Potash - The Other Macro-Nutrient In Manure

Is it possible to get nutrient deficiencies when manure is routinely applied? In some areas, there has been a higher incidence of potash (K) deficiencies, even where manure is applied on a regular basis. There has also been a downward trend in the soil test potash levels in some areas of the province. How can this be?

Nutrient management rules and increased awareness have changed how manure is managed. Manure application rates are determined by considering nutrient analysis from the livestock type with the needs of the crop considering environmental risk.

N & P Determine Manure Application Rates

Nitrogen and phosphorus are the two main nutrients considered in determining manure application rates. Over-application of nitrogen can lead to nitrate leaching and can lead to economic losses. Over-application of phosphorus can lead to increased phosphorus reaching watercourses and increasing eutrophocation. Potash is seldom mentioned, and never triggered in nutrient management planning for municipal bylaws or provincial regulations. As a result, potash removal by crops, over the entire crop rotation, may be higher than the total potash applied.

The ideal soil test range for potash is between 120 and 150 ppm (150 to 180 ppm for rotations that include legume forages). In this range there is no additional requirement for potash. Greater than 250 ppm K is considered excessive. Potash levels lower than 100 ppm will require additional applications of K to maximize yield potential.

There is a large range in potash levels in manure, due to the amount of potash in the animal feeds. On dairy farms, potash soil test levels are often very high when commercial K is applied to forages to help winter survival. This can cause problems in dry cow reproductive health. As a result, most dairy farms are more conscious of potash in their nutrient management planning.

In recent samples of liquid hog manure, potash ranged from 2.25 to 7% on a dry matter basis (average near 4.5%). Liquid manure, on a dry matter basis, is higher than solid manure for potash. This is due to the potash being associated with the liquid portion of the manure, while phosphorus is connected with the solids portion of the manure.

There is also a large range in the potash removed in crops. The rate is low in crops where just the grain portion is removed. However, when the whole crop is removed (silage, forages, cereal straw), then the potash removed increases significantly.

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Using Commercial K Sources

To ensure that soil test potash levels stay in the range where deficiencies are less likely, it is essential to include potash as a nutrient that is balanced in the nutrient management plan. If manure application rates are limiting potash replacement because of nitrogen or phosphorus content in manure, then it may be necessary to add commercial sources at some point in the rotation. Refer to Table 1 – "Approximate Application Rates to Meet Specific Nutrient Needs for Common Field Crops" for examples of application rates to meet specific nutrient needs. When utilizing manure, it is crucial to have:

• an analysis of the nutrients being applied,
• a record of actual rates applied to crops, and

• uniform application across the field.

When combined with regular soil testing, potash deficiencies should no longer be a concern.

Table 1. Approximate Application Rates to Meet Specific Nutrient Needs for Common Field Crops

¹ Maximum NMP rate does not take p-index n-index or liquid loading into account since they are situation specific

* brackets mean N is removed but not required

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Assumptions:

• Rates are rounded to the nearest 100 gal/ac
• Manure application occurs in spring into standing crop or incorporation occurring within 24 hours
• 125 bu corn (6 dm ton silage) in SW Ontario (137 lb N requirement)
• 75 bu wheat with straw removed; spring cereals will have similar rates for K, lower for N and P
• 3.3 ton DM mixed legume-grass hay
• 40 bu soybean yield, 45 bu canola yield
• Average hog, dairy and poultry manure values from NMAN databan

   

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