Over the years I have made several attempts in my article to clarify how to interpret a fertilizer recommendation.  It is pretty easy to understand once you have the basics but it is hard to explain, especially in writing.  I continue to get questions and confusion from many people and so I will attempt to explain it again.

Over the years I have made several attempts in my article to clarify how to interpret a fertilizer recommendation.  It is pretty easy to understand once you have the basics but it is hard to explain, especially in writing.  I continue to get questions and confusion from many people and so I will attempt to explain it again.

We’ll use a typical recommendation for a spring application of nitrogen on fescue as an example.  That typical recommendation would be 70-30-30 and this is on a per acre basis.  Just exactly what does that mean?  This means that the blend must supply 70 pounds of actual nitrogen, 30 pounds of actual phosphorous and 30 pounds of actual potash per acre.  The first number represents nitrogen , always.  The second number represents phosphorous, always.  The third number represents potassium, always.  So when you see a recommendation, you will always know what nutrients the first three numbers represent.

Some people refer to actual nutrient amounts as units of nitrogen or other nutrient. This term is another way of referring to actual pounds of plant nutrient.  I always refer to amounts of nutrient in pounds and I think the use of the term units is confusing and shouldn’t be used.
                   
The numbers in the example above do not represent the actual amount of material to be applied.  What the numbers represent are the pounds of actual nitrogen, phosphorous and potash required on a per acre basis.  The actual pounds of total material used is going to depend on the material that is being used and will always be greater than the actual plant nutrients required.

The four commonly used nitrogen sources and their nitrogen content includes urea at 46% nitrogen, ammonium nitrate at 34% nitrogen, anhydrous ammonia at 82% nitrogen and UAN(urea-ammonium nitrate solution) at 28%.  It is important to note that ammonium nitrate is going to be hard to find.  Each material contains a different amount of actual nitrogen per hundred pounds.  For example, anhydrous ammonia contains 82 pounds of available nitrogen per hundred pounds and urea contains 46 pounds of actual nitrogen per hundred pounds.

The two most commonly used sources of phosphorous in our area includes diammonium phosphate(DAP) at 46% phosphorous.  This is a dry material and it’s analysis is 18-46-0.  Ammonium polyphosphate is a liquid and it is 34% phosphorous and its analysis is 10-34-0. It provides 34 pounds of phosphorous per 100 pounds of material.  Remember that the first number of a fertilizer analysis is always nitrogen. Based upon this fact then 18-46-0 is 18% nitrogen and 10-34-0 is 10% nitrogen.  So a hundred pounds of 18-46-0 provides 18 pounds of nitrogen and 100 pounds of 10-34-0 provides 10 pounds of nitrogen.

The only commonly used source of potash in our area is 0-0-60.  This material is actually potassium chloride which is a salt.  This material has no nitrogen or phosphorous in it at all as you can tell by the analysis but it is 60% potassium. Incidentally, chloride is also a plant nutrient but is seldom talked about as a useful component of 0-0-60.

The lime recommendation is also sometimes misunderstood for the same reason. Normally, a lime recommendation is made on the basis of effective calcium carbonate or ECC.  This recommendation represents the actual pounds of effective calcium carbonate needed per acre to neutralize soil acidity.  The actual pounds of liming material to be used is going to depend upon the liming material being used but would nearly always be greater than the pounds ECC needed per acre.

Most ag lime in our area is about 60% effective calcium carbonate.  The ECC of the liming material is determined by the fineness of the liming material and the calcium carbonate content of the liming material.  So in our area the actual lime requirement is going to run about 1.67 times greater than the recommended effective calcium carbonate needed.  Just divide 100 by 60 and that is 1.67.

By now this should all be as clear as mud.  The main point that I wanted to make is that the fertilizer recommendation that I make does not reflect the amount of actual material that is needed.  The pounds of actual material needed will be greater than the total of actual nutrients needed and the total amount of fertilizer needed is going to depend upon the the fertilizing material that is being used.  Call me at 724-8233 if you have any questions about this.