Know the Cost Per Pound of Nutrients in Organic Fertilizers
Posted: March 14, 2012
Dr. Grubinger points out that if your soil test indicates that a field has adequate phosphorus (P) and potassium (K), then you should avoid blended fertilizers and only use those that are high in nitrogen (N) content, but if you need multiple nutrients, i.e., N-P-K, then a blended fertilizer is usually a better deal.
A lot of fields where you have been using manure or compost are high in P and K. In that case, you are mostly looking for nitrogen and so that makes the math easy. For example, an 8-1-1 at $19.50 for a 50 pound bag would be $4.80 per pound of nitrogen. A 5-1-1 at $19.50 per 50 pound bag would be $7.80 per pound of nitrogen. Bloodmeal (13-0-0) at $53 per 50 pound bag would be $8.80 per pound of N. Feathermeal (13-0-0) at $32.25 would be $4.96 per pound of N. Composted poultry manure pellets (6-2-2) at $8.75 per 50 pound bag would give you nitrogen at $2.92 per pound of N. Keep in mind that the price per pound should not be the only consideration. Some of the single nutrient products might be hard to spread, due to their texture, with a tractor- mounted spreader.
If you are confused, remember that the numbers on a fertilizer bag or analysis mean the percent N-P-K in the product/substance. For example, bloodmeal (13-0-0) has a guaranteed analysis of 13% nitrogen (13 pounds per 100 pounds of product). If it is a 50 pound bag there are only 7.5 pounds of nitrogen in that bag.
Other fields are high in P, but need both N and K. So custom mixes make sense. Grubinger talks about a custom 6-0-6; at $25 per 50 pounds the cost is $8.33 per pound of N or K. Alfalfa meal 2.6-0-2.3 at $21 per bag has an N cost of $16.15 per pound and K at $18.26 per pound. I have not seen those available around here. The Natural No Phos 4-0-4 at $17 per 50 pound bag would be $8.50 per pound of N or K.
If you only need K, then potassium sulfate 0-0-51 is the best deal; $51 per bag equals K at $2 per pound. Sul-po-mag 0-0-21 at $31 per bag has K at $2.95 per pound, but it also supplies magnesium (Mg), so if that is low and you will not be liming with high mag lime, it may be a better choice.
Keep in mind that nutrient availability varies among organic fertilizers. For example, 50 pounds of greensand 0-1-7 at $11.50 has a price per pound of K of $3.30, but most of this will not be available to plants in the short term.
I generally cost out a couple of different sources and combinations for what is recommended by the soil test for a field. Sometimes it is easier to think about the cost per bed instead of per acre. Either way, take a couple of recommendations into consideration, do the math and if it conflicts with a recommendation ask them why. There might be a difference in the product, like release rate, you have not thought about.
Of course this article only refers to granular organic nutrient sources. Most of us are also using compost, manure and/or cover crops. Make sure you take the nutrient contributions from these sources into consideration. If you need help calculating credits from your cover crops and compost, let me know.
Always check with your certifier if you have any doubt about the status of an organic amendment.