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Soil Health for Horse Pastures: Part 5 - Calcium & Magnesium

This is the fifth in a series of blog posts aimed at helping you better understand soil nutrients & testing data for improved horse pasture management, healthier horses, and a better environment. In this series, we will explore traditional soil data like N, P, and K levels and soil pH, along with less well understood data including micro-nutrient levels and biological measures of soil health.


Previous posts

Part 1 – Nitrogen

Part 2 – Phosphorous

Part 3 - Potassium

Part 4 - Sulfur


In the first four blog posts, we discussed four of the six soil macronutrients, nitrogen, phosphorous, potassium and sulfur. In this post, we discuss the last two macronutrients, calcium and magnesium.


Calcium (Ca)

In its plant available form, calcium in soil is found as a cation (Ca++) adsorbed to soil colloids (clay particles and humus). It is also found in soil in relatively unavailable mineral forms. Calcium plays important roles in plant growth & nutrition and also improves overall soil structure.


In terms of plant health, calcium is key for proper cell division and cell wall development, nitrogen uptake, enzyme activity and starch metabolism. In simpler terms, it helps move things in and out of plant cells.


With respect to soil health and structure, calcium again plays multiple essential roles. It helps soils form stable aggregates essential for preventing soil erosion and enabling water and air to move through soil. It also acts as a nutrient filler, maintaining balance among nutrients and acting as a buffer against soils becoming too acidic. This latter behavior is an example of why cation exchange capacity and % base saturation (two soil properties that will be discussed in more detail in future blog posts) are two confusing, but important soil metrics. For now, suffice it to say that soil contains a reservoir of mobile cations, the most common of these being calcium (Ca++), magnesium (Mg++), potassium (K+), ammonium ( NH4+), hydrogen (H+) and sodium (Na+). The storage and flow of these cations in soil is measured in terms of cation exchange, and informs the availability of these nutrients to plants (remember, just because a nutrient is present in the soil does not mean it is available to be taken up by plants).


Calcium in soil is generally correlated with the amount of sand versus clay in soil as well as soil pH. Sandy soils tend to have calcium levels at or below 1000 lbs/acre, whereas high clay soils often have levels above 2,500 lbs/acre, Soils with a good pH level also tend to have plenty of soil Ca. Plant roots, however, are not very efficient at absorbing calcium from the soil (for example roots are about 10X more efficient at absorbing potassium). Visual symptoms of calcium deficiency in plants are less obvious than those of other soil nutrients, but include dieback of soil roots (short stubby roots) and unhealthy looking plant tips.


When testing indicates soils would benefit from addition of calcium, there are multiple options to choose from depending, in part, on other soil needs. For soils with low pH, the addition of different forms of lime, which contain calcium along with carbonate and, in the case of dolomitic lime, magnesium is typically recommended. Note that it is the carbonate in the lime that actually raises soil pH, not calcium. For soils deficient in calcium but already at an ideal pH, gypsum (calcium sulfate) is typically recommended, as gypsum does not raise soil pH. Common organic calcium amendments include animal manure, wood ash, seashells and lobster shells. Their use will obviously depend largely on local availability. See the table below for more information on various calcium containing soil amendments.


Amendment % Ca

Calcitic Limestone 32

Dolomitic Limestone 22

Hydrated Limestone 46

Gypsum 22

Poultry manure 0.4

Horse manure 0.5



Magnesium (Mg)

Magnesium, like calcium, is a key nutrient found in soil in its plant available form as a cation (Mg++). Magnesium’s most important role in plants is as the central core element of one of nature’s most amazing molecules, chlorophyll. Where magnesium is deficient, plants are not able to make enough chlorophyll to absorb light and generate the energy needed for photosynthesis. It’s no surprise, therefore, that where magnesium deficiencies occur, they are a significant limiting factor in healthy plant growth.


Fortunately, in most soils in the US, there is a sufficiency of magnesium. Sandy soils low in pH are the most common soils where magnesium is deficient. While ideal soil magnesium levels (like most any soil nutrient) vary by region and plant need, in general, soils with 1 to 100 lbs/acre Mg are considered low, 101 to about 500 lbs/acre medium, and 501+ lbs/acre high. For soils low in magnesium, dolomitic limestone (containing 8 to 10% Mg) is the most often recommended soil amendment. Horse manure typically contains about 0.25% Mg.


At Sustainable Stables, we believe that the key to healthy pastures starts with healthy soils. Our soil testing services go beyond simple macro-nutrient (NPK) and pH analysis to include micro-nutrient levels along with biological measures of soil health. Such soil data is key to developing a sustainable management plan to create healthy, resilient pastures. Our testing services can be done independently, or as part of our equestrian property planning and design services. Contact us today to learn more.

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