Does my Soil Need Cation Nutrient Balancing?

 

A practical guide to balanced  nutrition for soil and crops

 

        Cation nutrient balancing is the practice of adjusting the levels of potassium (K), sodium (Na), calcium (Ca) magnesium (Mg) and hydrogen (H) in the soil to promote soil, crop and livestock health.  These five elements occur in soil as cations, or positively charged ions, which are adsorbed (held in plant-available form) on negatively-charged clay and humus particles.  The soil’s ability to hold cations in this way is called the cation exchange capacity, or CEC.  Plants roots absorb K, Mg and Ca by releasing H (acid) in exchange for these essential nutrients.  In nature, the soil’s CEC is recharged with cation nutrients by decomposing leaf litter and droppings, and the gradual weathering of rocks.  Farmers must periodically replenish K, Mg and Ca by adding organic materials, lime and other mineral amendments.

 

        There are two ways to manage cation nutrients.  The first is the nutrient sufficiency approach, in which amendments are added to provide sufficient but non-excessive levels of each element.  This is the strategy that conscientious farmers use with regard to all plant nutrients, especially nitrogen (N), phosphorus (P) and K.  Soil N and P occur mostly as anions (negatively charged ions), which are not held on the CEC.   Farm nutrient management plans often focus on preventing N and P excesses that can cause water pollution.  Of the cation nutrients, plants utilize K in larger quantity than Ca or Mg.  Most growers replenish K annually with manure, compost, hay mulch, or K fertilizers.  Usually, crop residues, existing soil minerals, and lime applied to correct excessive soil acidity (low pH), provide sufficient Ca and Mg for crop nutrition.  Less frequently, other Ca or Mg sources may be required on certain soils. 

 

        The second strategy is the base saturation ratio approach, which is applied specifically to the cation nutrients K, Ca and Mg, and the non-nutrient H (the bearer of soil acidity).  Base saturation ratio is the percentage of the soil’s CEC occupied by each of the cations.  In this system, mineral amendments are used to adjust the relative amounts of Ca, Mg and K on the soil's CEC toward a ratio that is thought to promote good soil tilth and crop nutrition. 

 

        During the mid-20th century, soil scientist William Albrecht developed cation balancing guidelines based on his research into forage quality and livestock health in the central United States.  Since then, others have modified the "Albrecht formula" for lighter textured soils, which require somewhat higher Mg and K saturation to ensure sufficient available Mg and K for the crop.  Guidelines for desirable base saturation ratios are shown below.  Note that Albrecht recommended 10-15% H, which produces a soil pH of about 6.0, as he considered a mildly acid soil condition more conducive to nutrient availability than a neutral condition (pH 7.0). 

 

 

 

        Practitioners of cation balancing believe that soils with low Ca and too much Mg or K will show poor tilth, crusting, hardpan formation, poor drainage and aeration.  The unbalanced condition is thought to inhibit beneficial soil organisms, hinder humus formation, reduce availability of P and other nutrients, and promote soil-borne plant pathogens and noxious weeds.  These conditions can leave the crop more susceptible to pests and diseases, and yield produce, grains and forage of lower nutritional quality and shorter shelf life. This cation imbalance is remedied with applications of high-calcium limestone if the soil is acid (pH<6.0) or gypsum (calcium sulfate) if pH is higher.  Farm testimonials of dramatically improved soil tilth, crop quality and animal health as a result of these measures can often be found in Acres USA and other sustainable farming publications.

 

        Critics of the Albrecht formula consider base saturation ratios irrelevant as long as each individual nutrient is present in sufficient quantity, and pH is favorable (6.0 to 7.0 for most crops).  Although soil cation balance is important, there is actually little evidence to suggest that the soil’s base saturation ratio must be adjusted to the Albrecht formula in order to have healthy soil, crops and livestock.  This has raised the concern that growers may be spending money, labor and resources on amendments they do not need, and that will benefit neither the soil nor the farm’s bottom line.

 

        What do the research findings suggest?  Most crops can thrive under a fairly wide range of soil Ca and Mg levels, and tilth worsens only at extreme Mg levels.  Soil K levels may be more critical – it is important to provide enough K but not too much.  Finally, cation balancing needs are highly site-specific.  Optimum ranges for K, Mg and Ca vary with soil texture, the kinds of clays present in the soil, organic matter content, soil life, and what crops are grown.  For more details on research results, see the companion information sheet, Soil Cation Nutrient Balancing in Sustainable Agriculture – missing link or red herring?

12 Tips on Soil Cation Nutrient Management

 

1.  CARE FOR THE SOIL AS A LIVING SYSTEM

 

        Feed the soil life with a varied “diet” of cover crops, organic mulches and other residues to promote a vigorous organic matter cycle.  Use high-quality compost to inoculate the soil with beneficial organisms.  Light applications of one to three tons per acre every one or two years can accomplish this.  A healthy and diverse web of life in the soil will:

        · conserve and recycle nutrients from organic residues,

        · release nutrients in the root zone of growing crops,

        · reduce nutrient leaching losses,

        · slowly unlock nutrients from insoluble soil minerals,

        · stabilize soil pH so that much less liming is needed.

 

2.  MONITOR TRENDS IN SOIL NUTRIENT LEVELS

 

        Get a soil test every two to three years so you can observe changes in soil nutrient levels, organic matter and pH over time.  Be sure to use the same lab and the same sampling procedures each time you re-test, so that successive samples are truly comparable. Note possible nutrient deficiencies (levels rated “low” or “very low” on the test report) and excesses (“very high” or “excessive”), and whether or not these nutrient levels are moving toward a more favorable range (“adequate,” “optimal” or “high” on test report).  However, don’t take the numbers or the lab’s recommendations too literally, as some labs prescribe higher fertilizer or lime rates than actually needed. 

        For more information on soil testing see the companion information sheet, How to Use a Soil Test.

 

3.  IF IT IS NOT BROKEN, DON’T FIX IT 

 

        Your soil does not need to conform to the Albrecht formula to be healthy.  Signs of a healthy soil include:

        · rich, brown color (not black), pleasant earthy odor

        · loam and clay soil have distinctly crumbly appearance

        · adequate organic matter (2-6%, depending on texture)

        · pH remains stable at about 6.0 to 7.0

        · soil drains well after a soaking rain,  but remains moist

                for several days (sandy) to a couple of weeks (clay)

        · soft, easy-to-work (except when very dry), no hardpan

        · many earthworms and small insect-like organisms

                (may not be evident if soil is dry, hot or very cold)

        · surface residues and mulch disappear in a few months

        · incorporated green manures disappear in a few weeks

        · crops healthy and resistant to pests, drought, disease

        · good yields of high quality, tasty produce

 

        If your soil seems healthy and is producing good crops, yet Ca saturation is only 50-60%, and/or Mg is 20-30%,  don't worry. You probably do not need to spend money and time on calcium applications to “correct” the cation balance.

 

4.  WHEN IN DOUBT, ASK THE PLANT 

 

        A foliar nutrient analysis can tell you whether or not the soil is "in balance" from the plant's point of view.  The lab report will classify each nutrient level as deficient, low, sufficient, high, or very high (excessive).  This information can tell you whether a low soil Ca level, or high K or Mg level, might be hurting the crop.  On a biologically active soil, crops may obtain sufficient Ca, P and other nutrients even if these nutrients are “low” on a soil test. 

        To help ensure a valid foliar test result, be sure to follow laboratory instructions precisely in collecting the sample. Take the sample at the right stage of growth, take one leaf from each of a sufficient number of plants, and take leaves from the right part of the plant, are essential for valid results.  You can get sampling information for each crop from the laboratory.

 

5.  LIME APPROPRIATELY

 

        It is not necessary to make soils completely neutral (pH 7.0) through liming.  In fact, most crop nutrients are most available when the soil pH is about 6.5 (slightly acid).   Many crops can do well anywhere in the pH range of 5.5 (moderately acid) to 7.5 (slightly alkaline) if the soil life is healthy.  In sandy soils, be careful to avoid overliming, as this can make manganese (Mn) and other essential trace elements unavailable to crops.  Some liming guidelines:

        · on loamy or clayey soils, lime only when pH drops

                below 6.0, apply 2,000 lb/acre (45 lb/1,000 sq ft).

        · on sandy soils, lime only when pH drops to 5.5 or less,

                apply 500 to 1,000 lb/acre (12 to 23 lb/1,000 sq ft).

        · lime in conjunction with organic inputs (green

                manure, compost, etc).  This helps hold the cation

                nutrients.

        · small fruit prefer moderately acid soil (pH 5.5 to 6.0)

                and rarely need lime.  Blueberries like pH 5.0,

                and should never be limed.

        · use high calcium (calcitic) limestone if the Mg base

                is 20% or higher (25% on sandy soil).

        · use dolomitic limestone if Mg base saturation is below

                10% (below 15% on sandy soil).

        · if the soil’s Mg base saturation is near optimal, use a

                mixture of dolomitic and calcitic limes, or

                alternate types in successive applications.

        · re-test soil the following season to determine whether

                more lime is needed, and if so, what kind.

 

 

 

6.  CHECK FOR ACID SUBSOIL 

 

        Some soils in the southeastern US, have highly acid subsoils, which can be severely Ca-deficient and overloaded with toxic aluminum (Al).  If crops are not thriving despite reasonably good topsoil pH and nutrient levels, carefully dig down into the subsoil and observe crop root growth.  If roots suddenly stop at a certain depth and show the browning typical of root Ca deficiency, get a soil test or at least a pH measurement on soil from the depth at which roots are inhibited.  (Take samples from several spots and pool them.)  If it is highly acid (pH 5.0 or lower), and/or has extremely low Ca (less than 30% base saturation), you can supplement subsoil Ca in one of two ways.  If you have the needed equipment, apply pulverized limestone and chisel-plow or subsoil at least several inches into the acid layer.  Otherwise,   apply gypsum  at 500 to 1000lb/acre (11-23 lb/1,000 sq ft) to the surface.  It will leach down to the acid layer with rainfall.  Several applications every 6-12 months may be needed.

 

7.  WATCH OUT FOR HIGH POTASSIUM 

 

        "Enough but not too much" is the key with this nutrient!  Excessive soil K blocks plant uptake of Ca and Mg, and may contribute to blossom end rot and other Ca-stress disorders, poor flavor or keeping quality, or other vegetable production problems.  Ample Ca in plant tissues helps protect the crop against some fungal and bacterial diseases, but high tissue K levels can cancel this effect.  Soil K base saturation levels of 8% or more can make some soils more sticky and slower draining.  High soil K, with low Mg and Ca can also lead to grass tetany in cattle, or other livestock health problems. 

        If soil test K exceeds 350 ppm, or K base saturation rises above 6% on heavy soils, or 10% on sandy loams:

        · cut back on high-K inputs such as hay mulch, manure,

                wood ashes, manure-compost and NPK fertilizers. 

        · use cover crops to maintain soil organic matter and N.

        · to build organic matter, use tree leaves, chipped brush

                or straw in mulch or compost.  They are lower in K.

        · harvest-off excess soil K with vegetable crops (potato,

                sweet potato, brassicas and winter squash are heavy

                K feeders), or alfalfa.

 

        On the other hand, if soil K drops below 100 ppm, or below 5% saturation on a sandy soil, increase use of K-rich materials such as hay mulch or composted manure.

 

8.  MANAGE SOIL ORGANIC MATTER SUSTAINABLY

 

        It is important to restore soil life and soil organic matter (SOM) levels on a worn-out soil,.  However, once this has been achieved, cut back on organic inputs from off-farm sources.  Tillage accelerates SOM breakdown, and cultivated soils tend toward “steady state” SOM levels considerably lower than the 10% often found in virgin temperate forest or prairie.  The warmer the climate, the lower the “steady state” SOM level.  In the southeastern US, healthy cultivated soils might have the following organic matter levels:*

        · sandy soils and sandy loams 1.5 to 3%

        · loam and silt-loam soils            2.5 to 5%

        · clay-loam and clay soils           3.5 to 6%

 

    * Note that these are values for “wet chemistry” methods.  If the test lab uses the “loss on ignition” method, reported SOM values will be about 50% higher.

 

        Intensively managed organic farms sometimes achieve higher SOM through heavy annual applications of compost, manure and organic mulch from off-farm sources.  However, these farms often have excessive soil P, K and soluble N, posing a risk of water pollution and soil cation imbalance.  If soil tests show ample SOM and “very high” P and K levels, it is time to cut back on off-farm organic inputs, and use cover crops to replenish OM and N without adding P and K.  This will also save money, labor and resources.

 

9.  WATCH OUT FOR LOW MAGNESIUM

 

        Crops and livestock may show Mg deficiency if:

        · soil test Mg falls below 60 ppm,

        · Mg base saturation is <8% (15% on sandy soil), or

        · Mg base saturation is less than twice that of K.

 

        To supplement Mg, use dolomitic lime if the soil is acid (pH<6.0).  Otherwise, apply sul-po-mag at 200-500 lb/acre (4-11 lb/1000 sq ft) yearly until soil Mg levels are restored.  Caution: if soil K is high, be sure total K inputs do not exceed K removed by harvest.  Use Epsom salts if feasible.

        There is considerable evidence that moderately high Mg levels (20-30% base saturation) will not cause trouble on most eastern US soils, as long as the Ca base saturation is at least twice that of Mg.

 

10.  TREAT SOIL TILTH PROBLEMS HOLISTICALLY

 

          If your soil is sticky, hard to work, poorly aerated and drained, or if a hardpan is present, and the base saturation ratio differs considerably from the Albrecht guidelines, cation imbalance may be part of the problem.  Tilth problems respond best to a combination of measures:

        · fracture a hardpan with a chisel-plow or subsoiler set

                just deep enough to penetrate the hard layer.

                NOTE: do this when the soil is moderately dry.

        · follow subsoiling immediately with deep-rooted crops.

        · minimize heavy-machinery traffic in the field.

        · reduce tillage, and vary the depth of tillage.

        · implement a diverse crop rotation.

        · protect the soil surface with vegetation, mulch or

                residues as much of the season as possible.

        · promote healthy, diverse soil life by providing  diverse

                organic materials (cover crops, mulches, etc.).

        · use high quality compost to inoculate soil with

                beneficial organisms (1 to 3 tons/acre will suffice).

        · incorporate green manures and compost into the bio-

                logically active surface layer – the top 3-6 inches.

        · apply high-calcium lime if soil is acid and Ca is below

                60% base saturation, or K and/or Mg are very high.

        · apply gypsum at 500 lb/acre (11-23 lb/1,000 sq. ft) on

                non-acid loam or clay with Ca below 60%.  Several

                annual applications may be needed to improve tilth.

        · on sandy, low-CEC soils, use gypsum only if Mg or

                K is clearly excessive, and only at 300 lb/acre. 

                Gypsum aggressively leaches K and Mg from

                sandy soils, and can do more harm than good.

        · monitor the soil for improvements in tilth, and repeat

                the soil test to determine if more Ca may be needed.

 

11.  USE INTEGRATED MANAGEMENT FOR

        CALCIUM STRESS DISORDERS

 

        Blossom end rot, celery blackheart and other Ca-stress disorders are caused by a combination of factors, only some of which are within the grower's control (see sidebar on page 2).  The soil’s cation balance may or may not be contributing to the problem.  Use the following strategies to combat Ca stress disorders in vegetable crops:

        · adjust planting dates to avoid temperature extremes.

        · keep the root zone evenly moist through timely

                irrigation (drip irrigation works well).

        · avoid excessive soluble N and K fertilizers.

        · if soil K is very high, reduce K input to bring it down.

        · foliar Ca applications may help.

        · if soil Ca is below 65% base saturation, high calcium

                lime or gypsum may help.

 

        Dr. Albrecht suggested the following economical method for supplying Ca to crops: band or drill finely pulverized calcitic lime near the crop row at 100-200 lb/acre. This creates a small, Ca-saturated zone from which the crop can absorb Ca more effectively than if the same application were diluted across the whole field. You can also use 100 lb/acre of gypsum, which is more soluble.

 

12.  TRY IT OUT ON A SMALL AREA FIRST

           

        If you believe that an unbalanced cation ratio may be causing production problems but are not sure, try out the calcitic lime, gypsum or other corrective amendment(s) on a small area before investing in treatment for the whole farm.  Apply the material to a marked strip through a field.  Compare soil tilth, crop production and quality in the strip with an adjacent “control” strip over a few seasons to see if the cation adjustment seems to be helping. If you see an improvement in marketable yields or soil health that would justify the expense and effort, then go ahead! 

       

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.