Cation Exchange, pH and Base Saturation

Soil sampling is a critical tool for any farming operation.  It not only helps determine your fertilization plans for the following growing season, but it will also aid in determining the long term strategy for building sustained soil health.  This article will address three key parameters available in a soil test that best aid in improving agricultural soils over the long term, and also have relevance in the short term.  These parameters are pH, cation exchange capacity and base saturation.  It is important to ensure that your soil report includes all three of these categories.

Taking and deciphering a soil report is a relatively quick and easy task.  Post-harvest fall sampling is the best time to capture the information in a soil report, but any time of year is acceptable.  There are two distinct ways to sample by spatial means: grid and zone.  In basic terms, grid sampling is best conducted on properties that are relatively uniform (homogenous) when it comes to soil texture and type.  Zone sampling should be conducted when soils are varied (heterogeneous) in texture and type.  Grid sampling is done in equally spaced samples throughout the field where the field has consistent growing potential.  Zone sampling takes into account soils that will have drastically different growing potential in specific areas within a field and should be sampled as such.  Once the sample has been taken and sent off to the lab, it makes sense to not only test for the three key factors we are discussing, but to also test for macro nutrients, including nitrogen, phosphorous and potassium,  and secondary micro nutrients.  

The first item to evaluate should be pH.  The effects of pH are far-reaching; most nutrients will become either more or less available according to the pH level.  Phosphorous is a good example.  At a pH of 6 to 6.5, phosphorous is easily available to plants when there is a sufficient level of it in the soil.  However, as pH moves higher—becoming more basic—phosphorous becomes less available.  At a pH near 8.0, phosphorus becomes almost completely unavailable to plants.  The good news is that pH can be adjusted in both directions with little work.  In acidic soils with low pH, adding lime is a quick and easy way to adjust pH upward. Basic, or high pH, soils are a little more difficult to adjust.  It’s common in soils with pH near 8 for growers to add acids, such as sulfuric, or phosphoric acid, to irrigation water.  Adding elemental sulfur is another means of lowering pH in soils.

The next soil component to evaluate is cation exchange capacity, or CEC.  CEC is a relative measure of the soil’s negative charge and thus its ability to hold certain nutrients that are cations (positively charged in nature). There are only a few components that give soil a negative charge; the two most common are clay and organic matter.  Elements such as calcium, potassium, magnesium, and others are cations and have positive charge.  CEC levels will give a grower a good idea of the overall nutrient capacity as well as soil texture.  High CEC, in the range of 25 and above, represent clay dominated or fine soils.  Soils with a CEC of 10 or less are sandy or course in nature.  This can have great ramifications on farming practices such as fertilization and irrigation.  Fine, or clay soils retain nutrients better than sandy soils.  Course, or sandy soils have little charge and are worse at holding large quantities of nutrients.  Fine soils can be fertilized less often, where course soils should be fertilized in small, but frequent doses.  Fine soils hold a great deal more water than course soils.  Therefore fine soils can be irrigated less frequently than course soil and are more prone to becoming waterlogged.  CEC is very difficult to alter, but by utilizing certain tools one can work around most of the problems encountered.  As an example, an effective tool for high CEC is to add gypsum to create better porosity, thereby creating better drainage.  Another mitigating technique can be to add drain tile to alleviate water logged soils.  In sandy, low CEC soils, the annual addition of organic matter in the form of manures, composts and compost teas helps build better charge, better water retention and increases microbial numbers in the soil.  

Base saturation is the final item to focus on as it also has ties to CEC.  The importance of base saturations cannot be overstated.  In general, the term refers to the level of permeation of soil surfaces by five cations, calcium(Ca), magnesium (Mg), potassium (K), hydrogen (H) and sodium (Na). Each of the cations will be represented as a percentage on the soil test.  What is important is how these percentages relate to one another.  Ca should be 60 to 65%, K 4-6%, Mg 12-25%, H 10% or less, and Na less than 1%.  If any of these cations are out of range than the grower should work to alter the percentages.  For example many soils in California are high in Mg and lower in Ca.  An easy way to alter this is the application of gypsum, calcium sulfate or lime (calcium carbonate).  The addition of these materials will not only aid in the addition of elemental Ca, but facilitate the removal of excess Mg from the soil profile.  Excessive H on the base saturation generally points to acidic soils which can be altered by liming the soil.  The addition of a host of mineral nutrients to the soil to aid in plant nutrition is of little use if your base saturation percentages have not been addressed before hand.

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