How Can the Nature of Clays in Your Soil Affect Plant-available Potassium?
Cristie Preston, Ph.D.
Agriculture has always been an integral part of Dr. Cristie Preston’s life. She grew up in southwest Virginia and had interest in crop and animal agriculture since an early age. Once she began college, she initially chose to study animal science but switched to soil science. Dr. Preston attributes her decision partly to an influential professor who told her, “You can’t understand animals until you understand what they eat.” She received a Bachelor of Science degree in animal science and a Master of Science degree in crop and soil environmental science from Virginia Tech. Dr. Preston holds a Ph.D. in agronomy, focusing on soil fertility from Kansas State University. While completing her advanced degrees, Dr. Preston conducted more than six years of field and lab research. Dr. Preston has experience in laboratory research measuring volatility loss from urea-based fertilizers. Her field research has focused on phosphorus availability and the interactions with tillage and placement. She also has extensive experience in working with large data sets and analysis. Her main priority is helping growers to identify yield-limiting factors and fix those issues as cost efficiently as possible.
Many universities use various factors to determine potassium management recommendations. Of those, soil test potassium concentrations, yield goals and removal rates are the most widely used to make these recommendations (IPNI, 2014).
Soil sampling is not a perfect science; it is actually quite imperfect. However, it is the best practice we have for measuring potassium in the soil solution and, therefore, exchangeable potassium in the potassium soil cycle (Figure 1). Proper soil sampling and using the most correlated Soil Testing Method can assist with making fertilizer recommendations.
Yield goals are also a good factor to consider and by applying removal rates, farmers can at least assure that they are replacing the nutrients that are removed at harvest. See our eKonomics Nutrient Removal Calculator to determine these values.
There are, however, other soil properties which may play a big role in affecting potassium availability to plants. Soil clay can play a huge role in this availability. The term clay can mean three different things:
- Soil particles: clay particles are less than 0.002 mm size
- Texture class name: the concentration of sand, silt, and clay within soil; see the Texture triangle (Figure 2), a clay soil that contains more than 40 percent clay particles, and therefore, less than 45 percent sand and less than 40 percent silt.
- Specific group of alumino-silicate minerals: soil mineralogy is the branch of soil science that deals with the homogeneous inorganic materials found in the earth’s crust to the depth of weathering or of sedimentation.
Soil particle and texture analysis can vary by farm and can easily be measured. However, silicate minerals do not vary as often and can be identified within regions of the U. S.
There are two properties of clay minerals that can affect potassium availability: the structure of clays and the primary minerals present.
Structure refers to the organization of layers and their contents. Isomorphous substitution — the replacement of one atom by another of similar size in a crystal structure without disrupting or seriously changing the structure of clay minerals — leads to a net negative charge in soil. This occurs when a substituting cation is of a smaller valence than the cation it is replacing.
In simple terms, valence is the number of electrons available for chemical bonds. For example, Mg2+ and Fe2+ would both have two electrons and Fe3+ and Al3+ would have three.
If Mg2+ were to substitute for Al3+ in the soil structure, there would be a net negative charge created. This charge is important because it must be balanced by positively charged ions, such as potassium, from solution and aids in the retention of plant nutrients and cation exchange capacity (CEC). Recall, the CEC is the sum of exchangeable bases (cations mainly of potassium, calcium, and magnesium), plus the total soil acidity (hydrogen and aluminum).
There are three categories of clay in soils: 1:1 non-expanding, 2:1 non-expanding, and 2:1 expanding (Figure 3). The type of clay present is controlled by soil weathering.
Areas that are greatly weathered due to high moisture, such as the southeastern U.S., contain mostly kaolinite (1:1 non-expanding). Kaolinite clay layers are held tightly by hydrogen bonds and therefore, cannot be separated with wetting and drying. Kaolinite clay soils have low CEC and therefore, potassium is not held tightly on the soil surface.
The north central U.S. region has a high percentage of soils containing 2:1 expanding clays, like smectite and vermiculite. Smectite clays are 2:1 clays that shrink/swell. These clays have an interlayer that can be penetrated by water and hydrated ions, like potassium and ammonium, making them non-plant available (see how North Dakota State University is updating their potassium recommendations).
Illite is also a 2:1 clay, but does not shrink/swell as smectites do. Kaolinite is the most weathered clay, which does not swell and has low CEC.
In contrast to kaolinite, potassium availability in smectite and illite clays is not dependent on CEC at all. From illite clays, potassium is released from the clay surface and interlayers whether the soil is wet or dry. On the contrary, in smectite clays, potassium is only released when the soil is wet.
When smectites dry out, potassium is drawn back into the interlayer and becomes temporarily unavailable as the clay shrinks/collapses. Interlayer potassium remains unavailable until the soil is moistened again. Therefore, a soil dominated with smectites would potentially require more potassium (compared to illite dominated soils) due to the temporary fixation between clay layers, especially in drier times during the growing season.
Logistically, the university does not make an adjustment to the formula for determining recommendations due to differing clay types. Recommendations are still based upon soil test level, but smectitic soils have a higher critical level — the potassium level where you would not expect to see a fertilizer response. Ensuring adequate potassium levels for production can maximize yield and profitability.