The Importance of Potassium Fertilizer
eKonomics News Team
Potassium (K) is one of the 17 essential nutrients required by plants for growth and reproduction. Like nitrogen (N) and phosphorus (P), potassium is classified as a macronutrient. The term “potash” refers to a group of potassium bearing minerals and salts. There are multiple potassium fertilizer sources, expressed in units of K₂O on fertilizer labels, such as muriate of potash (KCl), sulfate of potash (K₂SO₄), double sulfate of potash and magnesium (K₂SO₄ 2MgSO₄), and nitrate of potash (KNO₃).
The importance of potassium in plants
Good potassium nutrition is vital to consistently improve crop productivity. Potassium’s role in the plant is primarily in plant/soil/air-water relations; it also activates certain enzymes, and it aids in moving captured carbon from plant biomass to reproductive material (grain, fruit, and fiber). Inadequate potassium nutrition leaves the plant more susceptible to different stresses, including water deficit, insect pressure, and pathogen pressure.
The signs of a potassium deficiency
A deficiency in potassium can greatly impact crop growth and overall yield. Learning the signs of a deficiency can help growers identify and correct the problem. Since potassium can be easily translocated within the plant, deficiency symptoms usually show up on the lower leaves of the plant first. Common signs of a deficiency include yellow or brownish scorching on the leaf margins beginning at the tip progressing toward the leaf base. In severe cases, the leaves may become necrotic and fall off. Along with the external problems, a potassium deficiency also affects the development of the plant’s root systems and stalks.
The potassium soil cycle
Click on the infographic image below for an interactive diagram demonstrating the three different factors that affect potassium within the potassium cycle – inputs to the soil, losses from the soil, and components in the soil. Once you are viewing the infographic, click on the MORE buttons below for further information on each factor.
Approximately 90-98 percent of total soil potassium is present in this form. Minerals contain the potassium and break down over time, releasing potassium. This long and slow process moves potassium in small amounts to the readily available pool.
Slowly available potassium
This form of potassium is trapped between layers of clay minerals and is oftentimes referred to as being fixed. Some of it can be released for plant use during the growing season, but some is fixed between the clay layers and slowly converted to available potassium. The actual amount varies with the type of clay in the soil.
Readily available potassium
Potassium that is considered readily available for plant growth is potassium that is dissolved in soil and held on soil exchange sites. Plants readily absorb the potassium dissolved in the soil water. As soon as the soil water’s potassium concentration drops, additional potassium is released into soil solution from the potassium attached to exchange sites.
Current soil testing techniques can actually quantify the amount of potassium held on soil exchange sites. This information is used to inform whether additional potassium supplementation is required, and if it is required, how much to supply. Expected crop response based upon soil test level is what is used to calculate return in the eKonomics ROI calculator.
Potassium uptake and removal
There are several variables that affect potassium uptake. Soil moisture, soil aeration, soil temperature, oxygen level, and the tillage system all impact potassium uptake. Plants usually absorb most of their potassium at an earlier growth stage than they do phosphorus and nitrogen though. Studies done on the uptake of potassium in corn show over 90 percent is absorbed before the plant initiates reproductive growth (tassel emergence).
Placement and broadcast
Most common potassium fertilizers are water-soluble and sometimes have a high salt index. Because of this, if placed too close to seed or transplants, they can decrease seed germination and plant survival, especially on sandy soils. The type of crop you are applying to also is something to consider. Crops such as peanuts, soybeans, and cotton are more sensitive to fertilizer injury than corn. The most common application is 2 inches to the side and 2 inches below. Band placement can be more efficient than broadcast application when soil test levels are low, thus lower rates can be applied to achieve similar yields.
To maximize the response potassium fertilizer has on crops, other nutrients are needed. High-potassium fertilization can decrease the availability of magnesium to the plant and may cause a magnesium deficiency if the soil is already low in magnesium. It also works the other way. Crops grown in soils with high magnesium can suffer potassium deficiency. Ultimately, it’s important to consider all factors and nutrients when crop planning. Nutrient balance is needed to get the most out of potassium fertilizer application, as well as fertilizing of any kind.