The Anion-Cation Connection in Soil

By Neal Kinsey

An-i-on / ´an-,i-en / n [Gk, neut. of anion, prp. of anienai to go up, fr. ana + tienai to go] 1) the ion in an electrolyzed solution that migrates to the anode; 2) a negatively charged ion 3 an acid forming element 4 opposite of a cation.

Cat-i-on / ´kat-,i-en / n [Gk kation, neut. of kation, prp. of katienai to go down, fr. kata- cata- + tienai to go] 1) positively charged base elements, either alkali metals or alkaline earths; 2) migrate to the cathode in an electrolyzing solution 3 the opposite of an anion.

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New words now command entry into the farmer’s vocabulary — cation, anion, exchange capacity, base saturation — also some new nuances attend the use of a soil audit that must be mastered. But all have to do with the clay of the soil and the electricity of nutrition, and how nutrients, or the lack thereof, govern everything from crop production to weed control.

In buying a farm, some people seem to prefer space and loca­tion to prime land. Land in the Missouri Ozarks — often seem­ingly quite useless for crop production — sells for more than prime land in the Little Dixie area of northern Missouri. There are some yellow, clay soils in the Ozarks of Missouri. I went there to sample a farm on one special occasion. This fellow wanted to develop his farm, but the soils were extremely poor. They grew scrub oak, not much more. As we were pulling the samples, I discerned a nice yellow clay soil, thinking this was great because it could be built up easily. When the analysis came back, it was immediately evident that the soil had no more ability to hold fertilizer than a sand dune in Florida. It was a yellow clay, but if it had ever been broken down into a fine colloidal soil, that por­tion had been lost. The finer particles of humus and clay which hold the plant nutrients in a soil were gone, leaving only a coarse yellow clay with very limited nutrient-holding capacity.

A soil colloid is a particle of clay that has been broken down to the point that it can’t be broken down further. Such a clay particle and humus carry a negative charge, much like the negative post on a storage battery. Fertilizers must have a positive charge to be held to the soil colloid. Calcium and magnesium from lime compounds have this positive charge. So does sodium. Hydrogen, as a gas, also has a positive charge. Negative sites on a clay particle will attract and hold positives, according to our scientific conceptualizations. The more clay colloids in the soil, the more negatives there are to attract positively charged ele­ments, much like a magnet. Positively charged elements are called cations. Negatively charged elements such as nitrogen, phosphorous and sulfur are called anions. Negative ions do not hold to the clay colloid. The bottom line is that clay has a nega­tive charge and the element being held on that clay has a positive charge. Most of the chemical reactivity of soils is governed by clay colloids. These colloids are extremely small, and can’t be seen with the naked eye.

It should be pointed out that some laboratories do not actu­ally measure the amount of clay in a soil. Some operators put it between their thumb and two fingers and rub it around as if to say, “Well, that feels like so much clay,” and so they put that num­ber down. If you start getting good, round numbers on a soil test, you can just about say, “Well, they are just estimating the exchange capacity.” That is a standard practice in Europe. It is a standard practice for a lot of soil testing classes. Obviously, if you cannot see the colloid with the naked eye, how are you going to determine how much of it there is unless you involve sophis­ticated instrumentation? Colloids are plate-like in structure. These plates lie upon one another, very flat, forming the clay soil.

Colloids come from clay and organic matter. In other words, there is a humus colloid and a clay colloid. Both have negative charges. They are very small, much like dust or talcum powder — only smaller and finer in makeup. These smallest pieces of clay — along with humus — attract and hold nutrients, but they are also easy to lose. If you could collect the dust that wind moves across a field and analyze it, you would find that it has the highest fertility of any part of the field. The most fertile part of the soil always leaves first, via either water or wind erosion. The longer erosion continues, the worse the soil gets.

When we pull soil samples, a lot of farmers think we overdo it. Take a flat field. Basically, if you can find out which way it drains, sample the low end and the high end. The biggest area we recommend being put in a sample the first time is around 20 acres. The low end will almost always have the highest nutrient content because the lion’s share of nutrients is held on the clay particles, which are that light colloidal dust. On upland soils, whatever way the water drains, the fertility goes that way, too. Soils can be built up. They can also be torn down. In the area where I live, soils that are considered the worst soils — the ones that nobody really wants, the ones that are sold for poor horse pasture — were considered the most fertile soils in southeast Missouri when my grandmother was a child. Growers farmed it and farmed it and let it erode away. They would raise wheat year after year and burn the stubble, then plant something else for a second crop. Nobody really wants to try to raise crops on it nowadays. Much of this land is now blown sand.

The first thing to do for your land is to correctly measure the amount of clay and humus the soil has in it. Nothing less than a detailed analysis will answer the questions. The procedure to rely on is atomic absorption. Technicians use a flame and actually measure the atoms, and how much the atoms will absorb in terms of color.

This test shows a different amount of light for each nutrient. That measurement has a name — cation exchange capacity, or CEC. As mentioned earlier, cations are nutrients with a positive charge. Exchange capacity is merely a measure of capacity of the soil to exchange nutrients. Whether the CEC is large or small, it affects the soil’s capacity to hold nutrients such as calcium, magnesium and ammonia nitrogen, and it also affects the quantity of a nutrient needed to change its relative level in the soil. A lighter soil will hold less of everything. Consequently, it doesn’t take as much fertilizer to get the right nutrient balance for total saturation. But that nutrient load can be lost or quickly taken up by cropping it. If you have an exchange capacity of, say, 5, that is like a sandy soil for certain. It is not going to hold very much fertilizer. Another soil may have an exchange capacity of 10. It will hold twice as many pounds of nutri­ents as the soil with a CEC of 5.

The term cation exchange capacity is not used on our soil tests. The nomenclature total exchange capacity fits much better. “Cation exchange capacity” means that the laboratory is mea­suring a certain part of the cation content. It may be measuring all, and it may not. We use the word total on the form to assure the client that we are measuring all the cations that could have a major effect on the soil analysis.

A number of soil tests do not report the sodium content in the soil. If you get a soil test that doesn’t measure sodium content, it does not measure the total exchange capacity, and therefore the exchange capacity will be expressed differently. The exchange capacity is not something you measure and then fill with nutri­ents. It is developed because the soil can hold a certain amount of calcium, magnesium, potassium and sodium. Each of these nutri­ents must be measured, or a valid answer will not be provided.

There is also a category called other bases shown on our soil test. It covers cations not usually singled out in terms of how many pounds are available in small amounts. Thus, use of the term TEC, or total exchange capacity, instead of CEC. The TEC shows the measured amount of “holding power” of the clay and humus in a soil.

Let me illustrate the point. Potassium has a single + beside it, meaning a single positive charge. So do sodium and hydrogen. But calcium and magnesium exhibit a double plus charge, thus a ++. The latter are strong-arm elements. They have the capac­ity to push single-plus elements aside.

Hydrogen is at the bottom of the pecking order. Then come sodium, potassium, calcium, magnesium. The positively charged nutrients obtained from the use of lime and manures and fertilizers are called cations. Their positive charge is attracted to the colloid because it has a negative charge.

As mentioned, the clay colloid has a plate-like structure. This plate may be hexagonal, square, chunky or blocky, but it basi­cally maintains a plate shape of some type. All of the cations are attracted accordingly. For every plus charge there is a negative, or minus charge.

That is great as long as we have enough open negatives for the single plus-charged elements like potassium, but when we start saturating a soil to achieve pH 7, not enough room will remain for weaker cations, and therefore additional nutrients with a single positive charge will not be easily positioned on the soil colloid. This is likely part of the reason why potassium will not be built up in clay soils when the pH is above 6.5.

Adsorbs is a term that needs to be added to every farming vocabulary, with special emphasis on the “ad.” It means held on the surface, in this case on the surface of the clay particle. When a plant root releases its acids, an exchange between hydrogen and a cation nutrient takes place.

Sand has a low exchange capacity because it contains smaller amounts of clay and humus and holds less nutrients than other soil. Gumbo, on the other hand, has a high exchange capacity. A Florida sand used to grow leather leaf fern probably has a 3 or 4 exchange capacity. Some heavy clay soils have a 40 to 50 exchange capacity, or ten times more ability to hold nutrients. If you started out with nothing in either soil, it would take ten times more fertilizer to balance the high exchange capacity soil com­pared to the low capacity soil. That is why we have to measure the soil and mark the nutrient equilibrium, or lack thereof. High TEC soils therefore hold much larger amounts of fertilizer and moisture because they contain higher amounts of clay and humus.

In a Mississippi survey, 82% of all farmers questioned said that they thought soil tests should be taken on farms to deter­mine how much fertilizer should be used. Only 28% of the sur­veyed farmers actually used soil tests. Many farmers it seems, do not really believe in using soil tests.

Until Hands-On Agronomy was first published, virtually all of my clients came to me by referral. Since most farmers and growers do not really trust soil tests, I cannot properly convince anyone of the value of soil testing in an hour or even in a day. If you start reading all the literature on soil tests, it might seem appropriate to ask, “Why should they?” At a meeting in Illinois, the head of the state Extension Service and I served on the same panel. One of the farmers asked if it was possible to use a soil test to determine the fertility of a soil. The head of Illinois Extension said, “No. You only use a soil test to determine roughly how much fertilizer to put on to feed the plant.” When my turn came I explained, “I tell every farmer that from the analysis I do, I can sit down with him — not knowing what his soils have produced previously — and rate the samples from the best to the worst.”

Farmers give various reasons why they do not trust soil tests. One fellow, in his early sixties, said he hadn’t used soil tests for years, but in a sense he had. He said way back when the AAA program was in effect, and the government paid for the limestone, he had some very good pasture and some very poor pasture. He went out and took soil samples and he said, “You know what? I noticed the poor pastures needed two tons of lime and the good pastures also needed two tons of lime. One day I was digging post holes and got down to some old yellow clay, and I thought, “I wonder if I had soil like that how much it would need.” The next time I sent in some soil samples, I reported that the yellow clay was from one of my pastures. It came back needing two tons of lime just like the rest of them. I decided, why should I walk all over these fields and take these soil samples to get the government to pay for the lime. From then on, I just kept a bucket of soil in my barn. When I thought I needed to put on lime, I put some of that soil in the sample bag and sent it in. They would tell me I needed two tons of lime.”

I recall another farmer. His fertilizer dealer called me. He said, “We have a major problem in this area.” This guy had over-limed his fields. He took a soil sample one year and sent it in to the university. They told him to put on two tons of limestone. He took a sample the next year and sent it in to the university. He didn’t tell them that he had put lime on the year before. He thought that they ought to be able to pick that up, which is not the case. The recommendation came back, two tons of lime. He got so much lime on his fields he tied up the other nutrients, and his yields dropped. The fertilizer dealer said he took a soil sam­ple from another client’s land and split it up into three parts. He did this because he said, “Always, if a sample comes from the northwest part of the county it needs three tons of lime. If it comes from the northeast part of the county, it needs two tons of lime. If it comes from the south part of the county, it only needs one and a half tons of lime.” He just wondered if it really made any difference. So he split up this one soil sample and reported that it came from each of the three areas. Each of the three came back with the lime recommendations as “needed” in the three different areas.

Right in the area where we lived, there was a fellow who graduated from Purdue University. He wanted to have a soil test made. It didn’t have anything to do with Purdue at the begin­ning, but in the end it did. Anyway, he wanted to have his soils tested, but he also went out and pulled samples and sent them to the University of Missouri. The soil test results came back from the University of Missouri. It revealed that the pH was some­thing like 5.0 to 5.6. The results came back from our lab. The pH was around 6.5. He said, “Everywhere I look, I am a point to a point and a half lower on the university tests compared to the tests that you people do. How do I know which one is right?” So he sent samples to Purdue. When those levels agreed closely with our lab tests, he was satisfied.

University of Missouri uses salt pH tests. The tests we use are water pH tests. There is an important difference. The salt pH test will generally read a point to a point and a half lower, maybe sometimes a half point lower. Herbicide instructions call for a pH of such and such, but that is not for a salt pH test. It is for a water pH readout. So if your pH shows a 5.5 on a salt pH test and I show 6.5 on a water pH test, it could affect the herbicide program. If you are going to use herbicides, it is important to know which is which. In farm periodicals, they almost always fail to tell you which one is being used.

There are several different ways to run trace element tests. There are different extracting solutions. Even if the same extract­ing solution is used, and the same shaker from the same com­pany to shake the solution is employed, if one laboratory shakes it fifteen minutes and another shakes it thirty minutes, the results are going to be different. A higher concentration will result for the one that was succussed longer.

The laboratory that I use feels that you get a more accurate reading if you shake it longer. They don’t cut down the time of shaking in order to get more samples processed. The question is, how do we get the most accurate readings, the ones most helpful to the farmer and the fertilizer dealer? The laboratory I use buys the more expensive extracts because it wants to do the best job. There’s a lab close to where I live that runs three times more samples in an eight hour shift than the lab I use could run with three shifts a day. They supposedly measure all the same things. They just cut down on all the time involved.

I have clients who have been with me since the mid-1970s. I have seen a soil look great one year, and the next the bottom falls out. There seems to be no explanation. It doesn’t fall out on every field. It’s not the rain. It just happens. I don’t know what the scientific explanation is. If I don’t catch it, and there are crops that really benefit from calcium, the yield will suffer. Calcium is the one thing that I notice most of all, but there will be other nutrients—phosphates, for instance.

I met one farmer, now retired — still one of the most interesting clients I have ever had — who decided to use our testing services. It was late in the spring when the samples went in. It got dry early that spring. He said, “Look, we need to start to work, and this field is the driest, and we can go ahead and get it ready for cotton. Would you use this test from this other lab and tell me what to do? I know you can’t tell me that you know exactly what to do, but at least that is better than what I would otherwise do.” That particular lab simply said he needed to apply two tons of limestone. Although Dr. Albrecht always cautioned, “Don’t do it,” we sat down and used the formulas. We went through and calculated out how much limestone was needed. According to the formulas from the numbers on his lab test, he needed high magnesium limestone. Magnesium deficiency is basically not a problem in most soils, but some of the lighter soils do need it if they have never had it. His had never had it. When our sample came back, it turned out that he didn’t need high magnesium lime. He should have used high calcium lime. I did him a disservice. I can’t use somebody else’s numbers and know the right thing to do. I have to use the soil test I understand, and not one from another lab.

Now, why should anybody believe that my service is better than somebody else’s? For one thing, I try to find out what is the next limiting factor, and then try to review the principles of physics and chemistry in order to answer the questions. I know that all of these things have an effect—sand, insects, disease, variety, drainage, placement, weeds—but increasing the soil’s fertility does more to take care of problems with yield than any­thing else. It also helps to moderate some of these other factors.

The answers contained in what I call The Albrecht System are based on over 40 years of practical experience in the field. Not my 40 years, but 40 years that this system has been applied before I ever started.

André Voisin, a member of de l’Academie d’Agriculture de France, distilled his years of research into the Law of the Maxi­mum. This law states that if you put on too much of a given nutrient, it is going to tie up something else that is needed. He found that if you put on too much potassium, it ties up boron. If you put on too much phosphorus, it ties up zinc and possibly copper. If you put on too much nitrogen, it ties up copper and sometimes some of the other elements, even zinc. If you put on too much calcium, it could tie up all the other nutrients, depend­ing on their level of availability.

Thus our lessons fall into place. When the pH is 7 or higher, the exchange of hydrogen will be zero. As you come down the scale from a pH 7, then hydrogen begins to increase in direct proportion (as long as a water pH test is used to measure the phenomenon). If pH goes from 7 to 6.9, exchangeable hydrogen will go up by 1.5%. If pH goes from 7 to 6.8, exchangeable hydrogen will go up by 3%. For every 0.1 that pH is dropped, exchangeable hydrogen from pH 7.0 downward will go up by 1.5% until you reach pH 6.0.

When micronutrients are present in the soil in adequate amounts, and the soil has the right base saturation percentages, then they are most available, but not necessarily in adequate amounts. At the right percentages of calcium and magnesium—if the micronutrients are in that soil—these are going to be pres­ent in their most available form. Still, there are a tremendous number of soils that can be balanced in terms of all major nutri­ents, and be missing micronutrients in bare minimum amounts. They are in the deficient category even after we have done every­thing we can to balance the soil. It is not correct to say balance the soil, and micronutrients will take care of themselves. Some soils simply do not contain adequate minimum amounts of micronutrients. But if they are already there and tied up by excesses, they will be released as the excesses are brought under control.

Source: Hands-On Agronomy