Determining the Best Fertilizer Choice


Agricultural producers are taking a hard look at the upcoming production season. Of all the inputs that our hay producers are studying, the cost of fertilizer is a significant factor in the final cost and profitability of this summer’s hay crop.

As we would expect, hay producers are examining the various kinds of fertilizers and trying to come up with the best option for the hay field. Historically, the dry fertilizers made the most sense. And while they still may be the best option, the tremendous hike is prices over the past several months has prompted interest in liquid and organic fertilizer options.

Of course, no discussion about fertilizer cost should be had without a good soil test to lead you to the right ratios and right amount to apply per acre. When asked, “What fertilizer should I use?” I love to respond with “I have no idea!” It’s true. While generalizations may be given, there is no way for even the most educated to estimate the fertility found in a field.

Thus, before making any fertilizer decision, one must first properly collect a sample and send it off to a lab to be analyzed. From there, you’ll know exactly what nutrients are lacking.

Looking at the label, those three numbers represent the nitrogen (N), phosphorous (P), and potassium (K) that the fertilizer contains. True, there are many more nutrients than N, P, & K that plants need, but certainly those are the three nutrients needed in the largest amount. Students of soil science will know them as the macro-nutrients.

All fertilizer products are legally bound to correctly display the nutrients found within the package. The Texas Fertilizer Control Act details this in Chapter 63 of the Texas Agricultural Code. In it, you will find all the definitions and legalities for fertilizers. Also in the Texas Administrative Code, Title 4 -Agriculture, Chapter 65, you can read all about the labeling, permitting, inspection, and more of fertilizers. The agency over this is the Texas Feed and Fertilizer Control Service.

Rest assured, Texas law provides guidance and teeth to enforce the labels and contents of any fertilizers sold. Overkill? I don’t think so. Having this safeguard keeps “snake oils” from being peddled, so long as we know our nutrients, do our diligence, and study the label.

If you buy a bag of the standard garden fertilizer, 13-13-13, then by law, the package must have 13% by weight nitrogen, 13% phosphorus, and 13% potassium. If it’s in a 50-pound bag, 13-13-13 is equal to 6.5lbs of N, 6.5 lbs. of P, and 6.5 lbs. of K. If it is a liquid 13-13-13 fertilizer in a 5-gallon container that has net weight of 40 pounds, then it must contain 5.2 lbs. N, 5.2 lbs. P, and 5.2 lbs. K.

Continuing on, if you buy a 50 lb. bag of 21-0-0, then we know that 21% of that 50 lb. weight is nitrogen. By doing the math we multiply 50 times 0.21. The result is 10.5 pounds of nitrogen is contained in that 50-pound bag. Pound for pound this simple math can be applied to any fertilizer product sold.

Taking this up a notch to commercial bulk ammonia sulfate (21-0-00) fertilizer sold by the ton, one simply multiplies 2,000 times 0.21 and learns that one-ton yields 420 lbs. of N.

Now marketing that fertilizer product is a different story! Shifty folks can claim it’ll double the amount of hay produced or double the amount of fruit that you get out of a garden. But when you dig down and study the label and we can determine the actual contents.

Research from the Texas A&M Research Station at Overton, Texas states that warm-season perennial grasses utilize nitrogen, phosphorus, and potassium at a ratio of approximately 4-1-3. Specifically, if you plan to produce 1 ton of dry hay, that bermudagrass hay must have absorbed approximately 50 pounds of nitrogen, 15 pounds of phosphorus, and 42 pounds of potassium. If these numbers are multiplied by the number of tons of forage desired across your hay meadow, the result you need will equal approximately the pounds of nutrients needed. For example, for 3 tons of production, it will take approximately 150 pounds of nitrogen, 45 pounds of phosphorus, and 82 pounds of potassium.

So, if you see an ad that claims their 5-gallon jug of fertilizer can give you tremendous hay production, just run the numbers. The net weight they say to use multiplied by the percentage by weight on the label will give you the pounds of nutrients. Does it even come close? One cannot imagine how.

The last step to determining the best fertilizer is dividing its cost by the nutrients is says it contains. From there you should be able to make the best decision for what should be used on your operation.

Nutrients are the fuel to grow plants. If I tried to sell you one gallon of super-duper fuel that will make your truck run for 1,000 miles, would you buy it? I’ve got a special going on right now. One gallon of “Super-Fantastic-Go-Fuel” – just a couple hundred dollars. Any takers?

All this brings to mind a song from one of the best country singers of all times, George Strait. He sang that he has some “ocean front property in Arizona”! Probably not.

Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is To receive a monthly newsletter about local Extension educational events and other offerings, e-mail

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