Corn planting progress continues, with 23% of the Illinois crop planted by April 25, and planting continues with some rain delays the last week of April. The warm temperatures that finally arrived will be good for emergence of both corn and soybeans, but there are some reports of damage to early-planted soybeans from frost on April 21 and 22.
Warmer soil temperatures will start the mineralization process by which microbes release N from soil organic matter, although with soil temperatures only in the 50s now, this will process will remain slow. Warm rain will help raise soil temperatures and provide water to help move this process along as the crop emerges and gets established. Nitrogen from starter fertilizer or from broadcast UAN will help as well.
A great deal of nitrogen fertilizer has been applied, helped along by having soils fit to apply N while producers wait for it to warm up enough to improve planting conditions. We can’t rule out some loss of N from surface-applied UAN due to urea hydrolysis as surface soils have stayed dry, but cool temperatures have slowed this process.
Because the N rate calculator that we use to set N rates is sensitive to corn and N fertilizer prices, many are wondering if rate decisions made last fall or early this spring should be revisited. Those who still need to make the last application of N needn’t revisit this, since the total rate is set by the last application of the season, and prices at the time of that application should be used to set the rate. All N fertilizer applied before the last application is sunk cost that can’t be changed.
The MRTN—the N rate predicted from previous N rate trials to be the amount needed to maximize profit from N fertilizer—changes considerably when the price of either corn (per bushel) or N (per pound of actual N) changes while the other doesn’t. But if the price of both change in the same direction and the ratio between the two doesn’t change, the MRTN doesn’t change. Both corn and N prices have risen in recent months, raising the question of how much change in N rates we should make for this year’s crop.
Let’s use the N rate calculator to look at how N rates responded to price changes since last fall. Setting last fall’s anhydrous ammonia price at $500 per ton and corn at $4.00, the MRTN for corn following soybean was 185 lb N/acre in northern Illinois and 192 lb N/acre in central Illinois. With current prices of $700 per ton for anhydrous ammonia and corn for fall delivery at $5.25, MRTN values are 181 lb N/acre in northern Illinois, 189 lb N/acre in central Illinois, and 209 lb N/acre in southern Illinois. These are pounds of N: multiply these numbers by 1.22 to give pounds of ammonia. Because the ratio of N price to corn price changed little, rates barely budged. If you have different prices to use, enter them into the calculator to find the MRTN. Corn following corn will have higher MRTN values.
Anhydrous ammonia is typically the lowest-priced N fertilizer, so produces the highest MRTN value if we keep the corn price fixed. As an example, using UAN 28 at $300 per ton and keeping corn at $5.25 per bushel produces MRTN rates that are 10 to 12 lb N/acre lower than those given above for anhydrous ammonia.
The MRTN rate is the total amount of N to apply, and it’s important to include all N applied prior to fixing the rate for the last application. That includes any N from MAP or DAP applied last fall or this spring, any N applied with the planter or as broadcast UAN used to apply herbicide, and any manure. Do not make a subtraction for corn following soybean, since the calculator already makes that adjustment. We don’t have evidence that having a legume cover crop (like hairy vetch or clover) before corn produces enough N or that the N becomes available to the corn crop early enough to cut fertilizer N rates. A grass cover crop like cereal rye or annual ryegrass might lower N availability, but if it’s killed a few weeks before planting it’s probably safe to use the MRTN rate without adjustment. If rye has made more growth before it is killed, moving to the upper end of the MRTN range provided by the calculator might be appropriate.
Does it really make sense to change N rates just because fertilizer or corn prices change, or when we change from a lower-cost to a higher-cost (per pound of N) fertilizer material? It helps that ammonia, which is the lowest-cost form, is also the best at staying in the soil after application compared to forms (like UAN) that contain some nitrate; urea is in-between. We also find that fertilizer N and corn prices tend to move in the same direction, as they have in recent months, although one usually begins to move (as the corn price did last fall) before the other does. This means that the N:corn price ratio is less volatile than the price of either N fertilizer or corn; as a result, the MRTN N rate doesn’t fluctuate greatly, at least once prices equilibrate.
The default setting in the N rate calculator is with prices that result in a ratio of 1:10—this ratio was chosen because the ratio really does tend to move towards 1:10. At that ratio, the last bushel of yield produced as N rate increases pays for 10 pounds of N; if that bushel can only pay for 5 pounds of N, we lose money. Although the weather can throw us a curve, we can depend on our productive Illinois soils to provide the rest of the N the crop needs for high yields.
We’ll continue to watch the crop and weather to see if any surprises come up, but relatively cool and dry soil conditions so far in 2021 has meant little N movement in soils. If we get normal rainfall and temperatures in May, there is little danger of having N move below the rooting depth by the time N uptake begins to accelerate in late May. Having mineralization kick in while N movement remains low will help set the corn crop up for a good start.