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What Happens if I Don’t Apply Potassium This Year?
 What happens if I don’t apply potassium (K) this year? The correct answer to this question is “It depends”. It depends on:
- Current soil fertility or soil test K levels
- Past K fertilization
- Growing season weather conditions
- Level of soil compaction
- Disease pressure
- Several other cultural practices such as crop variety or tillage system.
Current Soil Fertility
The loss in crop yield or quality experienced is very dependent on the soil’s ability to supply the K needs of the crop to be grown. That loss can vary from zero for highly fertile soils to over 40% for infertile soils. The following table is an example based on long-term calibration research in Illinois.
Soil test level | Relative yield loss, % | Loss in bu/A* |
Low | 47 | 85 |
Medium | 13 | 23 |
High | 2 | 4 |
Very high | 0 | 0 |
*Assuming 180 bu/A yield potential. Reetz et al., 1999.
To put this table in perspective, a summary of soil test levels for the eastern Corn Belt of the US based on over 350,000 samples collected for the 2001 growing season, showed that 60% are medium or below in K, mostly in medium (PPI/PPIC/FAR, 2001). Long-term trends indicate that this number is increasing because farmers have generally been applying less K fertilizer than is being removed by crops.
Past K Fertilization
Low crop prices of the last few years have induced many farmers to be quite conservative in K fertilization. Potassium budgets for the six leading corn-producing states for 1998-2000 show that K removal in crops exceeded K fertilizer applied by 62%; removal exceeded K fertilizer applied plus estimated recoverable manure K by 30% (PPI, 2002). Thus, if a farmer does not have recent soil test results available, it is likely that levels are less than the last time he or she sampled.
Growing Season Weather
Risk is a fact of life in crop production and weather is a major cause of that risk. The yield losses from not applying K shown above are based on long-term averages. Results in any one year could vary markedly depending on weather. For example, Ohio research has shown that yield response to K on a medium testing soil varied from 4 bu/A in a good year to 40 bu/A in a droughty year for corn and from 4 bu/A to 17 bu/A for soybeans (PPI, 1998b). Good K management is one means farmers have of reducing down-side risk related to crop stress.
Soil Compaction
Soil compaction is another factor that can contribute to crop stress and influence K response, especially response to banded K. Research by the University of Wisconsin showed that corn yield loss from compaction on a soil testing low to medium in K was 22 bu/A without applied K and only 5 bu/A when 45 lb K2O was applied in a band near the row (PPI, 1998b). Again, the importance of good K management in reducing the negative effects of crop stress is apparent.
Disease Pressure
Supplying adequate K can enhance a plant’s natural disease resistance mechanisms. Studies have shown that K can reduce the negative effects of leaf spot disease in cotton, stalk rot in corn and sorghum, stem canker in soybeans, leaf spotting in bermudagrass, several fungal diseases of wheat, and numerous other diseases in a variety of crops (PPI, 1998a).
Rate of Soil Test Decline or Build-up
Potassium in most soils is relatively well buffered which means that soil levels do not change quickly, either in an upward or downward direction. The exceptions are sandy soils and when high yielding forages are being grown. Many agricultural soils will increase or decrease 1 ppm of soil test level for every 8 lb K2O/A applied as fertilizer or removed by crops. To increase a soil testing in the middle of medium to the middle of the high category (a 40 ppm change in many systems) would require about 320 lb K2O/A in addition to the K removed by the crop. The following table illustrates the expected rate of change if no K is applied to various crops.
Crop | Removal, lb K2O/A | Annual soil test decline, ppm | Years to drop from high to medium |
180 bu/A corn | 52 | 6.5 | 6 |
50 bu/A soybean | 70 | 8.8 | 5 |
60 bu/A wheat | 21 | 2.6 | 15 |
5 T/A alfalfa (dm) | 300 | 20-37 | 1-2 |
Closing Thought
Whether your vocation is producing food to feed a growing world population, leading a company through difficult times, or using paint and canvas to capture the beauty of nature, you probably are not striving to be average. Your goal is to do something special … to be outstanding at what you do. Perhaps the most insidious result of the increasingly negative K balances showing up in so many agricultural regions is that it is gradually robbing crop producers of their ability to produce truly outstanding crops of truly exceptional quality.
References (all can be ordered or downloaded from http://www.ppi-ppic.org)
PPI. 1998a. Effects of Potassium on Plant Diseases. Better Crops with Plant Food 82(3): 37-39.
PPI. 1998b. Potassium Reduces Stress from Drought, Cool Soils, and Compaction. Better Crops with Plant Food 82(3): 34-36.
PPI. 2002. Plant Nutrient Use in North American Agriculture. PPI/PPIC/FAR Technical Bulletin 2002-1. Potash & Phosphate Institute, Norcross, GA.
PPI/PPIC/FAR. 2001. Soil Test Levels in North America. PPI/PPIC/FAR Technical Bulletin 2001-1. Potash & Phosphate Institute, Norcross, GA.
Reetz, H.F., T.S. Murrell, and T. W. Bruulsema. 1999. Maintaining High Soil Tests. News & Views. July, 1999. Potash & Phosphate Institute, Norcross, GA.
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