Soil and Manure Potassium Levels
from 2010 to 2015
Sally Flis, Ph.D. - Feed and Crop Support Specialist - Dairy One
Potassium (K) levels in the soil and in crop nutrition has come up at meetings and in conversations a lot this year. This led me to look into our soil testing and manure testing results. In the Agro-One lab, we perform the Mehlich-3, Modified Morgan's, and Morgan's extraction for testing nutrient levels in soils.
|Figure 1. Soil test K results by extraction from 2010 to 2015 from the Agro-One Lab.
Over the last 6 years, the average soil test K level has been high and has remained high for all 3 soil test extractions. The 6 year average for Morgan's was 213.4 lbs/ac K, for Modified Morgan's was 221.0 lbs/ac, and for Mehlich-3 was 241.2 lbs/ac. There is no trend for either increasing or decreasing soil test values. However, the standard deviation of all the extractions was almost equal to the average, indicating a very large range in sample results.
In order to get a better representation of the K status of soils we have tested, Table 1 shows the percent of samples that resulted in a recommendation of no K application for the crop. The Mehlich-3 extraction has the highest percentage of samples that would be recommending K application for crop production, this is also the largest percentage of samples that come into the lab. On average over the last 6 years, 63.3% of the samples submitted to the lab were for Mehlich-3 analysis. The Modified Morgan's extraction averaged 18.6 % and the Morgan's averaged 18.1% of samples per year over the last 6 years.
*In NY, soil test K levels are interpreted by soil management groups; this is the lowest of the high values for soil management group I.
Cornell K Guidelines
After seeing the high soil test levels, I also looked at the manure K values for the last 6 years. The average book value for liquid dairy manure K2O lbs/1000 gal is 20. In Table 2 are the average results for samples submitted to the lab and identified as Manure, cow liquid. These numbers are all lower than the average book value.
In conversations this year, I heard concerns that producers are seeing K deficiency problems. After examining the data from the Agro-One lab, here are a few thoughts on the discussions regarding K deficiency concerns in field crops:
- The majority of the samples we test would recommend some K application for crop production.
- Our average manure K test values are lower than book values.
- What values are being used for manure applied to crop field for K level?
- Are individual field balances done when determining K application?
If you are suspicious of a K nutrient problem, looking into the field history, nutrient application, and adding tissue testing to the management plan will help to pinpoint needs for management to address.