FOR more than a generation now American farmers have been urged by soil scientists, agronomists and crop experts to apply some form of “limestone” to their soils to “sweeten” them. They were told that crops, with few exceptions, do better in an alkaline soil and, in theory, “sweeter” the soil, the bigger the crops. Farmers learned a new term, “pH”; they the pH of their soils and applied limestone for the sole purpose of reducing acidity. In the last few years some scientists have questioned this, insisting limestone should be applied primarily a plant food, and that some acidity necessary to all plant growth. The time has come to get up-to-date on our thinking about lime because the yardstick by which we measure the need for lime has proven to be inadequate. For some crops, such as clover and alfalfa, the reduction of acidity is desirable, but complete alkalinity is not. And we should think of limestone as a plant nutrient rather than as a sweetener.
Chemistry Review
To understand why this is true let’s review our high school chemistry under the chapter on calcium, or limestone, as it is known agriculturally.
Limestone is actually a hard salt, the carbonate of calcium. We can make it in the laboratory quite simply by dropping a piece of metallic calcium into a jar containing carbonic acid (soda water), or by putting our breath, which contains carbonic acid, through lime water. Nature created an abundant supply of calcium carbonate, but she left us no instruction for its use on the soil. Consequently for many years as farmers we have used limestone with the wrong reason in mind.
Limestone and “lime” are not exactly alike. “Lime” is the oxide of metallic calcium, but since limestone and lime act so much alike in soil, what we say here applies to lime as well as to limestone. Limestone and lime are “Double Dealers”. They do two things, and they do both at the same time. First, limestone puts calcium, a plant food, into the soil; second, limestone diminishes acidity or produces alkalinity. The putting of calcium into soil is a vital, necessary, and intelligent thing to do, but the so-called “sweetening” of the soil, or complete removal of acidity, is, in almost every instance, a wrong and costly thing to do. Limestone has the habit of removing acidity or adding alkalinity to the soil. We cannot break limestone of this habit of reducing acidity while supplying calcium.
At this point it will help us to understand the real purpose of limestone applications if we remember that farmers have always used limestone for its alkalizing ability, whereas credit should have been given to the calcium portion of the limestone for its crop-nourishing ability. We grow crops, but not just for the sake of the crops; we grow crops for the sake of their food value to human beings and animals which live on those crops. Humans and animals cannot live without calcium and the products it helps plants manufacture; therefore, we must see to it that our crops have the opportunity to absorb from the soil as much calcium as they require.
Let us get back to the discussion of the bad feature of too much limestone—that business about adding too much alkalinity to soil. When limestone comes in contact with the water in the soil the calcium portion of the limestone unites with that water to form what is commonly known as “limewater”. Limewater is chemically known as calcium hydroxide with the formula Ca (OH)2. The carbonate portion forms carbonic acid with the formula H2CO3, but this is not so active in the water, and allows the (OH)2 of the limewater to dominate the activities. Water, in contact with soil, has the formula H(OH), and thus we begin to notice a similarity between (OH) in the soil water and the (OH) in the limewater. We have learned that acidity and alkalinity exist at the same time. If we have more acidity than we have alkalinity, we say that our soil is on the acid side. If we have more alkalinity than we have acidity, we can say that our soil is on the alkaline side.
That pH Scale
Soil water, with its formula of H(OH), is nicely balanced with one positively-charged H and one negatively-charged (OH). But when we begin to put limestone on the soil, the calcium portion of the limestone begins to associate itself with more and more of the (OH) portion of the soil water. The carbonate part is not so active; thus our soil becomes more and more alkaline. This is another way of saying that our soil becomes less and less acid. But where is the dividing line? Where is that point at which the soil stops being acid, and becomes alkaline, and vice-versa? In order to answer this question a scale was designed.
This scale begins at the figure 7.00 and runs in two directions. From 7.00 down to zero the soil becomes more and more acid; from 7.00 up to 14.00 the soil becomes more and more alkaline. The figure 7.00 indicates a neutral soil, which means that the soil contains exactly the same number of the alkaline OH units as it contains acid H units. The scale is called the pH scale.
Limestone does something else when, as a calcium carbonate it is added to soil. It forms some carbonic acid (soda water) as was pointed out above, but carbonic acid is a very unstable acid. It falls apart in a few minutes to lose part to the atmosphere, and this is why your glass of soda pop tastes flat if you let it get warm or if you spend more than five minutes drinking it. Thus we believe that attention should be focused upon the dangers of too much of the alkaline qualities of limestone, and action should be taken immediately to guard against bringing on too much alkalinity, for acidity in soil is vital and as necessary as a carburetor is necessary to an automobile engine.
All acids contain hydrogen; the “business end” of any acid is hydrogen. The chemical symbol for hydrogen is H, and the H in the formula of water, H(OH), is hydrogen. This is the element that is most active in taking the place of other elements to push them out into action. It pushes its way into a crowded molecule, other elements are forced out, and become food for plants.
Acidity Can be Added, Too
Now that we know that we need limestone in our soil, and we know that limestone has a bad habit of making soils less acid and more alkaline, what can be done about preventing or offsetting the undesirable alkalinity? There are several methods of accomplishing this necessary task. Some are quick; others are slow. Some are costly; others are cheap. But no matter what method we select we will be offsetting or preventing alkalinity by adding or by keeping some acidity, and when we keep or add acidity we are keeping or adding the vital hydrogen contained in that acid.
Prevention is simple. In putting limestone on the soil we must not add carbonate or alkalinity equivalent to the acidity originally present. We must keep some acidity on hand there. If excessive limestone is added to the point where no acidity exists, then acid must be restored to the soil. Theoretically, this could be accomplished by spraying dilute sulphuric acid on the soil. A slow method is to spread powdered commercial sulphur. Sulphur will do its work regardless of whether it is plowed under or left upon the surface. The sulphur slowly but constantly forms sulphurous acid, which is a milder acid than sulphuric acid; since we are after acid in our soil, to offset the alkalinity of the limestone, acid is what we are getting. Another method is the substitution of “gypsum” (calcium sulphate) for limestone. This salt will provide our soil with calcium, and at the same time will prevent the soil from going over on the alkaline side of the pH scale. Gypsum costs more than limestone, but gypsum needs no supplement to prevent the soil from becoming neutral or too alkaline. However, it is necessary to bring out the important point that limestone cannot be used excessively and successfully by itself; so let’s get on with the story, and we will see why limestone, alone, will put your farm on the shelf.
The type of agricultural lime added to the soil is also very important. Calcitic limestone, for example, has the tendency to make magnesium unavailable to plants. The use of dolomitic limestone gets around this fault and also supplies magnesium, a very necessary plant food.
Minerals Released by H Action
Acid soils are soils which contain more of the acid H units than they contain of the alkaline (OH) units. Soil contains minerals, and in the minerals are metals such as potassium and phosphorus, cobalt, manganese, magnesium, copper, zinc, and iodine. These metals, weathered out of minerals by nature, are packed into the clay of the soil. Metals and hydrogen do not get along well together. In school we are taught that the hydrogen in acids will go after a metal, but what really happens is that a metal goes after the hydrogen in that acid. So, when we have an ample amount of the H units in our soil we can be certain that the clay will make a “deal” with the H units, and the deal is to exchange a unit of metal for a unit of H. At this point the metal is no longer bound to the clay. It “goes on the road”, we might say, in search of a home. It has been pushed out to become active, and is thereby available for absorption by the root of the plant.
But look at what happens when our soil is neutral, which means there are just as many (OH) units in the soil as there are H units. Each H unit has a very strong liking for an (OH) unit, and since there are enough (OH) units to keep every H tied up, no deals for metals can be made with the clay, and no metals come out of the clay; therefore, the roots of the crops do not get as much metal as they need to feed themselves, and to feed humans and animals. Such crops are “empty” foods, because they cannot make sufficient protein, enzymes, vitamins and other essentials we have not yet discovered.
When our soil goes over to the alkaline side there is a surplus of (OH) units and even though the roots of a crop have the wonderful ability to excrete H units for the purpose of helping the plant to get metals into it, the plant starves because as fast as the roots excrete the H units, these H units are grabbed by the excess (OH) units to give nothing but water. Because limestone has the ability to create (OH) alkaline units, and because limestone puts vital, important calcium into the soil, the time has come for every farmer to pay close attention to the degree of pH of his soil whenever he applies limestone or lime to his soil, lest he make his soil neutral or alkaline, and the calcium he applied be less active than when acidity accompanies that calcium. An acid soil is a desirable condition. It is worth repeating here that acidity in soil is a fundamental and important principle upon which nature does her best to grow crops filled with food value.
High soil acidity, by nature, means low soil fertility. Consequently, it means also low-quality yields, and usually also low-quantity yields. Adding some single element of fertility may bring about high-quantity yields, but only plenty and well-balanced fertility additions to a soil left acid and not alkalized completely by that treatment can give high-quality as well as high-quantity yields. These are the facts that call for the help of soil testing. It is the soils of ample and balanced fertility with some acidity that make a high-quantity of high-quality products.
Litmus paper has been used, and still is being used, to tell whether soil is more acid than alkaline, and vice-versa. But litmus paper will not tell you how MUCH more acid than alkaline; neither will litmus paper tell you how MUCH more alkaline than acid. When your soil is tested an accurate pH testing kit which will show the difference between a pH of 5.5 and 5.6 and 5.7 and 5.8 and 5.9, etc. should be used. These close ranges are important because they will show you exactly what degree of acid is present to ward off the alkaline quality of limestone.
To apply limestone, or lime, in such quantities that we reach the point of excessive alkalinity, is very much like buying a new automobile, and then taking off the wheels to prevent the automobile from wearing out. It is true that the automobile will not wear out, but you have labored for something which will be of little use to you. Likewise, when we put limestone on soil, year after year, to the degree of bringing about the alkalinity of that soil, we are unknowingly reducing the yield, lower and lower, year after year, until we are operating at a loss instead of at a profit, and this is what we mean by the statement, “Limestone, by itself, will put your farm upon the shelf.”
Accurate Soil Tests Needed
This new approach to limestone application will result in some radical changes in farm practices. A soil testing service much more accurate than is available at present is one of the first recommendations. A second must is to apply just the right amount of lime for the crop to be planted. For alfalfa and clovers and other crops that like soil on the sweet side, a careful balance should be the goal, a balance which will leave some acidity along with the alkalinity. It may be that after the acidity is reduced, a change to gypsum should be made, or sulphur or other acid-producing material should be applied.
The main thing to remember is that too much limestone can be applied, either at one application, or that constant applications year after year, without offsetting the alkalinity, can accumulate too much and the soil will stop, or diminish, in production.
We might sum up the functions of limestone thus: (1) To provide plant foods, (2) to reduce the excess acidity for the benefit of a few special crops.
Some soils are so acid that a form of limestone should be applied to reduce the acidity as well as to supply plant food. However, a soil may be acid and still have a high exchangeable calcium, in which case it would be unnecessary and useless to add calcium except as a plant food.
It is difficult for a person who has only a layman’s knowledge of chemistry to understand how acidity and alkalinity can exist side by side in the same soil, since it is implied that one neutralizes the other. Pure water is an example of this; it contains exactly the same number of H units as it does (OH) units. There are many soils with a pH as high as 7.6 which raise fine crops. The Red River Valley in Minnesota and the Black Land soils of Texas and Alabama are alkaline and they grow excellent crops. In these soils, however, there is sufficient acidity present for good plant growth. It is when soils are excessively acid or when not enough alkalinity exists, that calcium should be added to supply the alkalinity or reduce the acidity.
Few soils can be over-limed for field crops. Truck gardens and greenhouses sometimes are over-limed, but this is a question of imbalance rather than acidity destruction. The goal towards which every farmer should aim is the proper balance of acidity and alkalinity for the crops he is growing and the application of the proper amount of calcium for plant food.