Identifying Potassium Fixation on Your Farm

Potassium Pools in Soil

Potassium (K) is one of the primary macronutrients occurring in agricultural soils, and it has a variety of roles in plant function, including drought resistance, carbohydrate translocation, turgor pressure maintenance and enzyme system activation. We are most familiar with the plant-available

pools of potassium: soluble K, which is K dissolved in the soil water, and the exchangeable K, which is the form that is weakly attached to the surfaces of cation exchange sites such as organic matter and clays. Both forms are considered readily plant-available. However, these pools only amount to ~1-2% of the totalK present in your soil. The vast majority of your soil K is locked away in the interlayers of clay minerals and held deep within the rock structure of the parent material. These two K pools are not readily available for plant use. Furthermore, certain clay minerals can scavenge plant-available K from the soil, including K fertilizers, and render it unavailable (Figure 1). This is called potassium fixation, and the degree of the fixation ‘strength’ can vary between regions and even in different areas of the field.

Understanding Potassium Fixation

Understanding the K fixation capacity of your field requires delving into the geological history and soil forming processes that characterize your field.  Strong K fixation capacity is typically associated with the presence of vermiculite and mica-based minerals in soil. These minerals have a high affinity (e.g., high negative charge) and vast internal space to attract and store the plant available K in your soil. These types of minerals are tied to granitic bedrock material of igneous intrusive origin. You might be picturing large granite boulders underlying your field, signaling the possible absorption of your K fertilizer. However, the presence of K-fixing minerals is much more subtle than that, with a K-fixing soil looking indistinguishable from a regular soil series. This begs the question: How are you supposed to identify areas of possible high fixation capacity in your fields? This is where your local Helena agronomist can help.

Managing Soil K Fixation with Agronomic Practices

If you have concerns about K fixation, contact your local Helena representative.  He can access information that can help make an informed diagnosis of K behavior in your soil. Once the field is examined, K fixation can be dealt with using sound agronomic practices and various methods of applying K to crops.

Suggested Reading

Pettygrove S., T. O’Geen, and R.Southard. 2011. Potassium Fixation and Its Significance for California Crop Production. Better Crops with Plant Food Vol. 95 (2011, No. 4).

- Dr. Karl Wyant, Desert Division Agronomist