CROPS

Potassium deficiencies widespread again
By Dr. Paul Tracy

While trying to decide a topic for this column, I asked Lyndon Brush, MFA staff agronomist, if there were any key issues that need to be discussed. Without hesitation, he responded, “Potassium deficiency.” Potassium deficiency seems to be a recurring theme.

What bothers me about potassium deficiencies is that they are one of the easiest crop production problems to avoid. Through soil testing, crop yield record keeping (tracking nutrient removal) and a fertilization program developed around optimum soil potassium levels, potassium deficiency should rarely, if ever, be your crop’s yield limiting factor.

In previous issues, I have listed several reasons for increased potassium deficiencies across our region. They have not changed and include: increased number of short-term farm leases that discourage planned long-term soil nutrient management; less university research and emphasis oriented toward optimizing crop yield through improved and site-

specific recommendations; increased potassium fertilizer costs. Plus, at times producers don’t account for increased nutrient removal associated with higher crop yields.

Many producers use maintenance only (crop removal) programs. Maintenance programs assume 100 percent potassium fertilizer use efficiency. They rarely succeed and require that all other crop production factors that influence potassium availability are optimum. The combination of weather, crop diseases, insect damage, soil properties, soil fixation, time of fertilizer application, crop grown and other factors always drive crop potassium fertilizer use efficiency below 100 percent.

The most important, but often overlooked component of crop nutrient maintenance programs is that soil nutrient values must be at or above optimum before starting.

Maintenance-based programs can work. In fact, MFA Incorporated’s Precision Agronomy Services group recently launched our Nutri-Track Program, which is designed to manage crop nutrients spatially based upon removal values associated with geo-referenced yield monitor field maps. Rick Greene, MFA’s Precision Agronomy Services manager, will be discussing this program in future Today’s Farmer articles. MFA Incorporated will not initiate this program until we are sure that soil test levels have been accurately assessed and lie within the optimum soil test range. Even then, periodic soil sampling is necessary to keep the system in check.

A maintenance only program assumes uniform and consistent crop removal factors. Under agronomic field situations, this simply does not occur. Variability in grain and forage potassium removal is extremely large.

For example, a recent Agronomy Journal article expanded upon information that I presented here in 2003. It showed corn potassium removal from 150 bushels per acre ranged from 31 to 64 pounds per acre. This range occurred over a fairly small geographic area and included an average variability between locations of 13.9 percent. What was interesting from this research is that even within the same corn hybrid across similar environmental conditions, potassium removal ranged from 31 to 50 pounds per acre with an average variability between locations of 15 percent.

The concept of variable crop nutrient removal is not new. Data from Pennsylvania State University Forage Testing Laboratory, representing several thousand samples referenced in 1975, showed the range in potassium removal was 53 to 83 pounds per ton of legume forage and 28 to 62 pounds per ton of grass removed.

Another key factor in potassium management in soils is the “build factor.” This refers to how much potassium fertilizer is required to build the soil test level by one unit. Both University of Missouri and MFA Incorporated soil test recommendations call for 3 pounds of K2O to raise the soil test potassium level by 1 pound per acre. However, research has shown this ratio is soil type/environmental condition dependent and actually ranges between 1.5 and 5 pounds of K2O, which are required to raise the soil test value by 1 pound per acre. Similar to the crop removal discussion above, build factors are extremely variable across locations and especially across soil types.

Soils high in clay tend to have higher build factors compared to sandy soils. I firmly believe that each field or sets of fields have their own potassium build factors. Until we have enough local information to provide you with a prescribed build factor, the generic 3 will be used.

In the past, I have discussed cash rent, high fertilizer prices, corn versus soybean fertilization philosophies, luxury crop consumption/yield removal, soil compaction, drought stress and several other factors associated with potassium management. The bottom line is that an aggressive nutrient management program consisting of building low testing soils and maintaining optimum testing soils is still the best case scenario for eliminating potassium deficiencies. By doing so, you will eliminate potassium as a yield limiting factor in your farming operation.