How to calculate the REAL cost of fertilizer. Fertilizer costs aren't limited to the price on the bag!
So far we have considered the nutrient content of fertilizer materials in percentages. If you want to apply 4 pounds of actual nitrogen to 1,000 square feet of lawn, it is easy to figure that this can be supplied by 100 pounds of a 4 per cent material or 25 pounds of a 16 per cent material.
The problem of calculating fertilizer costs seems more confusing though when mixed fertilizers are used because the bags carry three figures instead of one. However, remember that these are still percentages. Thus a 10-8-6 fertilizer contains 10 pounds of nitrogen (N), eight of phosphorus (P) and six of potash (K) in every 100 pounds of fertilizer.
When I first began using commercial fertilizers, figuring percentages was easy: everything was packed in 100 pound bags. This was hard on the back, of course. We had no handy small cartons, no 50 or 35-pound bags, but at least we knew, without figuring, that a 100-pound bag of 5-10-5 would supply 5 pounds of nitrogen, 10 of phosphorus and 5 of potash.
Today it takes a wizard to figure out actual weights and percentages. Despite the need for doing a little paper work, the gardener who wants to be careful with his pennies should take time to work out fertilizer costs.
Fertilizer Costs - Per Pound
The most important figure is the cost per pound of nitrogen. A product which contains 40 per cent or 40 units of nitrogen and costs $20 per 100 pounds sounds expensive by the pound (50 cents per unit). It would, however, be cheaper than sheep manure which sells for $2.10 for a 50-pound bag but contains only between 1 per cent and 2 per cent nitrogen. Compare the 50 cents per unit cost of nitrogen in the first product with the cost - between $2 and $4 - of each nitrogen unit in the second product, sheep manure.
Another way to figure costs is by the total plant food in a product. A dried sheep manure, for example, that contains 2-1-1 units of the "big three" nutrients, usually sells for $2.10 for a 50-pound bag; this sounds cheap, yet the cost per nutrient unit is over $1. A mixed urea-form fertilizer analyzing at 20-5-5 and selling for $9.95 for a 50-pound bag would at first glance seem many times costlier, yet the cost for all the units is less than 67 cents per unit.
In figuring fertilizer costs you should give some thought to the form in which the plant food occurs. While urea would be cheaper on a cost-per-unit basis, it is inferior to ureaform as a long-lasting turf fertilizer that provides an eight-month feeding period and greater safety in application.
Learn More About Soil Fertilizers & Additives:
Chemical Nitrogen In Soil - understanding nitrogen in soil, and how it is used to make fertilizer products.
Fertilizer Formula - feeding rates of various fertilizer formula.
Organic Nitrogen And Soil - organic forms for getting more nitrogen into your soil.
Phosphorus And Soil - phosphorus is vital to your soil of course. But bone meal is so overrated.
Mixed Fertilizers - the gardeners preference, so what do NPK values refer to?
Potassium And Soil (Potash Fertilizer) - potash is the common term for the fertilizer forms of potassium.
Fertilizer Burn - how to avoid burning your plants/lawn with fertilizer.