Calcium-Bearing Versus Neutral Fertilizers
OUR INCREASING KNOWLEDGE of nutrition has been compelling us periodically to list newly recognized requirements in the diets of plants, of animals, and of humans. If fertilizers are to be helpful in meeting these newer demands, then their composition must be shaped to improve the plant’s synthetic performances and thus to initiate, by way of the soil the improvements in kind and quality of their products to be used for the better nutrition first of animals and finally of humans.
Our knowledge about plant nutrition was long content to list but ten requisite chemical elements, so ably arranged in a simplified memory picture around his own name by the late C.G. Hopkins. Today that list has been extended, at least by four. It may soon be further extended. For animal and human nutrition, the essentials were listed for a long time as proteins, carbohydrates, fats and minerals. Only lately accessories were added, referring not to a single item but to many as this all-inclusive term indicates. With the increasing number of vitamins now recognized under this category, our minds are prepared for an extension of this list, as in the work of H.H. Bunzell of New York, when he finds that wheat germ gives effects on yeast growth beyond those of any, or all, of the vitamins yet listed.
Fertilizer thinking was long tied down to the three ingredient concept. Later it was adjusted to include the micro-nutrient necessities. This was a very easy adjustment when these were already passing into the goods mainly as unavoidable accompaniments or contaminants. Now the soil fertility has declined to the economic danger point, with resultant disturbances in animal and human nutrition manifesting themselves in many deficiency diseases some constructive thinking must be applied to revised fertilizer composition. This is especially important if fertilizer use as a soil treatment is to make crops yield not only more tonnage, but also feed of the quality and of the nutritional effectiveness it must eventually have. On the basis of better plant performance, the fertilizers and their functions may well be given critical examination in the light of (a) the various and the intricate soil processes, and (b) the plant’s complicated physiology, as science is rapidly elucidating them. It is from this viewpoint that the question of “acidity” of fertilizers deserves to be viewed. We need to raise the question whether this so called “acidity,” is so simple as a mere matter of the physico-chemical condition relative to the hydrogen-ion concentration of the mixture itself, or of the ash resulting from its ignition.
The introduction of the hydrogen electrode as a laboratory tool, brought a greater refinement in measuring the degree of acidity than had previously been possible. But when scientists can see better, or farther, objectively, it does not necessarily follow that they can see more clearly subjectively. Soil scientists seized quickly upon the hydrogen electrode for more critical study of soil acidity. They found that when limestone was applied to the soil for improved legume growth, it also corrected soil acidity. From these observations they drew the conclusion that soil acidity was the enemy of the leguminous nitrogen-fixing, protein-rich forage crops. This conclusion that calcium carbonate can remove, or reduce, soil acidity was objectively correct. But to say that it is the correcting of the soil acidity by the limestone that improves growth of the legume crop, is subjectively incorrect. Such observation demonstrates that nature is consistent, but such reasoning illustrates that human logic may be decidedly inconsistent. Reliance on the effect of limestone as calcium carbonate, or as magnesium carbonate, on the acid soils for the neutralizing effects by the carbonate radical rather than for the plant nutrient significance of these two dibasic cations, calcium and magnesium, may be leading us astray in fertilizer thinking. It has been leading our soil thinking astray where it has been diverting our attention from the function of calcium as a fertilizer to that of its carbonate as an antidote for hydrogen presence.
That soil scientists and plant physiologists should have neglected the role of the element calcium and should have given the carbonate premier significance in soil improvement by liming for crops, seems strange. As early as the days of Benjamin Franklin a story of his experiences with gypsum, or land plaster, for clover improvement was widely told. That gypsum, as calcium sulfate, with no acid neutralizing power should benefit red clover points suggestively to the possible importance of the calcium. It emphasizes calcium all the more when chemical analysis of this crop tell of its lower sulfur but high calcium content, and when young animals demonstrate it as an excellent growth-promoting feed.
It was the observations by farmers that cast doubt on the accuracy of the deduction that liming is beneficial to the crop through acidity removal. Farmers using limestone to get clover on heavy soils with high buffering capacities found crop establishment and improvement where no change in degree of acidity could be shown. They showed that acid soils need calcium to get a clover crop even if the acidity is not corrected. Their suspicion of the inaccuracy of the belief that the degree of soil acidity was the disturbing factor provoked experimental tests to separate the effect of the calcium from that of the carbonate. For this purpose use was made of such compounds as calcium chloride in commercial Dow Flake, and of calcium silicate in the form of ordinary cement. Both of these gave beneficial results in legumes on lime deficient soils. Here, then, the legume growth was improved by applications of calcium that carried no carbonate, and did not reduce the degree of acidity.
Similar confusion apparently prevails about fertilizer acidity and fertilizers are about to undergo universal acidity correction. Again, very fortunately, the economic aspect has come to our rescue unbeknown to us. The cheapest carbonates in the form of calcareous and dolomitic limestones are going to correct the fertilizer acidity. Unwittingly they are providing extra nutrients as calcium and magnesium to deliver the beneficial effects when in our confusion or misunderstanding of the facts, the acid neutralizing value of the carbonate is given the credit. If sodium carbonate were not more costly, and were more convenient, it might have gone in and given disastrous disturbances to our acid neutralizing belief. Its substitution of sodium for calcium would have revealed the wrong interpretation of the value of making fertilizers “neutral.”
Up to the present time, fertilizer analyses have paid no attention to available calcium. This nutrient of a fertilizer deserves consideration since in the humid region the increasing calcium deficiency in the soil is roughly the reciprocal of the increasing soil acidity. That liming the soil is beneficial because it delivers calcium to make up this soil shortage rather than because it removes the hydrogen ions was clearly demonstrated by some research using carefully prepared clays of different degrees of acidity. By mixing different amounts of clay with sand, the soybean plants could be given more or less calcium at any degree of acidity or pH. The plants grew better with less acidity. But they also grew better at any degree of acidity as they had access to more clay and to more calcium.
As a more accurate test, a series of clays with increasing amounts of calcium (representing increasing degrees of saturation of the clay by calcium) and accompanied either by decreasing amounts of hydrogen as acidity or of barium as neutrality were used to grow the same crop. The amounts of clay for addition to the sand were chosen so as to deliver the same amount of available calcium in every case. Strange as it may seem, the crop growth followed the degree of calcium saturation. The crop content of calcium did likewise, regardless of whether hydrogen or the degree of acidity was decreasing, or whether all the soils were neutral.
Here the lime addition demonstrated clearly that its beneficial effect was not one of fighting the soil acidity by means of its carbonate addition as a neutralizer, but that it was one of supplying calcium as a plant nutrient. Viewing fertilizers in the same light, we may well raise the question whether it is the neutralizing of their acid that is significant, or whether it isn’t the addition of the calcium as an extra nutrient, or possibly calcium as a mobilizer of other nutrients, that makes the so-called “neutral” fertilizers of more value as crop producers.
The increasing degree of calcium saturation of the soil makes the treatment more effective in delivering its available calcium to the crop. According to these studies, as the calcium was put on less clay to saturate it more completely, a larger percentage of the exchangeable calcium moved from the clay into the crop. With the clay carrying 40% saturation, the crop used but 12.3% of the available supply. With 87.5% saturation of the clay the crop got 29.3% of the applied lime. The efficiency of use increased more than twice. The concentrations of calcium in the crop for these two degrees of calcium saturation of the soil were .50% and .76% respectively, or the crop was made about 50% richer in calcium concentrations by what corresponded to drilling the calcium into the soil. In relation to our need for calcium-bearing feeds, this improvement may be significant.
Drilling fertilizers with seedings may at times be detrimental to germination. This effect is reduced, and germination may be improved, by adding calcium chloride which is a salt that has no neutralizing value. This effect has been shown for such non-legume seeds as tomatoes and bluegrass. Whether this effect is one of lessening fungus injury to the seed or one of improved germinating physiology has not been determined. INevertheless, increased calcium offerings have been shown to lessen the fungus attack on soybean seedlings in what resembled the more common “damping off” troubles. Even if the exact function of the calcium in the improved seed germination is not understood, its beneficial effect is recognized and its use becoming an adopted practice.
Such results point out that by putting the calcium, as has been done with the other nutrients, into the limited soil volume to saturate a part of it more completely, as is the practice by drilling, there is a much greater efficiency in getting the nutrients into the crop. This is the fundamental reason for drilling, rather than broadcasting, even limestone. Drilling makes the calcium carried by the fertilizer so much more effective than we have been wont to believe and brings calcium up for consideration as a significant nutrient within the fertilizer mixture.
For purposes of illustration let us imagine that the soybeans used in these studies were drilled with a 4-12-4 fertilizer at 200 pounds per acre. Let us suppose they made a ton of hay. According to their analysis they would contain the calcium equivalent of 40 pounds of calcium carbonate. With the gypsum of the fertilizer contributing the calcium equivalent of about 32 pounds of calcium carbonate, and with the neutralizer for the ammonium sulfate acidity adding about 60 pounds, then this drilled fertilizer would be delivering 92 pounds of carbonate. At a 30% efficiency figure found in the experimental studies, this fertilizer treatment would provide almost three-fourths of the calcium required for the ton of the soybean hay. Drilling the “neutral” fertilizers makes their calcium content effective. It suggest that when fertilizers are neutralized by limestone or dolomite, this is simply a case of supplementing the calcium already in the acid fertilizer in order to supply more nearly the amount of calcium badly needed by the crop. It is making for more effectiveness of the calcium, needed by the crop and hidden away in the fertilizer.
That calcium is of service in mobilizing other nutrients into the plant was suggested over 20 years ago by Professor True of Pennsylvania. That it mobilizes soil nitrogen was shown by growing some soybean seedlings for ten days in sand only, and some in sand given calcium carbonate. These were removed, washed and transplanted into an acid soil in order to test the significance of the calcium they could get and take with them in consequence of contact with the carbonate for but ten days of their seedling life. The calcium so gotten carried over to influence their growth. Those plants with calcium made more growth by 50% in the first ten days after transplanting. They excelled in growth for the rest of the time. During this same initial period after transfer, and before they had nodules, they took 20% more nitrogen from the soil. The calcium that was carried within the plants served to mobilize the soil nitrogen into them.
Calcium serves also to mobilize phosphorus, according to studies with both leguminous and non-leguminous pasture crops. Phosphate used with limestone returned three times as much phosphorus in the crop as when phosphate was used singly. A critical study of the data from the outlying fields of Kentucky points out that the crop increases from the use of lime and phosphorus in combination are greater, in general, than the sums of their separate effects.
Calcium is not only instrumental in mobilizing nitrogen and phosphorus into the crop, but it has been demonstrated for soybeans that as its supply in the soil becomes low, the nitrogen, phosphorus and potassium may even move from the plant to the soil. Thus, the plant may contain less of these than was in the seed originally planted. Such movement in the reverse direction has not been shown for calcium. Apparently no growth occurs unless calcium moves into the seed and crop. Experimental results also give some suggestion that the same holds true for magnesium as for calcium and that this element always moves into the crop if growth occurs.
With calcium mobilizing both nitrogen and phosphorus, and with all three of these nutrients associated with protein, which is the basis of growth, calcium may be more significant in growth promotion than direct measurement of calcium contents of crops might lead us to believe. It is some of these seemingly indirect effects that may be responsible for much that we believe is the benefit in making fertilizers neutral by means of calcareous or dolomitic carbonates.
That the calcium in the gypsum of the superphosphate is of fertilizer value has been shown by some work with bluegrass. Superphosphate applied at the rate per acre of 200 pounds of 20 per goods served as the basis for using diammonium phosphate at equivalent rates. The same amount of nitrogen as was contained in this concentrated fertilizer was then applied as urea in combination with superphosphate of which the accompanying gypsum might exert its effects. These different treatments were used for bluegrass on a soil needing nitrogen, phosphorus and lime. Because of this latter requisite, extra calcium was used in amounts to equal that carried by gypsum. This was used in both the sulfate and carbonate forms. The results are given in table 1.
In terms of tonnage produced, the diammonium phosphate was no better than superphosphate coupled with a doubling of its own calcium content as gypsum, or about what one might expect from superphosphate on limed land. The nitrogen of the ammonia in the diammonium phosphate was not used by the plant to increase its growth as forage even though it moved into the crop. When, however, calcium equivalent to that in the gypsum of the superphosphate was added to the diammonium phosphate, then this nitrogen made plant growth, delivered nitrogen, and returned phosphorus in the crop on a par with that by urea in combination with superphosphate.
These facts indicate clearly that the calcium of the gypsum is serving to mobilize the nitrogen of the urea and the phosphorus of the superphosphate into the bluegrass. This was not done when nitrogen and phosphorus in combination as diammonium phosphate were applied in the absence of the calcium. In the light of such effects, calcium will soon take on importance as an ingredient in fertilizer. These data suggest the possible danger in going to the more concentrated nutrient carriers that omit the calcium. They indicate that we may well think of using fertilizers in combination with lime or certainly in terms of the calcium they carry, more than whether they are neutral or acid in reaction.
The significance of calcium for the non-leguminous crop of cotton has been emphasized by the experiment stations of the South. Its service in mobilizing phosphorus was announced by F.L. Davis of Louisiana, when, as a result of his studies on available phosphorus in connection with nitrogenous fertilizers, he said, “Calcium containing compounds apparently maintain soil phosphorus at the highest available levels.” The data from that station reporting mean cotton yields for the three years 1938–40 in a test of different nitrogen sources in a 5-8-5 at 600 pounds per acre show some interesting effects from the fertilizers made “neutral.” The increases in seed cotton in consequence of “neutralization” are given in table 2.
Table 2
Increases in Cotton From 5-8-5 Fertilizer With Different Carriers of Nitrogen (Louisiana Data).
| Nitrogen carrier | Increase by fertilizer Pounds seed cotton | Increase by “neutralization.” Pounds seed cotton |
| Nitrate of soda | 250 | |
| 16-20-0 Ammophos | -109 | |
| 16-20-0 Ammophos, neutral | -47 | 62 |
| Cottonseed meal | 116 | |
| Cottonseed meal, neutral | 196 | 80 |
| Sulfate of ammonia | -81 | |
| Sulfate of ammonia, neutral | 1 | 82 |
| Urea | 23 | |
| Urea, neutral | 121 | 98 |
| Calnitro | 147 | |
| Calcium nitrate | 165 | |
| Cyanamid | 184 |
If these small amounts of extra calcium used in neutralizing the fertilizers can give these increases, should not some significance be given to the calcium present in the fertilizer even before it was neutralized? The effects of fertilizer neutralization and those of the calcium-bearing nitrogenous compounds on the cotton, all suggest that emphasis on fertilizers should not go to their acidity but rather to their calcium-bearing aspect even for this crop. When Ammophos reduces the cotton yield in Louisiana, as the data show, this again brings the gypsum aspect to the superphosphate in the mixed fertilizers into importance.
Also, the Alabama Station has called attention to the need for limestone to neutralize fertilizer acidity, which may be as much as 500 pounds of limestone per ton of 5-15-5 fertilizer. Applying 600 pounds of such a fertilizer after it has been neutralized would be equivalent to applying 150 pounds of limestone. Drilling limestone is not common practice for cotton, but drilled as a fertilizer neutralized it may be given benefits much as an application as light as 300 pounds of limestone does when it helps red clover in some soils of the cornbelt. Professor Tidmore points out that “an application of limestone is the most practical method of correcting the acid condition of the soil and supplying calcium.”
Much of the variation in cotton response to nitrogen from different sources in a mixed fertilizer may be connected with the variation in calcium content of that fertilizer, according to the suggestion of W. R. Paden of the South Carolina Station. In speaking of their results he says “These data show clearly that no marked difference in yield would be expected from the various sources of nitrogen on limed soil. When the question of soil acidity has been taken care of, one might expect approximately the same yield from the various sources of nitrogen.”
L. G. Willis in a study of “the value of gypsum as a supplement to a concentrated fertilizer,” particularly for cotton, suggests that the more concentrated fertilizers are deficient in some nutrient. He used limestone and extra gypsum in a search for this shortage. He hesitated to attribute significance to calcium since he says “freshly applied limestone did not correct the deficiency.” But he follows with the report that “other observations, however, appear to indicate that exchangeable calcium in these soils is as suitable a corrective as are the neutral calcium salts.”
Most of the southern soils need nitrogen and phosphorus. They apparently fit into the category of the soil cited previously, within which the effects of the calcium were separated from those of the phosphate. The Louisiana soils show similar conditions for the concentrated Ammophos and poor crop yields. It is therefore most probable that much of the irregularity in the so-called acidity of the nitrogenous fertilizer is due to a shortage of calcium for the crop’s most effective use of the nitrogen and phosphorus as well as for the crop’s need of the element calcium itself. These conditions are aggravated more as the superphosphates are concentrated to reduce the accompanying calcium. Calcium deficiency is presenting itself more prominently because the long existent shortages are not being covered by heavier calcium dosages in the lower grade superphosphates.
Neutralizing a soil with calcium carbonate encourages manganese shortage in some crops. But as more limestone is put into a limited amount of surface soil to put more calcium into the plants, then more manganese is taken from the untreated lower soil. Accordingly, the limestone functioning as a carbonate plays a detrimental role, but functioning as a calcium contributor it has a beneficial role in the same crop.
Soils should not be neutral if the products grown thereon are to be rich in calcium in particular, according to research by R.A. Schroeder. His recent work with spinach demonstrates that more calcium and more magnesium moved into this common, mineral-carrying, dietary component when the calcium content of the soil was increased and the soil kept acid at pH 5.2, than when the same calcium fertilization took place and the reaction was changed to nearly neutral, or pH 6.8. The calcium treatment was much more effective in giving calcium returns within the spinach, when grown on acid reaction. Three units of calcium applied on soil at pH 5.2 delivered more calcium in concentration and in total, in the spinach crop, than when 12 units were put on the soil at pH 6.8. It narrowed the potassium-calcium ratio and suggests a composition nearer the proteinaceous, mineral containing vegetation rather than that of vegetation that is mainly woody matter. According to this, a fertilizer can have less calcium and use it more effectively by having acidity present. Perhaps spinach will be more effective as a mineral contributor, or as an antirachitic factor, in our diet when we learn that the effects of calcium as a plant nutrient must be separated from the carbonate effects in acid neutralization, and that very probably even some hydrogen may be required in the soil to mobilize the calcium into the crop most efficiently.
Now that virgin soils are no longer available, we are more cognizant of the fact that we have been farming the organic matter of the soil. The seriousness of this is evident, especially in the South, where the farmer experience still maintains that much of the fertilizer should be in the organic form. These facts remind us that the fertility in the organic matter is tuned with the growing season for delivery of the nutrients at the rates of decay suited to the needs of the growing crop. The use of strictly mineral fertilizers with the seeding does not fit into the picture of plant needs so well. Consequently, the leaching loss from fertilizers is being appreciated. It suggests that fertilizers may be used on grass sods or crops growing heavily enough to take up the fertilizers quickly and to reduce the leaching loss. With more fertilizer used on seed crops, particularly those with legumes mixed through them, the fertilizers will be held back against loss in leaching. Such will be building fertilizer into the organic matter to extend the season of its nutrient delivery when the sod is plowed up and put to a tilled crop. This will spread the fertilizer effect over a longer time, and make it less of a hypodermic one. In fact, it will be a case of fertilizing the crops whose organic matter will have an additional fertilizing effect on other crops following.
Fertilizers have an opportunity for greater service as the plant functions and soil processes are more clearly understood. Their use will increase only as their service becomes greater. No fertilizer manufacturer would want his goods to render other than maximum possible benefit. Attention to other ingredients besides the nitrogen, phosphorus and potassium bids fair to improve fertilizer effects and to extend their service.
The present concern about improving the reaction of fertilizer is testimony that manufactures are eager to make their goods better. When these good intentions are supplemented by more fundamental information from research in plant physiology and soil science, fertilizer improvement will shift from this concern about neutrality to one of concern about calcium content. It may even aim to deliver hydrogen. It may add many other revisions that will be of greater value in making soils more serviceable for better plants, for better animals and better health to humans.