April 1998

POTASSIUM FERTILIZATION IN CONSERVATION TILLAGE SYSTEMS: NEW DEVELOPMENTS


FOR GROWERS in a no-till program, new studies from Iowa State University show that deep band and planter band applications can provide some corn yield advantages, even at high to very high soil test potassium (K) levels.

What are the advantages?
Two sets of studies were conducted, one at research farms, the other on farmer fields. At the research farms, three K placement methods were evaluated: Bands, 6 to 8 inches deep placed 30 inches apart; planter bands, placed 2 inches to the side and 2 inches below the seed in 30 inch rows; broadcast application. After the deep band application, corn was planted directly over the bands. Broadcast and deep band treatments were applied in the spring for the first two years of the study and in the fall in the third and final year of the study. Potash rates were 35 and 70 lb K
2O/A for all application methods.

Eleven sites were also studied on farmer fields, but evaluated only the deep band and broadcast application methods. Fertilizer rates were 35 and 140 lb K2O/A. Two of the trials received fertilizer applications in the spring while the remaining sites were fertilized in the fall.

In an analysis considering all of the research center locations, K fertilization produced significantly higher yields than where no fertilizer was applied. Table 1 shows that deep band applications increased yields above those where K was broadcast in 13 of the 15 site-years. Deep banding also produced higher yields than planter banding in 12 of the 15 site years. Planter banding increased yields above broadcast applications in 7 of the 15 site-years.

Table 1. Placement affects corn response to K at research centers.

How much was corn grain yield increased by the various placement methods? When only the responsive site-years were averaged, deep banding increased yields by nearly 11 bu/A, broadcasting and planter banding by about 6 bu/A. If both responsive and non-responsive site-years are considered, deep banding increased yields by an average of 6 bu/A, while increases from broadcasting and planter banding were about 2 bu/A. These results show fairly consistent yield increases to deep banding across all sites, even those testing high to very high in soil test K.

Are these yield increases seen at producers’ fields? Yes. Analysis considering all sites showed significant responses to K fertilization, as in the research sites. See Figure 1. Summarizing the data at only the responsive sites revealed that deep banding increased yields by an average of 13 bu/A, while broadcast applications resulted in 5 bu/A increases. Considering all sites, deep banding increased yields by 9 bu/A, while broadcasting increased yields by 4 bu/A.


Figure 1. Comparison of K application method on corn response in farmer fields.

Table 2 shows that the yield advantage of deep banding compared to broadcasting was fairly consistent across all of the sites on farmer fields.

Table 2. Deep banding of K produces higher yields than broadcast in farmer fields.

Why it works
In no-till systems where fertilizer has been broadcast, the level of K tends to increase near the soil surface. This occurs because K does not move far from the zone of application. Because no tillage is used, the K applied to the surface is not mixed with the soil. Also, during the growing season, plants take up K from the entire root zone, including the deeper layers of the soil profile, provided moisture is available. At harvest, the aboveground portions of the plant (except the grain) are left at the surface and release K back to the top soil. Thus, the crop tends to deplete the levels of K in the lower parts of the soil but returns part of it to the surface after harvest. This nutrient cycling by the crop adds to higher levels of K near the surface.

Roots grow where nutrients and water are in good supply. In no-till systems, both water and nutrients tend to be located nearer the soil surface. In a year when moisture is adequate, roots proliferate near the soil surface. However, if the soil surface becomes dry during the growing season root growth deeper in the soil profile will increase, provided moisture is available. The roots that actively take up nutrients are consequently located below the surface zone where fertilizer has been applied. So under dry conditions, corn can become deficient in K.

Placing the fertilizer 6 to 8 inches below the soil surface creates a zone of K enrichment at a deeper position in the soil. In this study, drier conditions in June may have caused greater root growth deeper in the soil profile. Where K had been deep banded, roots may have encountered both water and K. Where K had been broadcast, roots may not have found enough K to maximize yields.

Potassium Placement: Ridge-till

What’s new?
Potassium deficiencies have been observed for several years in Minnesota and eastern South Dakota, even at high to very high soil test K levels. Research from the University of Minnesota has shown that band applications of K are beneficial in correcting these deficiencies in ridge-till systems. The University of Minnesota currently recommends that 40 to 50 lb K
2O/A be applied in deep band on soils testing below 180 ppm K. Above this level, banded applications of K may provide some insurance against stressful conditions.

What are the advantages?
Yield advantages to deep band applications of K have been shown in ridge-till systems.
Figure 2 shows responses at K2O rates of 40 to 80 lb/A. These responses depended upon corn hybrid and averaged approximately 10, 22, and 9 bu/A for Pioneer hybrids 3737, 3732, and 3902, respectively. In this study, potassium chloride (KCl) was banded with a knife in the fall in the center of the ridge at a depth of 3 to 3.5 inches. Fall banding allowed the disturbance created by the knife to heal by planting time in the spring. In other trials, typical starter rates and formulations, such as 100 lb/A of 7-21-7, have not proven adequate for correcting the observed K deficiencies.


Figure 2. Corn hybrid responses to deep banded K.

At what soil test K levels are responses to banded K no longer expected? Research has helped define the upper limits of response. In a study conducted in 1992 at the West Central Experiment Station in Minnesota, two corn hybrids were evaluated. Both responded to banded applications of K. The soil test K level at this site was 156 ppm, (Table 3). A cold, wet summer limited yield.

Table 3. Corn hybrids respond to banded K.

 

Table 4. Location determines corn response to banded K.

In another study, various rates of banded K were evaluated at two locations (Table 4). One site, located in Pope County, Minnesota, had a soil test K level of 239 ppm. At the Blue Earth County site, the soil test K level was 157 ppm. In Pope County, there was no significant yield response to any rate of banded K applications. However, in Blue Earth County, there was a significant yield increase from the first 20 lb K2O/A applied in a band.

The results of the above studies, along with other observations, have led University of Minnesota scientists to recommend that 40 to 50 lb K2O/A be applied in a deep band in ridge-till systems on soils testing below 180 ppm soil test K.

Why it works
Insight into the possible mechanisms behind yield responses to deep banding comes from several studies at the University of Minnesota. The causes, like the no-till case form Iowa, appear to be related to the distribution of roots within the soil profile. Reduced-till systems can result in more compact soils. Increased compaction, combined with typically higher soil moisture levels, can result in root growth restrictions if spring conditions are wet and cool. In addition, corn roots also take up more nutrients within and adjacent to the ridge, depleting K levels in these zones. If soil from these zones is combined with higher K soil between the ridges, the overall soil sample will test higher in K and will not reflect the true nutrient requirements of the corn crop. Thus, K-deficient corn can appear on soils that are believed to be high to very high in K.

Potassium Source: Ridge-till

What’s new?
Applications of starter fertilizer are often important in the establishment of spring planted crops. New research from Kansas State University has shown that sulfate of potash (SOP) and muriate of potash (MOP) produce different responses when placed in direct contact with the seed.

What are the differences between sources?
Field studies were conducted in 1996-1997 in north central Kansas for the purpose of evaluating the effects of SOP and MOP starter fertilizer applications on corn and soybeans grown in 30-inch rows. Starter fertilizer (7-21-7) was formulated using both K sources. The test consisted of two placement methods, in-furrow with the seed and 2 inches to the side and 2 inches below the seed (2x2). Five rates were applied (50, 75, 100, 150 and 200 lb/A of 7-21-7). Soil test levels in the top 6 inches were: Bray P-1, high; exchangeable K, very high. Sulfur (S) rates were balanced. The corn received 200 lb/A nitrogen (N); the soybeans received no additional N.

When liquid starter fertilizer containing MOP was placed in-furrow, grain yield, plant stand, V6 stage dry matter, and K uptake were reduced in both corn and soybeans. In the corn experiment, starter fertilizer containing MOP applied in-furrow at the 50 lb/A rate reduced yields by 20 bu/A compared to the same rate applied as SOP (Figure 3). Corn yield was reduced by 43 bu/A when starter fertilizer containing MOP was applied in-furrow at 200 lb/A. When fertilizer containing SOP was placed in-furrow with corn seed, no yield or population reduction was seen except at the 200 lb/A rate, where there was an 11 bu/A decrease compared to the 2x2 placement. When starter fertilizer containing MOP was placed in-furrow with soybean seed, yields and plant populations were reduced regardless of rate (Figure 4). Yields and population of soybean declined when in-furrow rates of starter fertilizer containing SOP exceeded 100 lb/A.


Figure 3.  Effect of placement, rate, and
source of starter fertilizer on corn grain yield.


Figure 4. Effect of placement, rate, and
source of starter fertilizer on soybean yield.

Placing starter fertilizer away from seed in a 2x2 placement was safe at the highest rates of application, regardless of K source (Figures 5 and 6). However, there are hazards associated with in-furrow placement of starter fertilizer containing MOP (and SOP at higher rates). Understanding the potential for damage from fertilizer placed in contact with seed is critical in achieving the maximum benefits of starter fertilization.


Figure 5.  Effect of placement and rate of starter
fertilizer containing MOP and SOP on corn grain yield.


Figure 6.  Effect of placement and rate of starter
fertilizer containing MOP and SOP on soybean yield.

Summary

The frequency and magnitude of these responses to K fertilization reflect the importance of paying attention to K management. Don’t allow inappropriate K management to reduce crop yields and profitability in conservation tillage systems.