Soil Acidity (Low pH) Spells Fertility Deficiencies

Acidity is a common soil condition in many parts of the temperate zone. It occurs where the rainfall gives water enough to go down through and to wash out much of the fertility. In general, if the rainfall is high enough to provide plenty of water during the crop growing season, there will also be enough water, with its carbonic acid, to leach the soil or to give it hydrogen in place of much of its supply of plant nutrients, and thereby make it acid.

Acid soils differ in their degrees of acidity, expressed as negative logarithms or what are known as pH values. Accordingly, then, a higher degree of acidity is represented by a lower pH value, or figure. This merely means that as more hydrogen has come in, less fertility or other positively charged nutrients are left there. Timbered soils of the eastern United States are acid; also those of the eastern edge of the prairie are sour. Acidity is a natural condition where soils have had rainfall enough so they have been growing much vegetation. Such soils have therefore been subjected (a) to a leaching force taking the fertility downward, and (b) to the competitive force of the vegetation with its roots taking the nutrients upward. In the latter case, those nutrients are built into organic combinations of them above the soils of the forests and within the surface soils of the prairies. Consequently acid soils have distinct surface soil and subsoil horizons in their profiles. They are naturally low in fertility, especially in the subsoil, and have been growing mainly carbonaceous or woody vegetation. It is no small soil’s problem to have such soils grow protein-supplying, mineral-rich, highly nutritious crops.

Natural soil acidity is in reality, then, mainly a shortage of fertility in terms of many plant nutrients reflected in the physiological simplicity of the vegetation. This is the situation because the soil has been under cropping and leaching for ages. This was true before we took over to intensify these fertility-depleting effects. This, then, is the condition of the soil that prompts the common question, “How can we grow mineral-rich, fertility-consuming forages, like the legumes, of good feed value for such high-powered, protein-producing animals as cows?”

A Little Science Led Us Astray

It was the growing agricultural science of the early decades of the 20th Century that brought liming of the soil back as a more general agricultural practice. We cannot say that liming was an art carried over from colonial days. It had been pushed out when fertilizers came into use. Liming the soil has become an extensive practice under the encouragement of an embryo soil-testing service. That service was guided by the belief that the applications (a) of limestone, which is a carbonate of calcium; (b) of hydrated lime; which is an alkaline calcium hydroxide; or (c) of quicklime, which is a caustic oxide of calcium, are all beneficial for crop growth because each of these is ammunition in the fight against soil acidity, or against the high concentration of hydrogen in the soil, or the soil with a low pH figure, as the degree of acidity is now regularly expressed.^*

This struggle to drive the active hydrogen ion, or acidity, out of the soil was aided by the technological advancements giving us instruments and equipment that measured the hydrogen ion to a finer degree than known before. The ease and speed with which soil acidity could be detected and measured encouraged the widespread testing of soils. This activity discovered soil acidity almost everywhere in connection with extensive agriculture. Through the help of the pH-measuring gadgets we were impressed by the apparent universality of soil acidity. Only a few humid soils were not seriously stocked with acid.

We discovered that for acid soils, in general, the productivity was lower as the degree of acidity was higher, or as the pH value for the soil was a lower figure. From such a discovery we might expect to conclude–even though it was later found to be the wrong conclusion–that the presence of the large concentration of hydrogen ions in the soil, or a certain low pH value, was the cause of the poor crops. This conclusion would be expected also from the bigger troubles in growing the proteinaceous, mineral-rich legumes of higher feeding values and of more physiological complexity through which these values alone are possible.

The extensive use of limestone in the corn belt has now multiplied itself into the millions of tons of these natural rock fragments that are annually mixed through the soil. This increased use was prompted by the belief (a) that limestone is beneficial because its carbonate removes the acidity of the soil, and (b) that soil is most productive if it is neutral, or when it has no active hydrogen ions in it; that is, when the soil has a pH of 7.0, or near that value. Under these beliefs (now known to be poorly founded), we have become belligerent foes of soil acidity. Limestone has become the ammunition for fighting this supposed enemy hidden in the soil. With national financial aid, we have been prone to believe that in putting limestone on the soil we can follow the old adage which says, “If a little is good, more will be better.” We are just now coming around to a fuller understanding of how nature grew crops on the acid soils before we did, and that crops are not limited to growth within certain pH values when they are well nourished by ample, well-balanced fertility.

Drilling the limestone (white streak, upper circle with nodules) to feed the young red clover plants some extra calcium helped them to develop nodules (lower circles) in the unlined soil with a pH of less than 5.0.

By putting calcium chloride (Dow Flake) into the right-hand half of the fertilizer drill, the better soybeans resulted in the right-hand streak in the field. The next streak resulted from calcium nitrate applied similarly. Both made the soil more acid. The rest of the improvement resulted from calcium hydroxide, making the soil less acid. All applied the plant nutrient calcium, which caused the crop improvement.

 

Science Now Shows Limestone Feeds Crops–Doesn’t Fight Soil Acidity

Only recently have we recognized the fallacious reasoning behind the conclusion contending that it must be the presence of acidity in the soil that brings crop failure when liming lessens both (a) the degree of and (b) the total of the soil acidity while making better crops at the same time. While the convenience of soil testing gadgets for refined points of pH was encouraging this erroneous belief about soil acidity as an enemy, it was the diligent study of the physiology of plants, of the colloidal behavior of the clays growing them, and of the chemical analyses of them all that finally pointed out the errors of such hasty conclusions. It indicated that the presence of soil acidity is not detrimental, but that the absence of fertility, represented by the acidity, is the real trouble. On the contrary, some acidity can be, and is, beneficial.

We now know, of course, that in applying the limestone, which is calcium and magnesium carbonate, there is possibly some reduction of the degree and the total acidity by the carbonate portion. At the same time there is applied also some calcium and some magnesium–nutrients highly deficient in the leached soils–to nourish the calcium-starved and magnesium-starved crops. These nutritional services come about both directly and indirectly. We have finally learned that it is this better nourishment of the crops, rather than any change in the degree of acidity, or any raising of the pH value, of the soil that gives us the bigger and better crops. Unwittingly we have been fertilizing the crops with calcium simultaneously while fighting soil acidity with the carbonate, the hydroxide, or the oxide of lime.

Regardless of our ignorance of how lime functions, we have unknowingly benefited by using it. However, an erroneous understanding of what happens to crops and to the soil when we apply lime, cannot successfully lead us very far into the future. We dare not depend forever on accidents for our good fortune. We cannot continue to grow nutritious feeds under the mistaken belief that we do so merely by changing the pH, that is, the degree of acidity, or by the removal of the soil acidity through the use of plenty of any kind of carbonates on our humid soils. Wise management of the soil to grow nutritious feeds can scarcely be well founded on facts so few and so simple.

Simple Tests Show Lime Is Beneficial through Calcium

Should you decide to demonstrate for yourself the truth of what has been said above, you might apply some soda-lime, or sodium carbonate, to acid soil. This will increase the pH of the soil. It will reduce its total acidity. But while this soil treatment will rout the enemy, i.e. soil acidity, and raise the pH toward 7.0, it will still not give successful crops. Merely removing the acidity by a carbonate (of sodium–a non nutrient–rather than of calcium, in this case) does not guarantee the successful growth of the crop.

As proof that it may be calcium as plant nourishment that is the helpful factor in liming a soil, one can repeat Benjamin Franklin’s demonstration and apply calcium sulphate, that is, gypsum, to the soil. One might even apply some “Dow Flake”, a calcium chloride. Either of these calcium-carrying compounds will make the soil more acid; either will lower the pH decidedly. In spite of this fact and because they add calcium, the gypsum and the “Dow Flake” will improve the crops on the initially acid soil either left so, or made more acid. We are now resurrecting the ancient art used by Benjamin Franklin, for whom liming the soil was a matter of fertilizing it with calcium sulphate, and not one of fighting soil acidity with calcium carbonate.

Soils Made Neutral Are Not Necessarily Made Productive

While we were fighting soil acidity, we failed to notice that most of the populations of the world are concentrated on acid soils. They are not in the humid tropics, where the soils are not acid or where the clay doesn’t absorb much hydrogen or even much of any nutrient cation. Nor are they on the arid soils that are alkaline (high pH values)–a reaction opposite to the acid (low pH values). Soils that are not acid are not necessarily the supporters of many peoples. Yet in fighting soil acidity we labor under the belief that if a soil were limed to the point of driving out all the acidity, such a soil should be highly productive.

We now know that even while a soil may be holding considerable acidity or hydrogen, it may be holding also considerable calcium or lime. To a much smaller extent of its exchange capacity, it is also holding nutrients other than calcium. Among these are magnesium, potassium, manganese, and others. But these in total are held in much less quantity and by less force than are either the calcium or the hydrogen–the former a nutrient and the latter a non-nutrient cation, or a positively charged ion. Should we put on lime or calcium enough to drive all the acidity out of the soil, that is, to make it neutral or to bring it to a pH of 7.0, by putting calcium in place of the hydrogen, all the other nutrients would be more readily driven out than would this acid-giving element.

Liming the soil heavily, then, does not necessarily drive out only the acidity, i.e. the hydrogen cations. Instead, it would also drive out all other fertility cations except calcium. It might load the soil with calcium so completely that it could offer only calcium as plant nourishment. Plants would then starve for other nutrients even though on a neutral soil. Plants on such a non-acid but calcium-saturated soil would be starving for all the same nutrients, except calcium, as they do on the acid soils. Making soils neutral by saturating them with calcium does not, therefore, make them productive. This is the situation of some of the neutral (pH 7.0 and higher) semi-arid soils of our western states. In our struggle against soil acidity we need to remember that neutral soils are not the productive soils. Instead, productive soils are the acid yet fertile ones that feed us and nourish the major portion of the other peoples of the world.

Some Soils Need Their pH Changed Much, Others But Little

By considering the increasing degrees of soil acidity simply as increasing deficiencies of fertility, we find in Nature, in general, that as the degree of acidity is higher (pH figure is lower), the adsorption or exchange capacity of the soil^** is saturated to a higher degree (larger percentage) by the positive ion, or cation, hydrogen. With this higher saturation by hydrogen, there are more hydrogen ions per unit of exchange capacity active or not held inactive by the soil; consequently the degree of acidity is higher. The acid is stronger. There are more hydrogen ions to make contact with the measuring electrode, and the pH value is therefore lower.

The same holds true for the degree of saturation of the soil’s exchange capacity by calcium, or magnesium, or potassium. As the degree of saturation by any one of these nutrient cations goes higher, more of it is active in making contact with the plant root and in getting into the growth activities of the plant. These cations are nutritional helps coming from the same source as the hydrogen cation, namely, the soil colloid. The hydrogen coming from there is not. Because we have had the gadgets to measure the hydrogen, we have emphasized the pH, or the presence of a certain degree, of acidity. We have not emphasized the absence of all the many fertility cations resulting because the hydrogen has replaced them on the negatively charged exchange complex or colloid of the soil. We have not had simple gadgets to measure them.

If, them, we should have a sandy, soil, for example, (low in clay content and thereby low in exchange capacity) with a low pH value or a seriously high degree of acidity, or hydrogen saturation this would conversely represent a low degree of saturation of its exchange capacity by calcium, magnesium, potassium, and other nutrient cations. Accordingly, then, the addition of but a small amount (two tons per acre) of limestone (calcium and magnesium carbonate) would move the pH value up decidedly or shift the degree of acidity to neutral.

This is similar to changing the degree of heat of a cup of scalding hot coffee by putting an ice cube into it. Here a little ice lowers the degree of heat very much where there was little total heat. With the sandy soil’s low exchange capacity there could not be much total hydrogen, even if the degree of activity by it is high; consequently, the pH is changed decidedly toward neutral and the acidity is completely removed. The exchange capacity is loaded very highly, in turn, by the calcium or magnesium of the limestone. Then, accordingly, these two nutrients become highly active in plant nutrition to make legume crops succeed well where on this sandy soil they may have previously failed.

By doing no more than using the gadgets, in this case, to measure the change in pH (or in the degree of acidity), resulting from liming the soil, one would conclude readily that the crop grew better because the pH was changed or the acidity was neutralized. One would not concern himself very commonly about the increased amounts of calcium, magnesium, and other nutrient ions which became so much more active to nourish the plant better. We have no simple gadgets to measure these effects; hence we attribute the crop improvements to the wrong causes. Grazing animals probably make no such mistakes in their choices of forages, judged according to nutritional values in terms of calcium and magnesium, rather than in terms of pH of the soil.

If, on the other hand, we should have a heavy clay soil (high in exchange capacity) with a seriously low pH value, or a high degree of acidity, and conversely, of a low degree of saturation of its exchange capacity by calcium, magnesium, potassium, and other nutrient cations, the addition of the same amount of limestone as was applied to the sandy soil would not change the pH value or the degree of acidity significantly.

Red clover failure was prevented in this division strip between the plots by trimming out the nurse crop early, or by eliminating competition for potassium. Clover was a partial success on the right, where potassium was applied. It was a failure on the left, without potassium. Clover failed there because of fertility deficiency, and not because of soil acidity. Sanborn Field, Columbia, Mo.

Clear glass jars of white sand given increasing amounts of a very acid calcium-hydrogen clay grew healthier soybean plants as more clay gave more exchange capacity and more total calcium to nourish the plants. The healthier plants resulted from better nutrition. The sick plants resulted from poorer nutrition and not from the low  pH value or the soil acidity, which was the same in all of them.

 

This would be similar to dropping an ice cube of carmine solution into a bathtub of scalding hot water. The cooling effect would not be recognized, but the coloring effect would. Because the larger exchange capacity and larger total amount of hydrogen would keep almost the same amount of it active in spite of the relatively small amount of limestone, there would be no measurable change in the pH of the soil as it is commonly sampled and tested. Yet the calcium (magnesium) carbonate would react with the soil to be adsorbed by it on the soil’s colloidal complex. It would exchange from there to the plant roots to improve the legume crop, even if much active hydrogen left in the soil maintained the pH of the soil near the initial value. There would be focal points of calcium (magnesium) in the soil (significantly so if the limestone was drilled) to exchange these nutrient cations more actively to the plant roots than before the soil was limed. Hence the two tons of limestone on the clay soil, initially considered of seriously low pH value, may not have changed the pH, although it established the legume where it failed previously.

Thus clover may be established on the sandy soil by two tons of limestone where the pH was changed to a decidedly higher value. Also, clover may be established on the clay soil by two tons of limestone where the same pH value was not changed significantly. Plant behaviors tell us that the changes in the pH values are not contributions to the improvement of the crop growth. On the contrary, it is the liming as a remedy of the fertility deficiency in its application of calcium (magnesium) for the crop, and not the change of the pH or degree of acidity by the carbonate that is the responsible factor in crop betterment. Different soils may differ widely, then, in the extent to which their pH values (or degrees of acidity) are changed by soil treatment for growing better crops, when it is the deficiency of fertility–and not the degree of acidity–that is the cause of the trouble on so-called “acid” soils. 

Experience Is a Thorough Teacher 

It was just such a case of confusion about what pH really means and how liming the soil serves plants as cited above, which put a county agent of Missouri into an embarrassing predicament many years ago. This occurred when the campaigns for “Lime for Clover and Prosperity” and “Lime for Legumes and Livestock” were at their height. A farmers’ meeting under his leadership was held one autumn day on a farm where the soil was about to be prepared for wheat serving as a nurse crop for red clover. The crowd assembled at the first field on the river hill, a windblown bluff of very fine sandy loam. After testing the soil and finding it seriously acid (of low pH figure), it was agreed that, according to this pH report on the soil, two tons of limestone per acre were needed to grow red clover.

The farmer crowd then moved down into the river bottom to the second field, made up of a heavy clay soil. This soil, under acidity test, revealed the same degree of this trouble. The same amount of limestone per acre, two tons, was deemed necessary as for the field of fine sandy loam.

After both fields had been plowed the two tons of limestone per acre were applied. The soils were disked, the wheat was seeded and followed by the clover seeding the following early spring. The clover stands in the wheat stubble the next autumn were excellent in both fields. This was reason, and considered a good setting, for another farmer meeting on liming and soil acidity. This one started again with the fine sandy loam soil, and the test of its pH to reveal the absence of acidity and the good clover there, supposedly because the pH value of the soil had been changed to that of neutrality. But in the second field, with its clay soil, the degree of acidity under re-test was the same as before the limestone had been applied. Yet the crowd observing this test was standing in an excellent stubble crop of red clover. The county agent was in a predicament. Here the liming treatment of the soil established clover in the second field without changing the pH of the soil, when on the first field he had just pointed out that liming the soil established clover because, as he erroneously explained, it had changed the degree of acidity, or raised the pH of the soil. The pH of these two soils was not shifted to the same extent, nor was the acidity changed by the same degree; yet the clover shifts were the same, namely, from failure to good stands. 

Plants Are Sensitive to Degrees of Fertility Deficiency, Not to Degrees of Acidity 

That plants are not “sensitive to, or limited by, a particular pH value of the soil” was demonstrated by experiments at the Missouri Agricultural Experiment Station some years ago. The clay fraction was taken out of the Putnam silt loam, and electrodialized to make it completely acid (replacing the nutrients by hydrogen) which gave it a pH value of 3.6. Six lots of this soil were set apart. Each was titrated with limewater to reduce its acidity to a certain particular degree, or specific pH figure. The lots represented the following series of pH values, namely, 4.0, 4.5, 5.0, 5.5, 6.0, and 6.5.

Enough clay was taken from each lot of this series of clays to represent .05 milligram equivalents (M. E.) of exchangeable calcium per plant for a total of 50 plants, and put into pans of equal amounts of quartz sand. In a second series, enough clay from each lot was taken to provide .10 milligram equivalents (M. E.) of calcium per plant in each of the six pans of sand, or doubling the amounts corresponding to the different pH values in the first series. A third series of these same different pH values of the soil was set up similarly, except that enough clay was used to produce .20 milligram equivalents of calcium per plant, or four times as much calcium in each pan as in the first series. Thus the three series were triplicates in pH values of the soils, but there was more clay, more adsorptive-exchange capacity, and more exchangeable calcium by two and four times in going to the second and third series from the first.

Observations of the soybean crops grown on these pans suggested different so-called “sensitivities to pH values” by this crop. By observing the first series containing the least amount of clay in the sand (the most sandy soil), and thereby the least amount of calcium for the crop (.05 M. E. per plant), one would have concluded that soybeans are sensitive to a pH of 5.5 but are not so to a pH of 6.0. Had one observed only the second series, the corresponding figures in one’s contention would have been pH 5.0 and 5.5. But had only the third series been open for observation, one would have put the “sensitivity” value at pH 4.5 and not at pH 5.0.

Here, then, a lower pH value (or a higher degree of acidity) by as much as ten times was the difference in sensitivity brought about by offering four times as much exchangeable calcium as nutrition for the plants. This was brought about merely by giving more clay, a heavier texture, to the soil at the same degree of acidity in the series, or at the same pH. More exchange capacity offset the significance erroneously ascribed to the pH.

These series were triplicates as to their respective pH values of the soil at the outset. But their exchange capacities were made the double (0.10 M.E. per plant) and the quadruple (0.20 M.E. per plant), respectively, in the middle and upper series, of that in the lower one by adding double and quadruple amounts of calcium clay. Thereby the corresponding extra exchangeable calcium was added. By this increase of calcium as nutrition, the plant’s sensitivity to pH was moved from pH 5.5 (lower series) to pH 5.0 (middle series), and to pH 4.5 (upper series), or a shift to 10 times as much acidity (31.6 to 316). Plants are much more sensitive to fertility deficiencies than to degrees of soil acidity (PH).

 

Still more significant was the change in the pH values of the clays and soils as the result of growing the soybean plants on them. The three more highly acidic soils of pH values, 4.0, 4.5, and 5.0 in the three series had all become less acid. The growth of the crops had made their pH values in these nine cases move upward, or shift toward neutrality. The three less acid soils of pH values 5.5, 6.0, and 6.5 in the three series had all become more acid in consequence of growing the crop; or this crop growth had moved the pH values downward in these nine soils, away from neutrality. These shifts in pH values were as much as 1.5 pH, where the original pH values were 6.5. The soils in these instances were made 32 times more acid by only the partial or limited growth of the crop. Surely, then, when the growth of the crop, or the activities of the plants by way of the roots’ contact with the soils make these soils 32 times more acid, one would scarcely say that it is the pH of the soil to which the crop is sensitive, or that a crop will grow only when a certain restricted pH or degree of acidity of the soil prevails. Plants are very sensitive to minute degrees of fertility deficiencies, but certainly not to degrees of acidity of very wide ranges.

Wise Soil Management Aims at Plant Nutrition Rather than Acidity Removal

Liming the soil, then, is a matter of putting active calcium and active magnesium into the acid soil, or even into one that is not necessarily acid. These two elements need to be in certain ratios of their respective degrees of saturation of the exchange capacity of the soil, if that soil is to grow legumes or protein-rich crops. For calcium, this may well be 75 percent, while for magnesium it may well be near 10 percent. For potassium the percentage saturation of the exchange capacity of the soil occupied may be from 2 to near 5. Just what percentage the other cations, especially the trace elements, should each occupy has not yet been specifically suggested. We need, then, some gadgets to measure the activities of the calcium, the magnesium, the potassium, and other nutrients. Instead of becoming so serious about the pH of the soil, we need to become much more serious about pCa, pMg, pK, etc., since these are activities of the nutrient ions and would help us get a picture of the dynamics by which these move toward the plant root for entrance there and nutrition of the plants.

Since the hydrogen ion, a non-nutrient, is positively charged, as are the nutrients calcium, magnesium, and potassium, it is significant to consider hydrogen along with these as the combination representing almost the total exchange capacity of the soil. From this total capacity the ratios of the percentage saturation of that capacity by the nutrients (and also by hydrogen, a non-nutrient) may be calculated and adjusted by fertility treatments for most efficient plant nutrition.

The pH, then, serves to suggest the degree to which the total potential stock of nutrients in the soil has been replaced by hydrogen, a non-nutrient; but it gives no suggestion as to which nutrients are grossly deficient or to what degree the nutrients are imbalanced. It is not an indicator of what kinds or amounts of fertility are required to make the soil productive, and has therefore been a hazard to keeping soils productive and in nutritional balance. Undue emphasis on, and attention to, the pH of the soil to the extent of disregard of the soil’s fertility saturation for plant nutrition suggests itself as a case where “a little knowledge can be a dangerous thing”.

 

Notes:

*The pH figure is the logarithm of the concentration of the active hydrogen or ions, not molecules. Distilled water has one millionth of a gram (.000001 or 10 to the minus 7 gm.) of active hydrogen per liter, said to be a pH of 7.0. Such water tastes flat. With a bit more acidity or ionized hydrogen (.00001 or 10 to the minus 6 gm.), or more acidity at a pH of 6.0, it tastes better. Thus, as the pH value is smaller, the degree of acidity is higher because there are more active hydrogen ions per unit volume as the hydrogen ion concentration is higher.

**The exchange capacity is expressed as milliequivalents (M. E.) per 100 grams of soil. One M. E. means 1 milligram (.001 gms.) of hydrogen or its equivalent. One M. E. per 100 gms. equals 1 gm. in 100,000 gm. or 10 in a million, or 10 pounds in a million or 20 pounds in two millions of soil (acre 6 ⅔ in. deep).