Post-harvest field fertility maintenance may feel like busy work for farmers worn out from hours in the combine. Instead, consider it next season’s foundation to higher yields. Begin with soil sampling and reevaluating field fertility. This year means balancing the decisions against high costs.
“If you have fields or field portions with higher than optimum phosphorus (P), potassium (K) or pH then don’t apply there and use variable-rate technology to apply only in the areas of the field that are low,” says Antonio Mallarino, nutrient management research and Extension specialist with Iowa State University.
The P or K rates to maintain desirable soil-test levels should be based on nutrient removal with harvest in the given year. The weather was a big part of that mix in 2021. Areas with less rain this season, such as in Iowa and North Dakota, may mean soils still have residual fertility.
“When you sample some of these wheat fields, they’re coming back 50 lb. to 80 lb. of nitrogen (N) higher than they normally would,” says Jason Hanson, of Rock and Roll Agronomy in Webster, N.D. “You don’t really want to put soybeans on that. So this upcoming year we might see more soybeans on soybeans, wheat on wheat or corn on wheat.”
On prices and availability, experts say everything is a guess from now into 2022.
“A month or two ago my advice was don’t buy your phosphate potash because I think there’s a chance we see values down by the springtime,” says Josh Linville the director of fertilizer with StoneX while on AgriTalk Radio explaining how higher prices could lead to demand destruction and ultimately lower prices. “Now with some of the actions being taken by China and other countries out there, if we have that bad fall, and we do go into the wintertime with very full supplies, I think U.S. producers will just say, fine, if North America doesn’t want it, we’ll export it.”
FACTOR IN WEATHER
Absent prices, on acres that do need fertility attention, Mallarino suggests making the investment. It may not be a year to build up the soil “nutrient bank”, but it should be maintained. P, K and lime make good fall applications.
“What we tell farmers in Iowa is that for both P and K, the placement is not an issue for yield (other than starter needed in some conditions) but could be for losses from the field,” says Mallarino. “I have data in Iowa where fall broadcast of P and K before soils freeze is actually less of a problem for water quality than if it’s done in the spring because in the spring there is a high likelihood of high rainfall intensity. So, the losses could actually be higher.”
That could be a different story in other states depending on when fields typically see their biggest seasonal rains.
FALL INPUT DECISIONS
Lime applications often cut or reduced in times of lower commodity prices, may be worthwhile in fall 2021.
“If you have not been applying lime or you have a pH of about 5.5 or less, then you urgently need to apply lime,” Mallarino says. “If lime costs are reasonable and you haven’t been applying lime then maybe this is a time to switch. For corn and soybean, optimum pH of 6.0 or 6.5 is recommended for different state soil associations.”
Sulfur(S) is a nutrient to consider seriously, he says, as they’ve recorded widespread deficiencies in Iowa soils. There is no reliable soil test for available S, so we and most states suggest applying mainly in soils with low organic matter or highly eroded where deficiency is most likely.
“There are many low-cost S sources (gypsum, binary fertilizer, or mixtures) that can be applied in the fall or spring, but elemental S (also a good source) needs to be applied in the fall because it takes time to be transformed by microorganisms into sulfate,” he says. “The beauty of S is that you don’t need as much, just anywhere from 25 lb. to 35 lb.”
On the N front, consider your local supply-and-demand situation.
“If you have the opportunity to put on anhydrous it is screaming for demand to go there,” says Linville. “It doesn’t look like we’ll be seeing a lot of relief through next April, May or June.”
“We just have to get it on because our retailers don’t have the capacity to store it overwinter and there are going to be charges to store it,” Hanson says.
Be it anhydrous, manure, or something else, Mallarino says a year’s worth of N doesn’t need to go down now in the fall to minimize nitrate losses.
“If you want to apply in the fall, apply a conservative rate and be ready to do some adjustments if needed in the spring, for which several diagnostic tools are available,” he says. FJ
