Soils, crops and livestock require balanced nutrition for health, just as people need a balanced diet to stay healthy. Organic farmers strive to provide a 'balanced diet" for their farms by adding a variety of organic materials and natural mineral amendments to the soil. Soil management practices that ensure enough but not too much nitrogen (N), phosphorus (P) and other plant nutrients help prevent water pollution, and also promote balanced nutrition for crops and animals.

Some farmers and consultants pay close attention to the relative amounts of certain nutrients, particularly calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na). These four chemical elements occur in the soil and plant as cations, or positively charged ions. They are held electrostatically in plant-available form on the soil's cation exchange capacity (CEC), which consists of negatively-charged particles of clay and humus. The CEC also holds small amounts of some micronutrients, and hydrogen (H) ions, the bearer of soil acidity. The relative proportions of the soil's CEC occupied by Ca, Mg, K, Na and H is called the base saturation ratio.

Proponents of cation balancing consider base saturation ratios of about 65-75% Ca, 10-15% Mg, 2-5% K, 0.5-3% Na and 10-15% H optimal for soil, crop and livestock health. This guideline is sometimes called the Albrecht Formula, after soil scientist William Albrecht who researched and developed it. For sandy soils with low CEC, the formula is modified slightly, to about 60% Ca, 20% Mg and 6-8% K.

Soils whose base saturation ratios depart significantly from these ranges are considered out of balance, and specific mineral amendments are applied to restore the balance. Production problems that have been attributed to improper base saturation ratio include:

• poor soil tilth, crusting and hardpan
• poor aeration and drainage, increased runoff
• inhibited or unbalanced soil life – more pathogens
• reduced humus formation
• reduced availability of other plant nutrients such as P
• crop susceptibility to pests, diseases and drought
• increased weed problems
• poor nutritional quality of produce and forages
• low soluble solids content (Brix), shortened shelf life
• increased health problems in livestock

Growers who utilize this system of soil balancing apply high-Ca limestone (calcium carbonate) or gypsum (calcium sulfate) to restore low Ca and/or displace excessive Mg or K. Dolomitic limestone can correct low Mg, while sul-po-mag (langbeinite) provides both Mg and K. However, some soil scientists reject the cation balancing theory because research findings often do not support the claims. They caution growers not to spend needless money simply to "correct" their soil's base saturation ratio to arbitrary ranges.

Many soils in the southeastern US have low Ca levels, and high K or Mg. Could this be limiting vegetable production or farm profitability in our region?

Virginia Association for Biological Farming (VABF) has recently conducted a literature review and a three-year field experiment at five farms to explore this possibility.

History of Soil Cation Balancing Theory

The concept of balancing soil cations began with research conducted from 1920 to 1970 by Dr. William Albrecht, a soil scientist at University of Missouri. In greenhouse experiments, he showed that liming does not merely correct soil pH, but also provides plants with the essential elements Ca and Mg, and can enhance plant uptake of N, P and trace elements.

Dr. Albrecht observed that cattle and wild bison thrive best in the tall grass prairie regions of the United States, whose soils are naturally rich in Ca (60-75% base saturation). In field experiments, liming increased forage yields only slightly, but substantially improved forage protein content and livestock weight gain. He also found that pasture soils should have about 10% Mg base saturation, and not more than 5% K. Too much K relative to Mg and Ca can cause "grass tetany," a potentially fatal condition in cattle. Finally, Albrecht noted that mildly acid soils (pH 6.0-6.5) have more available nutrients than neutral soils (pH 7.0). These findings led to the Albrecht Formula shown on the previous page.

During the 1940s, Firman Bear and colleagues developed a similar base saturation ratio formula for optimal crop production on New Jersey soils. In their experiments, alfalfa performed well over a wide range of Ca:Mg and Ca:K ratios, as long as Mg and K base saturation did not fall below 10% and 5%, respectively. About 20% H (pH ~ 6.0) was needed to ensure adequate available manganese (Mn), especially on sandy soils. They assigned the remaining 65% of the CEC to Ca, partly because Ca amendments were inexpensive. This suggests that somewhat lower Ca and higher Mg or K might be acceptable on New Jersey soils.

Many farm consultants consider base saturation ratio to be an important factor in soil tilth. They base their reasoning on the fact that Ca ions promote aggregation (crumb structure) in clays, whereas K, Na, and (to a lesser degree) Mg promote clay dispersion, which leads to crusting. Therefore, on soils with high K or Mg, and Ca below 65% saturation, high-calcium lime or gypsum (calcium sulfate) is often recommended to add Ca and flush-out excess Mg and K. Some growers feel that these measures can improve soil tilth, relieve compaction, improve aeration and drainage, and restore beneficial soil life. Other benefits claimed for cation balancing include fewer weeds, pests and diseases; and better-quality produce and forage with higher soluble solids content (Brix), more nutritional value and longer shelf life.

Where is the evidence?

Critics of the cation balancing approach question its practical validity for several reasons.

First, most crops show little yield response to variations in base saturation ratio. In greenhouse and field experiments, millet, corn, wheat, barley, soybeans, alfalfa, lima beans, collard greens, lettuce and citrus nursery stock have tolerated soil Ca:Mg ratios as low as 1:1 or as high as 10:1 or more with no loss in growth or yield.

In the VABF field study, Ca amendments have not improved Brix values of broccoli, cabbage or tomato; have not enhanced crop uptake of P or micronutrients; and have had no visible effect on foliar diseases, insect pests or weeds. Effects of Ca on marketable vegetable yields have been inconsistent, and require further study.

Second, Ca deficiency is rarely seen in most crops. Blossom end rot in tomato and pepper, blackheart in celery, tipburn in lettuce and cabbage, cavity spot in carrot and parsnip, and bitter pit in apple are “calcium-stress disorders” that result from a localized Ca deficiency in the affected part, and are often not prevented by liming or gypsum.

Third, Ca amendments improve tilth mainly on saline soils, and on soils with high Na or extreme Mg levels. Such soils are rare in the southeastern US. The dispersive effect of Mg is slight, and often negligible in the field.

In the VABF study, the two farms with the highest Mg levels (27-30% Mg saturation) have the best tilth, with high porosity, many earthworms, and no hardpan within 24 inches of the surface. At the other three sites where hardpans are present, the effects of adding Ca have been inconsistent.

Finally, once sufficient levels of each nutrient have been achieved, and soil pH is good, adding lime or gypsum to adjust the soil's base saturation ratio is economically unsound. Benefits sufficient to offset the cost of these amendments are unlikely.

Here is the evidence

Although soil tilth and crop yields can remain good over a wide range of base saturation ratios, the soil’s cation balance deserves attention for several reasons.

First, in high-rainfall regions like the eastern US, soils can lose 100 to 200 lb Ca per acre annually due to leaching. Mg and K can also leach. This may necessitate lime or other amendments to correct pH and replenish cations. Healthy, well-fed soil life, and deep-rooted crops in the rotation, can greatly reduces leaching losses, so that pH and cation levels remain stable for many years before liming is needed.

Second, the balance of Ca, Mg and K in most crops is influenced by the soil's base saturation ratio. Too much of one nutrient can hinder plant uptake of the other two. Plants actively absorb K along the length of their root systems, whereas Mg and Ca enter only at the root tips. Thus plants may contain more K than Ca, even when the soil’s base saturation is 5% K and 65% Ca. When soil K saturation equals or exceeds that of Mg, crops may show Mg deficiency, and forages may pose the danger of grass tetany to livestock.

Third, ample Ca in plant tissues can reduce the incidence of tipburn and other Ca-stress disorders. Plants can relocate K and Mg from older leaves into growing points and developing fruit, but they cannot do the same with Ca. Thus the plant must maintain a steady flow of Ca from the soil to newly forming tissue. This flow can be interrupted by dry soil conditions, sudden wet/dry fluctuations, rapid vegetative growth, excessive soluble nitrogen (N) levels, or high soil K, Na or ammonium (NH₄), which inhibit Ca uptake. Thus proper irrigation, and careful management of all of these nutrients may be required to control Ca-stress disorders.

Peanut is another crop with a high Ca requirement. After the developing pods "peg" into the soil, they must absorb Ca directly through the pod wall in order to develop normally. Peanut growers often apply 400 lb gypsum per acre at midseason to meet this need.

Fourth, ample tissue Ca enhances crop resistance to bacterial and fungal diseases, such as fusarium wilt and bacterial wilt in tomato, and botrytis mold in rose. Excessive tissue K levels may cancel the protective effect of Ca.

Fifth, plant root growth may become inhibited by Ca deficiency long before foliar symptoms develop. Roots may stop growing when they reach a highly acid subsoil with low Ca and toxic aluminum (Al) levels. Such acid subsoils are fairly common in parts of the southeastern US. Gypsum can relieve the subsoil Ca deficit, promote deeper root growth, and thereby enhance crop yield and drought resistance.

Sixth, although 20-30% Mg saturation may not hurt soil tilth, high K levels (>8%) may cause some clay-loam soils in our region to become more sticky. If K + Na + NH₄ total 15% or more, tilth is likely to deteriorate. In the VABF study, the one soil in which gypsum has reduced soil strength (resistance to root growth) started with a K saturation of about 8%, which decreased to 6.7% in the high-Ca (gypsum) treatment.

Excessive soil K is a common problem on intensive vegetable farms. Organic farms that use large amounts of manure, hay mulch or compost from off-farm sources for many years, often accumulate too much P and K in their soils. Conventionally fertilized vegetable fields may also have excessive soil P and K. In either case, growing cover crops and recycling on-farm residues can provide organic matter and N without adding more P and K.

Finally, the effects of soil base saturation ratio on soil life are not yet fully known. Although proponents of the Albrecht formula claim that adding Ca to correct base saturation ratio will markedly stimulate beneficial soil life and humus formation, we have not yet seen this in the VABF study. However, longer-term studies may be required to evaluate possible effects of base saturation ratio on soil life.

Cation balancing is site-specific

For several reasons, it does not make sense to apply a single cation balancing formula to all farms. First, some types of soil clays hold onto calcium much more tightly than others. Montmorillonite, bentonite, vermiculite and smectite clays have high CEC, but they bind Ca quite firmly. Soils of the midwest and prairie regions are rich in these minerals, and may need 60-70% Ca saturation to provide sufficient crop-available Ca. Kaolinte clay has a lower CEC, but it holds Ca more loosely, so that soils rich in kaolinite may provide sufficient available Ca at just 40-50% Ca saturation. Mica clays such as illite are intermediate in their Ca-binding behavior. Most soils in the southeastern US have primarily kaolinite and mica clays, although there are exceptions.

Humus has very high CEC and it releases Ca to plants as readily as kaolinite. Thus building organic matter generally enhances Ca availability. In the VABF study, vegetable crops have obtained sufficient Ca from two soils with "low" Ca (<60% saturation) but good organic matter levels.

Second, care must be taken on sandy soils with low CEC to ensure adequate Mg and K availability. Gypsum can aggressively leach-out these two nutrients on such soils. In the VABF field study, adding Ca (gypsum) to a sandy Tidewater soil tended to reduce tomato yields and soil microbial activity, possibly by making Mg and K less available. Surprisingly, the gypsum has also gradually tightened an existing hardpan. For sandy soils, many consultants use a modified Albrecht formula, with base saturation of about 60% Ca, 20% Mg and 6-10% K.

Third, plant species differ widely in their needs for Ca. As mentioned earlier, peanuts, cabbage-family crops and some other vegetables require plenty of Ca, and may be sensitive to low soil Ca or Ca:K ratios. Cereal grains, corn, forage grasses, sweet potatoes and most soft fruits have a lower Ca requirement and do well in a wide range of soil base saturation ratios. Buckwheat, vetches, some clovers, phacelia, and some perennial broadleaf weeds have high tissue Ca concentrations, but their roots extract soil Ca quite efficiently, even where soil Ca availability or pH are low. Deep rooted cover crops (and weeds too!) with this capability help recover Ca that has leached into the subsoil.

Finally, the crop may "see" a different nutrient profile than that shown on a soil test. For example, healthy, well-fed soil life can "buffer" soil nutrient levels so that crops have good nutritional balance despite "low" or "excessive" levels of some nutrients on a soil test. Conversely, a compacted or biologically depleted soil may not release nutrients effectively. At one of our study sites, soil tests showed low P, K, and Ca, but good biological activity, and broccoli was well supplied with all of these nutrients. At another site, the compacted clay soil had over 70% Ca base saturation, yet tomato showed Ca deficiency.

Furthermore, different soil labs use different methods, so that one lab might rate the Ca level of a certain soil as "sufficient" and another rate it as "low." Some soil consultants utilize the Morgan extraction method for estimating plant-available cation levels. Because soil-microbe-crop interactions are so complex, a plant tissue analysis often gives a better picture of the nutritional status of the crop than a soil test does.

Is Cation Nutrient Balancing Environmentally Sound?

Calcium amendments, like other inputs from off-farm sources, entail environmental costs, including fossil fuel use in transportation, as well as mining operations to extract limestone or gypsum. Ecologically conscientious growers seek to minimize their dependency on off-farm inputs, and to use primarily on-farm sources to maintain soil fertility.

Some soils are naturally lower in Ca and higher in Mg than recommended by Dr. Albrecht, because of the composition of underlying rocks and minerals from which the soils have developed. Adjusting such soils to conform to the Albrecht formula can require two to four tons of high calcium lime or gypsum per acre. For a larger farm, this could mean purchasing, mining and transporting (often from distances of 100 miles or more) many tons of mineral amendments – all at considerable cost to both farmer and environment. Good management of organic matter and soil life can keep many high-Mg soils productive and easily worked without the need for massive Ca inputs.

However, if a low-Ca soil is also prone to compaction, ponding, runoff or erosion, it makes ecological sense to correct the situation. Since calcium, organic matter and soil life work together to promote good tilth, several light applications (1000 lb lime or 500 lb gypsum per acre) in conjunction with green manure or other organic inputs, will give the best results. (1000 lb/acre = about 23 lb/1000 sq ft.) Adding a large amount of readily available Ca (gypsum or finely pulverized lime) at one time can temporarily inhibit soil life, and much of the applied Ca may leach away. A single heavy application of coarsely ground limestone is acceptable (since the Ca is released gradually over several years) and may be the most economical strategy for larger fields. Providing adequate organic inputs along with the Ca is essential, as a healthy, living soil retains Ca much better than does a worn-out soil.

For crops that are particularly dependent on plenty of available Ca, Albrecht sometimes recommended drilling or banding finely pulverized limestone near the crop row, at just 100 or 200 lb/acre (2-5 lb/1000 sq ft). This creates a Ca-saturated zone near the crop, which allows it to thrive despite the lower Ca levels in the bulk of the soil.

In conclusion

• Soil cation balance is important to soil, crop and livestock health. If the soil is managed to provide sufficient but not excessive levels of each nutrient, it is usually unnecessary to make the soil’s base saturation ratio conform precisely to the Albrecht formula.

• Cation balancing is site-specific, depending on soil type and crop mix. It cannot be prescribed for all farms by a single base saturation ratio formula.

• Moderately high Mg levels (up to 25 or 30% saturation) are usually not harmful to soil tilth or crop health. When acid soil pH warrants liming, it is wise to choose high-calcium lime if Mg is above 20% (25% for sandy soil), and dolomitic lime if Mg is below 10% (15% for sandy soil).

• High K (≥8% saturation) can make some clay-loam soils more sticky and prone to crusting or hardpan. Too much K relative to Ca may aggravate Ca-stress disorders in some vegetables, or reduce disease resistance. Soil K saturation equal or greater than that of Mg can lead to crop Mg deficiency, or grass tetany in livestock.

• Feeding and maintaining the soil as a living system helps maintain favorable cation levels, buffers pH, and reduces leaching losses of Ca, Mg and other nutrients.

• A crop foliar nutrient analysis can reveal the soil nutrient balance that the plant actually "sees."

• The effects of base saturation ratio on the web of life in the soil needs further exploration.

• The experienced grower's intuition and knowledge of his/her land may lead to better decisions than reliance on soil test results and base saturation formulas alone.

Mark Schonbeck

NOTE: this information sheet is a "work in progress." Comments or suggestions on clarity or accuracy of the information herein, are welcomed by Mark Schonbeck, 439 Valley Drive NW, Floyd, VA 24091; mschonbeck@usa.net