Soils: May 2001

Most articles you read discuss nitrogen fertilizer management. After all, nitrogen is directly related to yield. What about the importance of phosphorus and potassium fertilization, though? Both are primary nutrients, meaning that they are required in large amounts by the plant, yet rarely is information published with details regarding phosphorus and potassium nutrient management.

What is the function of these nutrients in the plant? What is their behavior in the soil? What are plant symptoms that indicate their deficiency? How do you know whether the soil is deficient in these nutrients and how do you correct the problem? I will attempt to answer these questions in this article.

Role in the plant
Phosphorus is a major component in plant DNA, the material responsible for the transfer of genetic characteristics in all living things. Phosphorus is also critical in root development, crop maturity, and seed production. Potassium is required for the activation of over eighty enzymes throughout the plant, and it increases water use efficiency and transforms sugars to starch in the grain-filling process. In general, the role of phosphorus and potassium in the plant is maintenance. Both are highly concentrated in new growth and are responsible for keeping the system operating smoothly.

Behavior in the soil
Both phosphorus and potassium are immobile in the soil, meaning they don't move readily with water. Let's compare the mobility of nitrogen, phosphorus, and potassium on a scale of 1 to 10, 1 being immobile and 10 being readily mobile. Nitrate nitrogen (NO₃-) is a 10. It is extremely mobile and can be lost to leaching, which is downward movement of water through the soil profile. Potassium is a 3. It has limited movement in the soil. Increasing soil moisture from 10 to 28 percent increases potassium transport by 175 percent. This movement of potassium is small and accounts for a small portion of the potassium absorbed by the plant. Phosphorus has a rating of 1. It is extremely immobile in the soil and is likely to stay wherever it is placed unless moved by erosion or crop export.

Now that I have defined nutrient mobility, you might wonder how it affects phosphorus and potassium availability to the plant. Mobile nutrients, such as nitrogen, are available from a large volume of soil. As a plant acquires water from the soil, mobile nutrients move with the water to the roots for uptake. Immobile nutrients are available from just a small cylinder of soil around each root. In other words, a plant's root system has to explore the soil to find available phosphorus and potassium. Only a small percentage of the soil's total phosphorus and potassium exists in a form available for plant uptake. The remainder is tied up with other elements, organic matter, and the like. However, these bound nutrients can become plant-available sometime during the growing season. As a plant takes up phosphorus and potassium, reserves readily replenish the soil solution.

Figure 1. Phosphorus deficiency in corn

Figure 2. Potassium deficiency in corn (above) and alfalfa (below)

Phosphorus and potassium deficiency symptoms
Historically, the soils throughout the region were rich in phosphorus and potassium. Over time, as land was farmed or hayed, more nutrients were removed than were replaced by fertilization. As a result, both phosphorus and potassium levels in some soils have dropped to insufficient levels. Phosphorus deficiency symptoms include purple coloring in the leaf (figure 1) or sometimes yellowing on the lower or oldest leaves. Potassium deficiency symptoms are chlorosis or dieback beginning at the leaf tip and moving toward the base along the outer edges (figure 2).

How to tell if soil is deficient
Soil testing is the answer. Unlike that for nitrogen and sulfur, phosphorus and potassium requirements are not based on yield goal but instead are based on soil test values and their corresponding sufficiency levels. Years of university research has been performed to correlate laboratory phosphorus and potassium test results to crop response in the field. A soil test measures the concentration of extractable phosphorus and potassium, not the total amount. In other words, a soil test predicts how much phosphorus and potassium will become plant-available through the growing season. A good analogy is a dipstick on a tractor. The dipstick measures the oil level, not the total amount in the crankcase. If the dipstick reads full, then there is no benefit from adding oil. If it reads one quart low, though, oil must be added to assure peak engine performance. The same is true with soil testing.

How to correct the deficiency
To correct a deficiency, apply phosphorus and potassium fertilizer according to the results of a field-calibrated soil test. Neither phosphorus nor potassium soil test numbers change much from year to year. It takes approximately 10 to 20 pounds of P₂O₅ per acre above crop uptake to raise the phosphorus level reported on the soil test by one unit. You can build phosphorus and potassium levels in the soil, but it is too expensive to attempt quickly. It is best to apply slightly more phosphorus and potassium each year than the crop requires. In time, soil-test levels will increase.

Phosphorus and potassium fertilizers are very water-soluble. Approximately 95 percent becomes plant-available shortly after application. Does this mean that the plant will use all of the available phosphorus and potassium? No, the plant will absorb only 10 to 15 percent. The leftover phosphorus and potassium will begin to build in the soil.

In wheat production, a soil-test phosphorus index of 65 and a potassium index of 250 are sufficient, which means the nutrient levels will not limit crop performance. Since the soil is providing 100 percent of the crops' requirement of these nutrients, there is no benefit in applying additional phosphorus and potassium fertilizer. If the phosphorus index is 20 and potassium is 125, then the nutrients available are sufficient to produce only 80 percent of the potential yield, so crop performance may be limited. In this situation, 40 pounds of P₂O₅ and K₂O per acre is needed to correct the nutrient deficiencies and prevent limited crop performance.