This is the sixth 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
In the first five blog posts, we discussed the six soil macronutrients essential for plant and pasture health. In this post, we discuss soil pH, a soil property responsible for determining the availability of these key nutrients to pasture forage.
What is soil pH?
Soil pH is a measure of the acidity or alkalinity of the soil. When the concentration of hydrogen ions (H+) are equal to hydroxyl ions (OH-) in soil, the soil has a neutral pH of 7. Acidic soils (pH < 7) have a surplus of H+ ions and alkaline soils (pH > 7) have a surplus of OH- ions. Soils in the U.S. generally range from a pH of 4 to 10, with most soils in the 5.5 to 7 range.
Soil pH is directly related to the amount of Ca+ and Mg+ cations present in the soil. Areas with high rainfall tend to favor acidic soil conditions over time, as the rain can leach away Ca+ and Mg+ from the upper soil profile. Such acidification is more prevalent in pasture soils where these cations are not returned to the soil via liming or manure applications (whether naturally or by manure/compost applications).
Why is pH important?
Soil pH directly influences the availability of key nutrients to plants. If a soil is too acidic, or too alkaline, the availability of these nutrients to plants will be limited. The ideal pH value for most soils is in the range of 6.2 to 6.7. As you can see in the figure below from the LSU College of Agriculture, at this pH range, the availability of the six key soil macronutrients to plants is optimal. In contrast, the availability of soil micronutrients to plants tends to be greatest under slightly more acidic soil conditions. Under extremely acidic soil conditions (pH < 5), elements that occur naturally in soil, like aluminum, are highly available--so much so that they can lead to plant and forage toxicity.
Ever wonder why Lexington, KY and Ocala, FL are such hotbeds for equestrian activity? One reason is that both areas are known for their limestone-rich soils. Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3), which naturally creates a higher soil pH, the ideal environment growing pasture forage to feed working performance horses. For an interesting read on this topic, see https://equestricon.com/blog/ky-soil.
Measuring soil pH
The most common method used to measure soil pH, and the one you will most likely see reported on a soil test, involves mixing a soil sample with water in a 1:1 ratio and measuring the pH of the solution using a glass electrode.
Some labs will also report a buffer pH. In this test, after measuring the soil:water solution pH, a buffer solution is added to the mixture and the pH is re-measured. In the lab I use for testing, they add a buffer solution at pH 7.8. The idea behind the buffer pH measurement is that it provides an indication of how your specific soil will respond to a lime application.
As an example, let’s say two soils may have the same water pH of 6. However, the buffer pH of one soil may be 6.8, whereas in the second soil it is 7.3. That is to say, when we add the same buffer solution to both soils, the pH increases by 0.8 in the first soil, and 1.3 in the second soil. What this tells us is that if we applied the same amount of lime to these soils, we would expect a greater increase in pH per pound of lime applied in soil two than we would in soil one. Knowing this data can help us provide more accurate recommendations in terms of lime application needed to increase soil pH to the desired level.
Adjusting soil pH
Generally speaking, most soils in the US tend to be a bit too acidic, and the corrective measure is to apply either calcitic or dolomitic lime. Calcite (a.k.a. calcium carbonate [CaCO3]) is recommended for soils where magnesium levels are sufficient, whereas dolomite (CaMg[C03]2) is best for soils that would benefit from additional Mg. Note that it is the carbonate in lime responsible for increasing pH.
Whereas lime increases soil pH, nitrogen fertilizers - especially ammonium sulfate and anhydrous ammonia - will reduce soil pH. This can be useful in the rare scenario that your soils are too alkaline. It also needs to be considered in cases where you want to both add nitrogen and increase soil pH, in which case you may need to slightly increase the amount of lime applied.
At Sustainable Stables, we believe that the key to healthy pastures starts with healthy soils. Our soils 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.