Appendix C. Acid And Basic Fertilizers

Fertilizers may alter the soil pH by either adding or removing acidity in the soil. The degree to which the pH changes is determined by cation exchange, in which an equivalent amount of one cation is exchanged for an equivalent amount of another.

Cation Equivalents

Cation exchange is an electrostatic phenomenon. It arises from an attraction between positively charged cations and negatively charged soil particles, or micelles. The soil particles may be either clay or organic matter. Exchange occurs in the displacement of cations of one species by those of another. Of the major cations, hydrogen and potassium have a charge or valence of +1, calcium and magnesium a valence of +2, and aluminum a valence of +3. A micelle has a high negative charge and attracts many cations. A calcium ion will neutralize two of those negative charges, potassium only one. Hence if the calcium drifts away from the micelle, its place can be occupied by two potassium ions or two hydrogen ions or one magnesium ion.

A mole is a unit of measure denoting a fixed number of ions, about 0.6 trillion trillion (known as Avogadro's number). A mole of potassium ions has the same number of ions as a mole of magnesium ions. The conversion factor that relates the weight of a cation to the number of moles is the formula weight, which is the same as the atomic weight for chemical elements. The number of moles of a cation is equal to the actual weight divided by the formula weight. Calcium has a formula weight of 40, and so the conversion factor for calcium is 40 gm/mole. Thus, 1000 grams of calcium contains (1000 gms)/(40 gms/mole) = 25 moles.

A mole, however, is not a good unit of measure. In cation exchange, a mole of potassium ions will not displace a mole of calcium ions; since it has only half as much electrostatic charge of calcium, the potassium will only displace half a mole of calcium. A better unit of measure is an equivalent. One equivalent of potassium ions will displace one equivalent of calcium ions. The number of equivalents of a cation is the number of moles multiplied by the ionic valence. So 1000 grams of calcium is 25 moles, or 50 equivalents.

If a fertilizer has a liming effect, it is able to neutralize some of the acidity in the soil. We can determine the liming value by calculating the number of equivalents of hydrogen which are neutralized. If the fertilizer has an acidifying effect, we can calculate the number of equivalents of hydrogen which the fertilizer adds to the soil.

Liming Fertilizers

Limestone

Since calcitic limestone is the most common material for neutralizing acid soil, it will be the reference. The calculations are easier with negligible error by assuming that it is pure calcium carbonate.

In neutralizing acidity, the carbonate in limestone reacts with hydrogen:

CaCO3 + 2H+ → Ca++ + H2O + CO2

The calcium replaces the hydrogen at a micelle. Since one molecule of limestone reacts with two of hydrogen, and since a mole of limestone and a mole of hydrogen ions contains the same number (Avogadro's number), it follows that one mole of limestone neutralizes two moles of hydrogen.

The formula weight of calcium carbonate is 100 grams/mole. One pound of limestone, or 454 grams, contains 4.54 moles. One pound of limestone will then neutralize 4.54 X 2, or about 9 equivalents of acidity.

Wood Ashes

The liming value of wood ashes is in its oxides and carbonates. Since this is only an estimate, we can assume that all of the calcium, magnesium and potassium exist as oxides.

Most ashes contain 30-35% calcium oxide, 3-4% magnesium oxide, and 3-8% potassium oxide. Assume average values: 32.5% calcium oxide, 3.5% magnesium oxide, and 5% potassium oxide. From the conversion factors in appendix A. Conversion Factors , the ashes have 23.2% calcium ions, 2.1% magnesium ions, and 4.1% potassium ions.

One pound of wood ashes, or 454 grams, then contains 105 grams of calcium ions, 9.5 grams of magnesium ions, and 18.6 grams of potassium ions. The formula weights of calcium, magnesium and potassium are 40, 24 and 78 grams/mole, respectively, and their ionic valences are +2, +2, and +1, respectively. The liming equivalents of one pound of average ashes is then, approximately:

Calcium: (105 gms) * (2 equiv/mole) / (40 gms/mole)   = 5.3 equivalents
Magnesium: (9.5 gms) * (2 equiv/mole) / (24 gms/mole)   = 0.8 equivalents
Potassium: (18.6 gms) * (1 equiv/mole) / (39 gms/mole)   = 0.1 equivalents

Total   = 6.2 equivalents

One pound of wood ashes neutralizes about 6 equivalents of acidity. Since one pound of limestone neutralizes 9 equivalents of acidity, a pound of ashes has the same liming value as about 2/3 pounds of limestone.

Sodium Nitrate

Experience shows that sodium nitrate has a liming effect, but the reason is not clear, because sodium nitrate is a neutral salt. Sodium ions have an alkaline effect, forming sodium hydroxide with water, and nitrate ions have an acidifying effect, forming nitric acid with water:

Na2NO3 + 2H2O → 2NaOH + H2NO3

One possible explanation for an imbalance is that the sodium hydroxide reacts with carbonic acid in the soil to form sodium carbonate:

2NaOH + H2CO3 → Na2CO3 + 2H2O

Sodium carbonate has a low solubility. What may be happening is that while the nitrate ions are taken up by plants or organisms or lost by leaching or denitrification, the associated hydrogen ions leach from the soil, and the sodium carbonate remains behind [94, page 340].

If this does occur, the sodium carbonate eventually dissolves and has a liming effect according to the reaction:

Na2CO3 + 2H++ → 2Na+ + Na2CO2 + H2O

Each mole of sodium ions replaces one mole of hydrogen ions. Since sodium nitrate contains one sodium ion, a mole of sodium nitrate should lead to the neutralization of one mole of hydrogen ions. Sodium nitrate has a formula weight of 85, and so a pound (454 gms) contains about 5.3 moles, and it can neutralize 5.3 equivalents of acidity. Since one pound of limestone neutralizes about 9 equivalents of hydrogen ions, one pound of sodium nitrate should have a liming capacity somewhat more than 1/2 pound of limestone.

According to the official method for determining the acidifying effect of fertilizers, the liming value of sodium nitrate is actually about 1/3 pound of limestone for each pound of sodium nitrate [61, page 178]. In the absence of any other plausible hypothesis to explain the liming effect of sodium nitrate, this model gives a reasonable explanation, except perhaps that a significant fraction of the nitrate and hydrogen ions remain in the soil.

Bone Meal and Rock Phosphate

The claim sometimes made for the liming capability of rock phosphate is exaggerated. It is based on an artificial chemical reaction splitting tricalcium phosphate into two products:

Ca3(PO4)2 → P2O5 + 3CaO

There are several problems with this:

Nevertheless, in an effort to at least estimate an upper bound to the liming capability of rock phosphate, it may be worthwhile to assume that it is indeed tricalcium phosphate.

Tricalcium phosphate can react with water in three different ways to produce calcium oxide:

Ca3(PO4)2 + H2O → 2CaHPO4 + CaO

Ca3(PO4)2 + 2H2O → Ca(H2PO4)2 + 2CaO

Ca3(PO4)2 + 3H2O → 2H2PO4 + 3CaO

The first reaction produces dicalcium phosphate, whereby one of the calcium moles has a liming effect; the second produces monocalcium phosphate with two moles of calcium having a liming effect; and the third phosphoric acid with all three moles of calcium having a liming effect.

In practice, all three reactions should take place to some extent. But the degree to which they occur depends on the soil pH. Dicalcium phosphate predominates when the soil is alkaline, monocalcium phosphate when the soil is acidic, and phosphoric acid only when the soil is extremely acidic. The last condition is unlikely in agricultural soils, and we shall ignore the phosphoric acid option.

At a pH of 7, monocalcium and dicalcium phosphate exist in approximately equal amounts. If liming were necessary at a pH of 7, the net liming capability of one mole of the combination would be equivalent to about 1-1/2 moles of lime. So at a ph where lime is adviseable, the liming value of the mixture of phosphates should lie in the range between 1-1/2 and 2 moles of lime; maybe a reasonable value is the average, or 1-3/4 moles.

Since a mole of tricalcium phosphate contains three moles of calcium, the portion of calcium contributing to a liming effect is 1-3/4 / 3, or about 60%.

Colloidal rock phosphate has a stated CaO content of about 20%. From Appendix A. Conversion Factors , the actual calcium content is 20 / 1.4, or about 14%. Of this, 60%, or about 8% of the calcium, should be associated with a lime value. A pound of colloidal phosphate then contains 454 X 0.08, or about 36 grams of calcium having a liming value. Since the formula weight of calcium is 40 and its ionic valence 2, one pound of colloidal rock phosphate neutralizes 36 X 2 / 40 = 1.8 equivalents of acidity. Thus a pound of colloidal phosphate has a liming value of almost 1/5 pound of limestone. A normal application rate of 1 ton/acre will supply the equivalent of about 400 lbs of limestone/acre. As stated above, however, this is an upper limit; some of the calcium is associated with one or more of the variables in apatite - most likely fluoride, which is common in colloidal phosphate from Florida, a principal source in the U.S.

Hard rock phosphate may have a stated CaO content of about 35%. A similar calculation predicts a liming value - also an upper limit - of about 1/3 pound of limestone.

Bone meal does contain limestone. Theoretically, 1/4 of its calcium is in limestone and 3/4 in tricalcium phosphate. If bone meal has a specified CaO content of 28%, 7% should be in the form of limestone and 21% of tricalcium phosphate. With this division, one pound of bone meal has a liming value similar to that of hard rock phosphate, or about 1/3 pound of limestone.

Magnesia

Magnesia is magnesium oxide. We can determine its liming value using the same reasoning as we did for wood ashes, because in that example we assumed that wood ashes contain 3.5% magnesium oxide. Now consider a pound of magnesium oxide, or 454 grams. The number of equivalents in a pound is 454 gms X 2 equiv/mole / 24 gm/mole, or 38 equivalents. Then one pound of magnesia should have the same liming value as approximately 4 pounds of limestone.

Acidifying Fertilizers

Ammonium Sulfate

Chemically, ammonium sulfate is a neutral salt. In soil, however, the ammonium is oxidized by various bacteria to nitrite and thence to nitrate. The net reaction is:

(NH4)2SO4 + 4O2 → 4H+ + 2NO3- + SO4-- + 2H2O

One mole of ammonium sulfate produces 4 moles of hydrogen ions.

The formula weight of ammonium sulfate is 132 gm/mole. One pound of ammonium sulfate contains 3.44 moles. Since each mole of ammonium sulfate produces 4 moles of hydrogen ions, one pound of ammonium sulfate will produce 3.44 X 4 = 13.8 equivalents of acidity. So each pound of ammonium sulfate theoretically requires about 1-1/2 pounds of limestone to neutralize its acidity.

According to the official method used to determine the acidifying effect of fertilizers, one pound of ammonium sulfate requires only 1.1 pounds of limestone to neutralize it [61, page 178]. So something else is also occurring which remains unknown (at least to me; maybe the oxidation of nitrite is more complex than is assumed in the above chemical reaction).

Muriate of Potash (Potassium Chloride)

Theoretically, potassium chloride is a neutral salt; it should not affect the soil pH. Whether it actually does is a matter of opinion. The official method used to determine the acidity of fertilizers predicts that potassium chloride has no acidifying effect, but in the field an effect has been noticed.

Potassium chloride has a potential to acidify a soil wherever leaching is prominent. The potassium is absorbed by the plant or fixed in the soil, and the chloride leaches out, taking with it any available cations, principally calcium and magnesium. Their place at the cation exchange micelle is taken up by hydrogen and aluminum; the result is a drop in the soil pH. This activity is, in fact, the predominant way in which soils in humid areas become acid, so it is a reasonable model for estimating the acidifying effect of muriate of potash.

Potassium chloride has a formula weight of about 75 grams/mole. One pound of potassium chloride then contains about 6 moles, and each mole produces one equivalent of acidity. So a pound of muriate of potash has a potential acidity which can be neutralized by about 2/3 pounds of limestone.

Sulfur

Soil bacteria oxidize sulfur with a net reaction:

2S +3O2 + 2H2O → 4H+ +2SO4-

The result is a release of two hydrogen ions for each sulfur atom.

The formula weight of sulfur is 32. One pound contains 454 / 32, or about 14 moles, and produces 28 equivalents of acidity. The only reason for adding sulfur to the soil in substantial quantities is to neutralize excessive quantities of limestone. Sulfur is sometimes dusted on trees as a fungicide, but the amount usually used is too small to have a significant effect on soil acidity. One pound of sulfur will neutralize 28 / 9, or about 3 pounds of limestone.

Acid Rain

Although it may supply nitrogen and sulfur, acid rain is not usually considered as a fertilizer. Its influence on our environment, however, is of great interest, and estimating its effect on the soil should be worth the effort.

The calculation is based upon the assumption that rain is unbuffered. This means that there is no reservoir of acidity: it is all in solution. Consequently, the acidity is the hydrogen ion concentration in solution. It is related to the pH by the equation:

acidity = 10-pH moles/liter

A neutral solution having a pH of 7 will then contain 10-7, or 1/10,000,000 moles of hydrogen ions/liter. For our purpose we want to know the hydrogen ion concentration in terms of equivalents/gm of water. This can be determined, since we know that the density of water is 1 gm/ml, and one mole of hydrogen ions is equal to an equivalent of hydrogen. The result is that the hydrogen ion concentration is given by:

acidity = 10-pH-3 equivalents/gm

Consider one inch of rain. Its weight over an acre is:

W = 1 gm/cm3 * 2.54 cm/inch * 43560 ft2/acre * (30.51 cm/ft)2

or

W = 103,000,000 gm/acre per inch of rain

Consequently the acidity of 1 inch of rain falling on an acre is:

acidity = 103 * 106 * 10-pH-3

As a reference, the liming value of limestone is 9 equivalents/pound. The limestone requirement (L.R.) to neutralize 1 inch of acid rain is

L.R. = 103 * 103 * 10-pH

L.R. = 11000 * 10-pH

For example, the lime requirement to neutralize rain with a pH of 4 is about l lb/acre/inch of rain. A season of 100 inches of this rain will need about 100 lbs of limestone/acre to neutralize it. So much acid rain is rare, but even then the resulting acidity is not much when we are accustomed to spreading a ton of limestone every few years.

The prediction that acid rain has a negligible effect on soil is due to the assumption that rain is unbuffered. It should, however, be reasonable, because otherwise the rain would contain a high content of buffering chemicals, which is unlikely.

This is not meant to infer that acid rain is not influencing our environment. There is little doubt of its impact on unbuffered lakes and streams, and it appears to affect plant foliage by direct contact. But its influence on agricultural soils should be insignificant.

Superphosphate and Triple Phosphate

In principle the acidifying tendency of synthetic phosphates can be calculated. In practice the effort is not worthwhile. Phosphate fertilizers contain several forms of calcium phosphate, each with its own chemical behavior. An experimental determination seem more feasible. I did this with a single sample of superphosphate and triple phosphate, by measuring the amount of alkali needed to neutralize a water mixture. The results were as follows:

Apparently the triple phosphate was treated with a large excess of acid in order to increase its solubility. Note that this test was done with only one sample of each fertilizer, and the results may be not valid for all samples.

© 2013 Robert Parnes

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