The dictionary defines “dynamic” as “ characterized by continuous change, activity or progress.” This is also a good definition of the crop nutrient situation on dairy farms in the Northeast. Soil analysis summaries show that soil fertility in this region is generally trending upward. That’s because farmers are applying more nutrients as manure and fertilizer than they’re removing as crops (or are being lost through pollution). Therefore, the amount of fertilizer being purchased by farmers in the Northeast should be decreasing—and fortunately, for both the environment and farmers’ bank balances, it is.
Going up ...
A good case in point is phosphorus. This nutrient isn’t volatile, as is the ammonia form of nitrogen, and unless soil P levels are excessively high, it doesn’t leach to any significant extent. Unless physically removed by water or wind erosion, phosphorus stays where it’s put. So, phosphorus doesn’t usually go down or up, but it can go sideways! What is going up is average soil test P value, especially on dairy farms. Farmers are responding to this increase in soil test P by limiting fertilizer phosphorus applications. Cornell University has a lot of data on the influence of reducing starter P applications to corn, and, as expected, the results show that if soil test P is high, there’s little or no response to starter P. Even with these changes, it’s likely that soil test P levels will continue to increase on dairy farms since most P comes onto dairy farms as feed, and therefore, manure is the primary source of phosphorus on cropland.
It’s difficult to generalize, but if you operate a dairy farm and haven’t significantly changed your phosphorus fertilization practices in 20 years or more, chances are you weren’t applying enough fertilizer P then, or you’re applying too much now. Most nutrients that are mobile in the soil are also mobile in the plant. Plants often take up more mobile nutrients than they need, including nitrogen and potassium. Nutrients that are immobile in the soil, including phosphorus, are also immobile in the plant. Plants only use as much immobile nutrients as they need. Therefore, applying much more P (as fertilizer or manure) than the crop needs doesn’t result in plants with a high phosphorus concentration. In fact, it’s almost impossible to tell the phosphorus status of a soil by looking at the concentration of P in the plant.
Going down ...
Then there’s sulfur. It’s external forces that are affecting sulfur status on dairy farms—and on all other farms, at least in the Northeast and probably elsewhere as well. Sulfur is an essential nutrient, and while it’s considered a “secondary” nutrient, some crops use a lot of it. In fact, alfalfa often uses more sulfur than it does phosphorus. Before the industrial era of the United States, it’s likely that many of our soils were low in sulfur. Then came the Industrial Revolution, which included the burning of coal for heat and power. Much of this coal was high in sulfur, and atmospheric sulfur depositions increased, eventually to worrisome levels. Sulfur is acidic, and acid rain has been blamed for the near-sterilization of some lakes and ponds in the Northeast.
This caused the Environmental Protection Agency (EPA) to begin regulating industrial emissions, and over time these programs were effective. Atmospheric depositions of sulfur started to decline, and are still declining. Now there are regulations on the sulfur concentration of diesel fuel, so we’ll probably see a continuation of this decline. Atmospheric sulfur depositions are carefully monitored by the government, and recent data suggests that crops that had more than enough sulfur 20 years ago may not be getting enough now. There’s confirmation to this, since crop responses to sulfur fertilization are starting to appear. Unlike phosphorus, sulfur readily leaches through the soil, so there’s not much stored for future use. Therefore, where it’s needed, sulfur fertilization should be, like nitrogen, an annual practice.
Alfalfa is generally considered to be one of the more responsive crops to sulfur. I participated in two years of Cornell University-sponsored research about 30 years ago that evaluated sulfur fertilization of alfalfa. Sulfur didn’t increase alfalfa yields at all, either at Miner Institute (where our trial was located) or at any of several other sites around New York state. However, that was then and this is now. This year, Cornell is doing another series of sulfur trials on alfalfa. Will we see the same results? I wouldn’t be surprised if there’s a yield response to sulfur fertilization, and perhaps a quality improvement as well. That’s what’s happening in the Midwest, where sulfur is increasingly resulting in increased field crop yields. Ten years ago, we applied ammonium sulfate versus ammonium nitrate to three cool-season grasses: timothy, reed canarygrass and orchardgrass. Using the same rate of nitrogen, ammonium sulfate increased grass yield of all species by an average of 17 percent—a statistically significant response. Not surprisingly, the grasses fertilized with ammonium sulfate also had higher sulfur concentrations.
Does this mean you should start fertilizing with sulfur in 2009? Not necessarily. I’d suggest waiting until we get some current yield and quality data from the Cornell University research. Quality data might be as important as yield, because there’s a relationship between sulfur status and protein production in plants. Ammonium sulfate is slightly more acidifying in the soil than other forms of N. It’s not as acidic as we once thought, however, and I certainly wouldn’t let this influence your sulfur fertilization decisions. We should be monitoring soil pH anyway, and the added cost of lime from using ammonium sulfate versus urea or another N source is pretty small. We’ve been using ammonium sulfate at Miner Institute for a number of years in our nitrogen-only starter fertilizer on high-fertility fields, and sometimes as an ammonium sulfate-urea blend for topdressing second-cut forage grasses.
Ev Thomas has worked as an agronomist in New York for 42 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 Forages column for over 10 years and has been an expert contributor on a number of other topics.