, the sodium ions will drift to the negative electrode, or cathode, and the chloride ions to the positive electrode, or anode. Consequently, positively-charged ions are called cations, and negatively-charged ions anions.

The common nutrient cations are nitrogen in ammonium form, calcium, magnesium, potassium, copper, iron, manganese and zinc; common anions are nitrogen in nitrate form, phosphorus, sulfur, boron and molybdenum.

Some of the properties of nutrients depend on whether they are present as cations or anions. Their net movement from the soil to the roots is such that the plant remains electrically neutral with respect to the soil. This requires an equilibrium between the net flow of cations and the net flow of anions. Consequently the total flow of cations is limited by the availability of anions, and vice versa.

The limitation on the total intake of cations limits each one: calcium, magnesium, potassium (and ammonium-nitrogen in acid soils)2. Plants, however, have a preference for potassium3 built into their behavior and will absorb as much as is available; the result is often a deficiency in the other cations but more likely magnesium.

Similarly, an interaction exists between nitrogen and phosphorus (except in acid soils). If both are in excess, phosphorus may be deficient, owing to the higher mobility of nitrogen.

In addition, the nature of the ionic charge determines how, if at all, the soil stores plant nutrients. Cation exchange4 is the mechanism for storing the major cations (calcium, magnesium and potassium).

Anion exchange also exists, but it does not function as consistently as cation exchange, and it is not beneficial. Nitrate-nitrogen is not held by anion exchange; sulfur is held to some extent; and phosphorus is held so strongly that it is not easily available.

Fortunately, anion exchange is not necessary for the conservation of anions, because anions are a significant component of organic matter. Soil microorganisms have a much greater requirement for anions to produce cell tissue than for cations5. As the organisms die and are attacked in turn by other organisms, a portion of these nutrients becomes available to plants.

Nitrogen has a balancing effect, since it can exist in the soil as either a cation (ammonium) or an anion (nitrate). In acid soils, calcium and magnesium are low, and nitrogen tends to be present predominantly in the ammonium form, which can be adsorbed and stored in the cation exchange mechanism. As a stored ammonium cation, nitrogen competes less with phosphorus for plant uptake. This is an advantage in an acid soil, where phosphorus may be strongly bound, and plants need all the help they can get to obtain a sufficient quantity. In a mildly acid, neutral or alkaline soil, nitrogen is predominantly in the nitrate form. As an anion it does not compete with calcium and magnesium for adsorption by cation exchange. This favors many plants which have a higher requirement for calcium and magnesium than those more tolerant of acid conditions.

Cation exchange is not important for the trace elements. Trace element cations are held by chelation, discussed in chapter 16. Micronutrients . The anion trace elements do not chelate but are bound to a small extent, perhaps by anion exchange, and they are present in the organic matter. Their storage in the soil, however, is less efficient than that of other nutrients. Molybdenum is rarely affected by the lack of a good storage mechanism, because it is required in such a small amount. But Nature seems to have forgotten boron, for the soil has no adequate mechanism for holding it; it is the most common trace element to be deficient.

1 K is for Kalium, which is Latin and German for potassium    [return to text]

2 We can ignore the effect on trace elements because the very small quantities required manage to slip in if the supply in the soil is adequate.    [return to text]

3 For an explanation why plants favor potassium over other cations, see chapter 12. Potassium - Potassium In The Plant     [return to text]

4 Cation exchange is discussed in chapters 14. Calcium And Soil Ph and 15. Magnesium     [return to text]

5 This tendency to absorb anions preferentially over cations may appear to violate the principle of electrical neutrality. It doesn't, because the microorganisms can release carbonate anions to compensate for absorbed mineral anions. A similar situation occurs in plants, which could release either carbonate anions or hydrogen cations to balance any difference in absorbed electrical charges. This compensation is the reason for the broad statements above inferring a relationship between the flow of cations and anions, but not an equality. The number of absorbed cations need not equal the number of absorbed anions, but the difference between them is approximately constant.    [return to text]

© 2013 Robert Parnes

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