This is the third in a series of blog posts aimed at helping you better understand soil nutrient & testing data for improved 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.
Potassium (K)
Potassium (K) plays an essential role in regulating metabolic activities, water pressure and water movement (osmosis) in pasture grasses. It is for this reason that K is referred to as the “great regulator.” K is also critical in enabling grasses to withstand environmental stress, including drought tolerance and winter hardiness (with the latter benefit revealing that fall applications of K may be particularly beneficial to pastures low in K).
Potassium occurs in the soil in multiple forms. In its more soluble and available form, it is found as a cation (K+) adsorbed onto soil colloids (clay and organic matter). In can also be found in both exchangeable and non-exchangeable mineral forms, where it is released slowly over time. Most soil tests do not measure the non-exchangeable component, meaning in some soils (particularly high clay soils), plants may be receiving enough K even when soil tests indicate a deficiency. This has led some to question the utility of soil testing for K, believing it has led to over-application of K where it is not needed (read The Potassium Paradox).
Healthy soil K levels need to consider more than just total K concentrations, as the ability of grasses to utilize K is dependent on other factors including soil pH and concentrations of other cations in soil that compete with K for uptake in grasses. For example, K has a unique relationship with magnesium (Mg), whereby grasses growing in soils with an excess in one nutrient are unable to take up the other in sufficient levels. Most commonly, it is high soil K levels that inhibit Mg uptake; however, some Midwestern soils have enough Mg to reduce K availability. Similarly, K can compete with Ca for uptake (typically an issue in higher pH soils), whereas, in acidic soils, excesses of hydrogen (H), aluminum (Al), or iron (Fe) can inhibit K uptake or utilization. The take home message is that the right balance (or ratio) of nutrients is key. For example, the ideal K:Mg ratio is around 1.5:1.
Pasture grasses that appear yellowish or stunted are evidence of low soil K levels (or ineffective K utilization due to an imbalance with other soil nutrients). Poor K utilization also impacts grasses’ ability to take up nitrogen from the soil.
Recommended rates of K application are highly variable, and can range from zero to 1,000 lbs K per acre depending on soil test results and other factors. It is usually best to apply no more than 300 to 400 lbs of K per acre at any one time. This is especially true for soils low in Mg (especially common in many soils in the eastern US), where any K applications should be spread out in smaller, more frequent applications. Horse and other animal manures, along with plant residues including hay and straw, are great K fertilizers. These organic fertilizers often supply all the K your pastures need.
Inorganic K fertilizers are generally referred to as potash, with muriate of potash (KCl) being the most common. KCl has an NPK ratio of 0-0-62. The table below summarizes potassium sources in more detail.
* K in animal manure ranges from 6 to 24 lbs K per ton manure, with K in horse manure about 9 lbs K per ton.
Other articles in this series
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.