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Spring is here and the dogwoods are in full bloom. Time is approaching to make that first pasture fertilizer application of the year on warm-season perennial grasses. As we have been talking about in our conversations these past few weeks, hopefully you are arming yourself with all the information required to make an informed and economically wise decision.

Last week we talked about selecting the proper liming material to fit your needs and the importance of controlling pasture weeds. This week we are going to discuss comparing apples to apples when evaluating which nitrogen fertilizer source to buy. Don’t let the price per ton be your only guiding factor. Some products, which have a higher cost per ton, can be the most economical in the long run.

Oh, and for those of you who prefer to attempt controlling pasture weeds through mowing, you are spending more time in the tractor seat and more money per acre than you would if you treated the weeds using herbicides. Shredding costs around $3.50 more per acre than spraying with herbicides, and you cover about 11.5 acres more per hour with a sprayer versus a shredder. Now, if mowing is your stress-relieving hobby or place to be left to your thoughts, I can sympathize, but it is not the most economical or time-saving practice. That is not to say that there are no times and places when it is appropriate to shred, it is just not as economical as spraying with herbicides to control pasture weeds.

Since nitrogen is the primary nutrient in most pasture fertilizer blends, let’s look at an example using some nitrogen sources commonly found locally and prices from earlier this year. Let’s say you can get urea (46 percent nitrogen) for $570 per ton, ammonium nitrate (34 percent nitrogen) for $435 per ton, or ammonium sulfate (21 percent nitrogen) at $305 per ton. The lowest cost per ton of fertilizer in this example, of course, is ammonia sulfate, but the lowest cost per pound of nitrogen in this scenario is urea.

The ton of urea supplies 920 pounds of nitrogen at about 62 cents per pound. Ammonium nitrate supplies 680 pounds of nitrogen per ton at about 64 cents per pound. Ammonium sulfate provides 420 pounds of nitrogen at about 73 cents per pound.

Important note: ammonium sulfate also provides 480 pounds of sulfur per ton, so if your soil test recommendation calls for adding sulfur, you might want to consider using this fertilizer source to meet your sulfur needs. If you do not need the sulfur and use ammonium sulfate, you are spending money on a nutrient that you do not need.

When taking into account the long-term effects on soil pH of these three nitrogen sources, it is important to note that for every pound of nitrogen from urea or ammonium nitrate you add to the soil, you will eventually need to add 1.8 pounds of limestone to counteract the drop in soil pH caused by the product. For every pound of nitrogen from ammonium sulfate you add to the soil, you will eventually need to add 5.4 pounds of limestone to counteract the drop in soil pH.

You can use this same methodology when trying to compare two or more fertilizer blends, but when working with fertilizer blends, you must first know the price per pound of all three major nutrients (nitrogen, phosphorous and potassium) in the blend. Remember that the numbers in the blend represent the percentage of nutrients found per 100 pounds of fertilizer. So a 21-8-17 blend, for example, contains 21 pounds nitrogen, 8 pounds phosphorous and 17 pounds potassium per 100 pounds of fertilizer (or 420 pounds, 160 pounds and 340 pounds each per ton). For those of you who garden on the side, that means that of the two more common garden fertilizer blends, 13-13-13 contains about 38 percent more of each of the three major nutrients per sack than 8-8-8.

Make sure to also factor into the price equation any per acre fertilizer application fees or fuel surcharges when comparing fertilizer prices.

Brian Triplett is the Smith County Extension agent for agriculture and natural resources. He can be reached at 903-590-2980, via e-mail at b-triplett@tamu.edu or on the Web at http://tceblogs.tamu.edu/mt/smith or http://smith-tx.tamu.edu.