Fertilizing Field Crops in 2012

By Everett D. Thomas

I'll start this column with what I believe are two changes in agriculture that if not permanent will be of long duration: First, a global population growing faster than the rate of increase in food production has resulted in considerably increased grain prices. Corn may not be as expensive in two or three years as it is right now, but we have almost certainly seen the last of $3-per-bushel corn. In fact, I think that $7-per-bushel corn, especially during short crop years here in the U.S., is a lot more probable than $3 corn. Second, higher grain prices will put upward pressure on fertilizer prices since farmers won't want to miss out on any yield potential. Much of the naturally occurring fertility has been "farmed out" of most U.S. cropland, so high crop yields require significant amounts of fertilizer, including both commercial products and animal manures. Though less significant perhaps, another change for farmers in the eastern U.S. is a huge decrease in the amount of sulfur that our cropland receives from precipitation. Environmental regulations have greatly reduced the amount of sulfur spewed into the air by midwestern smokestacks. This is good for air quality, but it will mean that sooner or later farmers will have to begin supplying what they used to get for free - whether they wanted it or not. An increasing amount of university research suggests that in some cases "sooner" is now. It's probably not coincidence that some forms of sulfur fertilizers are reported to be in short supply for 2012.

Prioritizing fertilization

Fertilizing crops is very different for livestock farmers than it is for cash crop farmers. Farmers without a livestock enterprise seldom have access to animal manures, which (as has been noted many times before in this column) is a valuable source of nutrients. It's almost impossible to intelligently discuss nutrient requirements for individual fields without access to recent soil analyses, in this case "recent" meaning analyses done within the past three years. Fields on cash crop farms generally are lower in fertility because the farmers have to purchase all the nutrients that the soil (and, in the case of legumes, nitrogen fixation) won't supply. We rarely see the sky-high phosphorus and potassium levels on crop farms that we find in fields lying close to livestock barns. Since it's your hard-earned dollars that will be purchasing these nutrients, it's essential that fertilization plans be made on a factual basis instead of "by guess and by gosh."

Potassium is a key nutrient for forage crops. Corn silage removes much more potassium than does grain corn, for an obvious reason: Harvesting corn for grain only removes the kernels while corn silage harvest removes all but the bottom 6 inches or so of the plant. Therefore, corn grain farmers don't have to apply as much potassium and other nutrients as corn silage producers do. Fortunately, most producers of corn silage have an ace in the hole: livestock manure. Potassium doesn't leach (except in very sandy, low-organic-matter soils), and it doesn't volatilize, so to some extent you can "bank" this nutrient in the soil, reducing potassium applications for a year when money is tight or fertilizer is unusually expensive. That's what many farmers did several years ago when muriate of potash skyrocketed to $1,000 per ton: They voted with their wallets, reducing or eliminating potash purchases for a year. This is a wise move only if you have adequate soil potassium levels.

Adequate potassium fertilization is important, but there's no nutrient that will affect yield and quality more than nitrogen. This nutrient considerably increases the yield of corn and forage grasses, and also increases the crude protein content. Forage legumes, including alfalfa and clover, make their own nitrogen (N) through nitrogen fixation, so they don't need N fertilizer. Soils don't store nitrogen to any significant extent, so you'll need to supply this nutrient each year by a combination of fertilizer, manure and plowed-down forages, the latter a practice sometimes called "green manuring." Reduce N rates and you'll reduce both yield and forage quality.

Like potassium, phosphorus (P) doesn't volatilize and it doesn't leach unless soil P becomes excessively high. Phosphorus is particularly important during germination and early growth. That's why most agronomists recommend band placement of this nutrient - to get it close to the germinating seed. Plants don't use nearly as much phosphorus as they do potassium or nitrogen, so the status of this nutrient in the soil doesn't change quickly. Once phosphorus levels are high in a field they may remain that way for many years with minimal additions. That's why soil analysis is so important as a determinant of soil fertility status.

Getting the most out of manure

How you use livestock manure depends to some extent on its form. Slurry and liquid manure, typically 10 percent or less dry matter, can be topdressed onto grass hayfields and older legume-grass fields. Solid manure and manure containing a considerable amount of bedding often need to be plowed down or otherwise incorporated into the soil. As I've mentioned before, the nutrients in manure are just about as plant-available as are the nutrients in commercial fertilizers and contain a wide range of secondary and minor nutrients. Dairy farmers have increasingly changed from daily spreading to storing and applying manure as a slurry or liquid. This has resulted in a lot more manure topdressed onto alfalfa-grass and grass, reducing the need for commercial fertilizers on these crops.

As you move into your manure storages this spring, consult your farm's soil analyses to determine which fields are lowest in fertility. Apply higher rates of manure to fields with lower fertility, even if they're a few miles further from the farmstead. Diesel fuel is expensive, but when you look at it on a per-acre basis, the cost of fuel hasn't increased nearly as much as the cost of fertilizer.

Parting shot

If your soil analyses are more than three years old, one of the biggest bangs for your buck is a fresh set of soil analyses. In fact, livestock farmers can often justify annual soil analyses on fields that they manure heavily. Why spend money on fertilizer when manure applications have already increased soil fertility to optimum levels?

Ev Thomas has worked as an agronomist in New York for 45 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has a written our Forage column for over 13 years and has been an expert contributor on a number of other topics.