Soil Testing For Potassium And The Fertilizer Value Of Potassium Chloride Challenged By New Study

In the chemical age of agriculture that began in the 1960s, potassium chloride (KCl), the common salt often referred to as potash, is widely used as a major fertilizer in the Corn Belt without regard to the huge soil reserves that were once recognized for their fundamental importance to soil fertility. Three University of Illinois soil scientists have serious concerns with the current approach to potassium management that has been in place for the past five decades because their research has revealed that soil K testing is of no value for predicting soil K availability and that KCl fertilization seldom pays.

U of I researchers Saeed Khan, Richard Mulvaney, and Timothy Ellsworth are the authors of “The potassium paradox: Implications for soil fertility, crop production and human health,” which was posted on October 10th by Renewable Agriculture and Food Systems.

A major finding came from a field study that involved four years of biweekly sampling for K testing with or without air-drying. Test values fluctuated drastically, did not differentiate soil K buildup from depletion, and increased even in the complete absence of K fertilization.

Explaining this increase, Khan pointed out that for a 200-bushel corn crop, “about 46 pounds of potassium is removed in the grain, whereas the residues return 180 pounds of potassium to the soil—three times more than the next corn crop needs and all readily available.”

Khan emphasized the overwhelming abundance of soil K, noting that soil test levels have increased over time where corn has been grown continuously since the Morrow Plots were established in 1876 at the University of Illinois. As he explained, “In 1955 the K test was 216 pounds per acre for the check plot where no potassium has ever been added. In 2005, it was 360.” Mulvaney noted that a similar trend has been seen throughout the world in numerous studies with soils under grain production.

Recognizing the inherent K-supplying power of Corn Belt soils and the critical role of crop residues in recycling K, the researchers wondered why producers have been led to believe that intensive use of KCl is a prerequisite for maximizing grain yield and quality. To better understand the economic value of this fertilizer, they undertook an extensive survey of more than 2,100 yield response trials, 774 of which were under grain production in North America. The results confirmed their suspicions because KCl was 93 percent ineffective for increasing grain yield. Instead of yield gain, the researchers found more instances of significant yield reduction.

The irony, according to Mulvaney, is that before 1960 there was very little usage of KCl fertilizer. He explained, “A hundred years ago, U of I researcher Cyril Hopkins saw little need for Illinois farmers to fertilize their fields with potassium,” Mulvaney said. “Hopkins promoted the Illinois System of Permanent Fertility, which relied on legume rotations, rock phosphate, and limestone. There was no potash in that system. He realized that Midwest soils are well supplied with K. And it’s still true of the more productive soils around the globe. Potassium is one of the most abundant elements in the earth’s crust and is more readily available than nitrogen, phosphorus, or sulfur. Farmers have been taught to think that fertilizers are the source of soil fertility—that the soil is basically an inert rooting medium that supports the plant.”

Khan and his colleagues pointed out that KCl fertilization has long been promoted as a prerequisite for high nutritional value for food and feed. To their surprise, they found that the qualitative effects were predominantly detrimental, based on a survey of more than 1,400 field trials reported in the scientific literature. As Khan explained, “Potassium depresses calcium and magnesium, which are beneficial minerals for any living system. This can lead to grass tetany or milk fever in livestock, but the problems don’t stop there.

Low-calcium diets can also trigger human diseases such as osteoporosis, rickets, and colon cancer. Another major health concern arises from the chloride in KCl, which mobilizes cadmium in the soil and promotes accumulation of this heavy metal in potato and cereal grain. This contaminates many common foods we eat—bread, potatoes, potato chips, French fries—and some we drink, such as beer. I’m reminded of a recent clinical study that links cadmium intake to an increased risk of breast cancer.”

While working in the northwestern part of Pakistan three decades ago, Khan was surprised to discover another use for KCl fertilizer. “I saw an elderly man making a mud wall from clay,” Khan said. “He was using the same bag of KCl that I was giving to farmers, but he was mixing it with the clay. I asked why he was using this fertilizer, and he explained that by adding potassium chloride, the clay becomes really tough like cement. He was using it to strengthen the mud wall.”

“The man’s understanding was far ahead of mine,” continued Khan, “and helped me to finally realize that KCl changes the soil’s physical properties. Civil engineers know this, too, and use KCl as a stabilizer to construct mud roads and foundations.” Mulvaney mentioned that he had demonstrated the cementing effect of KCl in his soil fertility class, and that calcium from liming has the opposite effect of softening the soil. He cautioned against the buildup philosophy that has been widely advocated for decades, noting that agronomic productivity can be adversely affected by collapsing clay, which reduces the soil’s capacity to store nutrients and water and also restricts rooting.

Khan and Mulvaney see no value in soil testing for exchangeable K and instead recommend that producers periodically carry out their own strip trials to evaluate whether K fertilization is needed. Based on published research cited in their paper, they prefer the use of potassium sulfate, not KCl.

Source: University of Illinois College of Agricultural, Consumer and Environmental Sciences

Role of Potassium in Crop Yield

Potassium is vital to many plant processes. A review of its role involves under-standing the basic biochemical and physiological systems of plants. While K does not become a part of the chemical structure of plants, it plays many important regulatory roles in development.

Enzyme Activation

Enzymes serve as catalysts for chemical reactions, being utilized but not consumed in the process. They bring together other molecules in such a way that the chemical reaction can take place.

Potassium “activates” at least 60 different enzymes involved in plant growth. The K changes the physical shape of the enzyme molecule, exposing the appropriate chemically active sites for reaction. Potassium also neutralizes various organic anions and other compounds within the plant, helping to stabilize pH between 7 and 8...optimum for most enzyme reactions.

The amount of K present in the cell deter-mines how many of the enzymes can be activated and the rates at which chemical reactions can proceed. Thus, the rate of a given reaction is controlled by the rate at which K enters the cell.

Stomatal Activity (Water Use)

Plants depend upon K to regulate the opening and closing of stomates...the pores through which leaves exchange carbon diox-ide (CO 2), water vapor, and oxygen (O2) with the atmosphere. Proper functioning of stomates is essential for photosynthesis, water and nutrient transport, and plant cooling. When K moves into the guard cells around the stomates, the cells accumulate water and swell, causing the pores to open and allowing gases to move freely in and out.

When water supply is short, K is pumped out of the guard cells. The pores close tightly to prevent loss of water and minimize drought stress to the plant. If K supply is inadequate, the stomates become sluggish – slow to respond – and water vapor is lost. Closure may take hours rather than minutes and is incomplete. As a result, plants with an insufficient supply of K are much more susceptible to water stress.

Accumulation of K in plant roots produces a gradient of osmotic pressure that draws water into the roots. Plants deficient in K are thus less able to absorb water and are more subject to stress when water is in short supply.

Photosynthesis

The role of K in photosynthesis is complex. The activation of enzymes by K and its involvement in adenosine triphosphate (ATP) production is probably more important in regulating the rate of photosynthesis than is the role of K in stomatal activity.

When the sun’s energy is used to combine CO2and water to form sugars, the initial high-energy product is ATP. The ATP is then used as the energy source for many other chemical reactions. The electrical charge bal-ance at the site of ATP production is maintained with K ions. When plants are K deficient, the rate of photosynthesis and the rate of ATP production are reduced, and all of the processes dependent on ATP are slowed down. Conversely, plant respiration increases which also contributes to slower growth and development.

In some plants, leaf blades re-orient toward light sources to increase light interception or away to avoid damage by excess light, in effect assisting to regulate the rate of photosynthesis. These movements of leaves are brought about by reversible changes in turgor pressure through movement of K into and out of specialized tissues similar to that described above for stomata.

Transport of Sugars

Sugars produced in photo-synthesis must be transported through the phloem to other parts of the plant for utilization and storage. The plant’s transport system uses energy in the form of ATP. If K is inadequate, less ATP is available, and the transport system breaks down. This causes photosynthates to build up in the leaves, and the rate of photosynthesis is reduced. Normal development of energy storage organs, such as grain, is retarded as a result. An adequate supply of K helps to keep all of these processes and transportation systems functioning normally.

Water and Nutrient Transport

Potassium also plays a major role in the transport of water and nutrients throughout the plant in the xylem. When K supply is reduced, translocation of nitrates, phosphates, calcium (Ca), magnesium (Mg), and amino acids is de-pressed. As with phloem transport systems, the role of K in xylem transport is often in con-junction with specific enzymes and plant growth hormones. An ample supply of K is essential to efficient operation of these systems.

Protein Synthesis

Potassium is required for every major step of protein synthesis. The “reading” of the genetic code in plant cells to produce proteins and enzymes that regulate all growth processes would be impossible without adequate K. When plants are deficient in K, proteins are not synthesized despite an abundance of avail-able nitrogen (N). Instead, protein “raw materials” (precursors) such as amino acids, amides and nitrate accumulate. The enzyme nitrate reductase catalyzes the formation of proteins, and K is likely responsible for its activation and synthesis.

Starch Synthesis

The enzyme responsible for synthesis of starch (starch synthetase) is activated by K. Thus, with inadequate K, the level of starch declines while soluble carbohydrates and N compounds accumulate. Photosynthetic activity also affects the rate of sugar formation for ultimate starch production. Under high K levels, starch is efficiently moved from sites of production to storage organs.

Crop Quality

Potassium plays significant roles in enhancing crop quality. High levels of avail-able K improve the physical quality, disease resistance, and shelf life of fruits and vegetables used for human consumption and the feeding value of grain and forage crops. Fiber quality of cotton is improved. Quality can also be affected in the field before harvesting such as when K reduces lodging of grains or enhances winter hardiness of many crops. The effects of K deficiency can cause reduced yield potential and quality long before visible symptoms appear. This “hidden hunger” robs profits from the farmer who fails to keep soil K levels in the range high enough to supply adequate K at all times during the growing season. Even short periods of deficiency, especially during critical developmental stages, can cause serious losses.