Sean,

You raise a subject that has interested me for quite some time, as I, too have often come across famers, salespeople and consultants who swear by Albrecht's theories of cation balance. Unfortunately, none of the people I have asked has been able (or willing) to provide any scientific basis or even replicated, reliable data to support their contentions. I have been told that they have "happy customers" not data. None of the farmers I have talked to have even tried replicated strips to test the high - Calcium system effects, so they might have been happy without it, too.

Certainly there is truth to the idea that Ca plays a critical role in soil flocculation and plant (and animal) nutrition. The cation exchange complex in mosat soils is dominated by Calcium ions (at least among the non-acid cations). If the ratio of divalent to monovalent ions gets unusually low, soil structure, permeability, and infiltration certainly suffer. While sodium is most commonly to blame, there are documented reports of excessive potassium applications having similar, but less dramatic effects. The effect of excessive Magnesium and Potassium on Calcium uptake by plants is also well-documented, and to be expected from charge balance considerations, among others. Actual Ca deficiency is quite rare, and occurs almost exclusively on very acid soils where it is difficult to distinguish from Aluminum toxicity. In solutions that are extremely low in Ca, other metal ions (such as Mg) actually become toxic to plant roots, for the Ca is needed to maintain cell membrane stability. Sean, if I recall, your rotations include processing tomato, a plant that is very susceptible to low Ca (which combined with uneven water) can cause blossum end-rot. Has this ever occured on your plots?

The calcium content of animal forages is also quite important to animal health. Grass tetany is an extreme case, sometimes associated with excessive potassium and reduced magnesium and calcium intake. likewise, earthworms have a definite requirement for Calcium in their environment, the amount depending on the species. Calcium probably also can enhance general heterotrophic microbial decomposition, as well as certain specific microbial species (some Rhizobia and nitrifiers come to mind). However, in most studies the effects of Calcium and pH are confounded. Lab and field evidence would indicate that pH is the overriding factor.

However, I am aware that some consultants recommend applying calcium even when pH is above 7.0. In humid regions this is generally not good advice, as calcium in the form of limestone would result in too high a pH and thus micronutrient deficiencies and other problems. However, in humid regions where pH has been long maintained with dolomitic limestone (about 35% Mg), the ratio of Mg to Ca can become an issue of concern. Magesnium has considerably less flocculating ability than does Calcium, so I would recommend calcitic limestone in most cases, unless available Mg by soil test is below "medium".

Low Magnesium is a problem more commonly than high. For example, Magnesium saturation should general exceed that of potassium by a comfortable margin (say 2.5:1 or 3:1 or higher). If the ratio gets close to 1:1, or certainly if there is more exchangeable K than Mg, deficiencies of Mg are quite likely in many plants. This rarely happens in nature (except in some humid tropical soils), but can result from heavy K fertilizer (or even poultry manure) application on some soils.

Except for those which evolved on serpentine-derived soils, most plants are adapted to a considerable predominace of exchangeable Ca over Mg and other non-acid cations. To a large degree, plant roots can selectively take up what they need, as evidenced by internal plant composition which can difer greatly from the composition of the soil solution. However, extremely out of balance soils can certainly affect plant uptake. I have seen no evidence that there is any problem with ratios are disparate as 3:1 or 10:1 (Ca : Mg) , but clearly the 1:1 or lower that you report could be a problem. So the upshot is, check those Ca and Mg saturation numbers again. Also, check the plant leaf tissue. Is the Ca level unusally low? If there really are about equal moles of charge from the two cations, yes, some gypsum might be in order (in both organic and conventional systems). But that is treating the symptom. I'd want to know where all that Mg is (or was) coming from. Magnesium uptake by corn and other grasses (not woody trees, or tomato leaves, though) is not too different from that of Calcium, so depletion rates are unlikely to explain it unless you have been removing all the tomato vines for years (the leaves and stems may be 5 to 10 times as high in Ca as in Mg, but the fruits have little of either Ca or Mg --but are vey high in K). Are your soils derived from unusually Mg-rich parent materials?

I am sorry this "essay" has been so rambling. I wish I had time to look up all the refernces for you. Many of the above considerations are discussed in 11th edition (and will be more so in the up-coming 12th edition) of The Nature and Properties of Soils (by Nyle Brady and myself). Do let us know what you find out on your soil and tissue analyses. (Oh yes, if you do apply gypsum, for all our sakes, can you please split the plots or try a side experiment so we can get some data on this!?)

Good luck,

Ray Weil

Last Updated on 2/12/98
By Karen Lutz
Email: karen@hillnet.com