T.S. Murrell, Director, Northcentral U.S., International Plant Nutrition Institute

21 Nov. 2009



Effects on crop yields: Crop yields may or may not see a decline. If soil tests have been built up to levels that are considered non-limiting, then there is a very small probability that yields will suffer if phosphorus (P) or potassium (K) is omitted. If P or K has been omitted in previous seasons and it has been awhile since you’ve taken a soil test, it’s time to re-evaluate the nutrient status of your soils. Fields or field areas with lower soil tests run a higher risk of yield loss if P or K is omitted.

Effects on nutrient budgets: Crops remove nutrients with every harvest. Failing to replenish what crops remove erodes soil nutrient supplies. When native nutrient supplies are high or if soil fertility has been built up, relying on soil supplies is a good practice but can be done for only so long. Use soil tests to monitor how quickly reserves are being drawn down and keep track of nutrient removals.

Effects on soil tests: Over time, soil tests will decline to reflect the drawn down of soil reserves. Soil tests are generally more sensitive to such changes at higher levels. Year-to-year noise in soil tests is less for P than it is for K. The K soil test is more affected by temperature and moisture conditions than is the P soil test, so don’t be alarmed if a K test comes back with a result you didn’t expect. Keep testing as often as you can afford to. Over time, the trend will become more apparent.

Effects on plant health: Proper nutrition is tied to decreased severity or improved resistance to a number of crop diseases. A commonly observed problem when potash applications are inadequate is lodging of corn as harvest nears. It has also been shown that aphids are more attracted to soybeans that are K deficient. Like humans, plants have the greatest chance of being healthy when they are properly fed.

Effects on crop response to nitrogen (N): Nutrients interact, meaning that their combined effects are greater than their individual ones. If either K, P, or both become insufficient, plant responsiveness to N gets compromised, resulting in reduced yield and/or quality. If only small quantities of P and/or K can be afforded, it is best to place them near the seed at planting, such as 2 in. to the side and 2 in. below the seed, as is often recommended for corn.

The need to be more watchful: Foregoing a management practice like a nutrient application requires frequent re-evaluation. Keep an eye out for signs of crop stresses during the season, by using visual observations for nutrient deficiency symptoms as well as tissue testing. Ensure also that soil tests are up-to-date.

Greater importance of starter applications: Applying nutrients near the seed at planting provides a more concentrated band of nutrients that is in a good position for early season access by young, limited root systems. Applying starter fertilizer is a good approach to managing nutrient variability, particularly if it is suspected that a soil test may not be capturing all lower testing areas or if it has been awhile since a soil test has been taken.

The need for a more complete starter fertilizer package: A lot of starter fertilizer formulations contain only nitrogen (N) and P. If one or more K applications have been skipped, consider adding K to the starter fertilizer mix. If K is added, pay attention to changes in the salt index of the fertilizer blend. It may be necessary to switch from placement in direct contact with the seed to a placement farther away, such as 5 cm (2 in.) to the side and 5 cm (2 in.) below the seed, to avoid seed injury.

The need for a future investment into the soil nutrient bank. Farmers who own ground often say that their fields are their retirement plans. A similar concept applies to nutrients. When prices are more favorable, soil nutrient supplies can be built up. That way, when markets become unfavorable, the less expensive soil nutrient reserve can be drawn down. But just like a bank account, withdrawals can be made for only so long. Once nutrients get below the “minimum balance,” the crop incurs a yield penalty. Managing the soil nutrient supply should be done with the same vigilance as managing a personal bank account. It’s okay to withdraw – just make sure your nutrient “check” doesn’t bounce.