Fertility

Across 35 Minnesota locations (2014–18), polymer-coated urea had a yield advantage over urea 26% of the time for corn following corn and 28% for corn following soybean, and never reduced yields relative to urea even though most sites had high N-loss potential. Near Becker (2014–15) the advantage on sandy loam was 39 bu/ac; the 2021 drought year showed little or no difference.

ESN held at 59°F for 40 days released only 29% of its N. Both sources at 120 lb N/ac in spring, ESN out-yielded urea by 39 bu/ac on Becker sandy loam, equivalent to 200 lb N/ac of pre-plant urea, while the same treatments on fine-textured southern Minnesota soils showed no yield difference. ESN costs $0.16 to $0.20 more per pound.

Compared with uncoated urea, ESN in the pre-plant application gave a consistent corn yield increase and lower N loss; fall-applied ESN increased yield only about 25% of the time versus fall urea and, averaged over the study, yielded less than spring pre-plant urea. A seven-year southwest Minnesota study found split application no better than pre-plant.

On irrigated Plainfield loamy sand at Hancock, WI (2003–06), pre-plant ESN averaged 185 bu/ac and 44% fertilizer-N recovery against 159 bu/ac and 19% for pre-plant urea; split-sidedressed ammonium sulfate averaged 189 bu/ac. In the wet 2004 season pre-plant ESN yielded 167 against 141 for pre-plant urea and 196 for split ammonium sulfate.

A three-year Iowa study at two locations showed a corn yield advantage for ESN over urea in only one of nine site-years, and a 2013–14 Iowa study found no agronomic or N-use-efficiency benefit. A 2009–11 Nebraska study on irrigated coarse soil found ESN applied shortly after planting consistently better than UAN; Michigan blend-ratio trials (2011–13) showed few yield differences.

Regression across published studies put cumulative ammonia loss at 31.0% of applied N for urea and 14.8% for NBPT-treated urea, a 52% reduction; it took 4.8 and 8.3 days for half the total loss to occur for urea and treated urea. Average crop yield rose 5.3%, but gains were limited in coarse-textured soils and at NBPT rates above 1,060 mg/kg.

Summarizes 78 experiments in 17 states over 5 years: NBPT raised corn yield 4.3 bu/ac with surface urea and 1.6 with UAN across all sites, 6.6 and 2.7 at the 59 sites with significant ammonia loss, with yield reductions of 10 bu/ac or more at 7% of sites. At Hancock, WI (1988), 70 lb N as urea plus inhibitor yielded 135 vs 127 bu/ac; at 140 lb N there was no difference.

NBPT delays ammonia volatilization from surface or near-surface urea for about 10 days. A 14-year southern Illinois study showed a 3 bu/ac corn advantage for urea plus Agrotain over broadcast urea in conventional tillage, and in no-till the advantage was 11 bu/ac over four years.

Mean yields from 1989 trials at four Indiana locations: urea broadcast on the surface 130 bu/ac, urea plus NBPT 143; UAN broadcast 135, UAN plus NBPT 140; coulter-injected UAN 147; a 20 lb N starter-only control 99.

The company claims more than 1,000 trials over 25 years, up to 40 percent N loss to ammonia volatilization when urea or UAN is not incorporated by more than 0.5 inch of rain or more than 2 inches of mixing, and an average corn yield benefit of +20 bu/ac. Marketing figures without a trial list on the page.

An independent literature review found average corn yield gains of 3.3% (±1.5) with nitrapyrin, just over 2% with anhydrous ammonia and almost 6% with urea, and a 13.1% (±9.3, n = 18) reduction in yield-scaled nitrate loss with anhydrous.

In three Indiana site-years with spring-applied N, nitrapyrin improved N recovery efficiency but did not significantly change grain yield; the accompanying meta-analysis found positive yield effects that did not carry through to N-use efficiency. Benefits with spring N are not always realized.

Across 16 paired Illinois on-farm trials, fall anhydrous with N-Serve and spring-applied N produced virtually identical yields (235 vs 236 bu/ac), but the fall system needed about 12 lb/ac more N at the optimum, a $9/ac advantage for spring application.

Seven Minnesota study years: fall anhydrous plus N-Serve yielded 15 bu/ac more than fall anhydrous alone, while spring anhydrous yielded 27 bu/ac more. The encapsulated Instinct formulation showed no yield increase in Illinois, Iowa, Minnesota or Nebraska (2008–09); N-Serve gains depend on rain causing leaching or denitrification.

Pooling trials across many environments, nitrapyrin raised yield 7%, soil N retention 28%, and cut N leaching 16% and greenhouse-gas emissions 51%. These are the figures the manufacturer quotes; the author was employed by Dow AgroSciences.

Eighteen Iowa trials (2003–05) with 0 to 180 lb K2O/ac: K raised yield at seven sites, all testing Optimum, and at none testing High or Very High. Responsive soybean yield rose linearly to 106 lb K2O/ac and corn to 122. Iowa’s published response probabilities are 80% Very Low, 65% Low, 25% Optimum, 5% High, under 1% Very High.

Twelve soybean trials (2018–19) following corn that had 0 to 135 lb K2O/ac: a 45 lb K2O/ac fluid sidedress added 5 and 4 bu/ac (2018) and 4 to 6 bu/ac (2019) after 0 or 45 lb before corn, 2 to 3 bu/ac after 90 lb, and nothing significant after 135 lb or on high-testing plots. University research funded by industry, not company-authored.

From 1979 to 2018 on Kenyon loam starting at 213 ppm K, unfertilized soil-test K fell to about 100 ppm after 25 years and it took 17 years before P or K raised yield. The 144 lb K2O/ac annual rate did not affect corn but lowered soybean yield relative to 72 lb once soil-test K reached about 300 ppm.

Sets the critical soil-test K level at 75 + 2.5 × CEC ppm for all crops (88 ppm at CEC 5, 150 at CEC 30) and lists soybean removal at 1.40 lb K2O per bushel against 0.27 for corn grain. At CEC 10 and a 50 bu/ac yield potential, the soybean recommendation runs from 165 lb K2O/ac at 50 ppm to 90 lb in the maintenance range and 0 at 150 ppm.

Minnesota recommends 60 to 110 lb K2O/ac for a 60–69 bu/ac soybean yield goal at soil-test K of 0–150 ppm, 35 lb at 151–200 ppm and none above 200 ppm, says building soil-test K above 120 ppm is not recommended, and advises no more than 100 lb of KCl per acre directly on soybean because small yield reductions have followed higher rates.

At 100 Wisconsin on-farm sites over three years, most with excessively high soil-test P and K, a 2×2 starter averaging 15+26+32 lb N+P2O5+K2O/ac raised yield by about 4 bu/ac statewide each year; site responses ranged from −10 to +42 bu/ac and 40% of sites had a positive economic response, more often where soil K was below 140 ppm and with later planting plus longer-RM hybrids.

Eleven no-till Iowa strip trials: liquid N-P-K starter increased yield in seven fields and reduced it in one (−231 kg/ha). Gains were 200–671 kg/ha where Bray-1 P was below 16 mg/kg and 80–194 kg/ha with in-furrow starter on high-P areas; early growth rose 32% against a 2.4% yield response.

Thirteen Iowa on-farm trials over three years compared in-furrow 3-18-18 at 10 to 14 lb P2O5 and K2O/ac with broadcast P-K (100–110 lb P2O5, 120 lb K2O). Fertilization raised yield at five sites, with starter matching broadcast at four of them; early growth rose at 9 of 11 sampled sites.

Sixteen Iowa trials with soil-test P from 5 to 77 mg/kg: fertilization raised yield at nine sites (0.80 to 2.11 Mg/ha), starter alone yielded less than broadcast P-K at five sites (0.30 to 1.48 Mg/ha less, four of them low in P), and starter added to broadcast did not raise yield at any site.

Summarizes 31 Iowa trials: early growth responses to starter P-K (5 to 25 lb P2O5 and K2O/ac) were far larger than yield responses, which are seldom seen when the recommended pre-plant P rate is broadcast. The in-furrow rule of thumb is less than 10 to 12 lb of N plus K2O per acre.

Forty Illinois trials (2009–11) found a significant yield increase from S at only 2 of 40 sites, where gains were large (+14.7% and +30.5%), while 34 kg S/ha reduced yield at two fine-textured sites. The 1977–79 Illinois survey found 5 of 81 trials responsive. Soil sulfate and ear-leaf S tests did not predict response.

Twelve central-Iowa experiments comparing ammonium sulfate, thiosulfate and gypsum at 34 kg S/ha: only 17% of trials yielded more than the control even though S raised early-season plant S in half of them. Economic return averaged up to $27.6/ha at favorable prices.

In 55 field-scale strip trials (2017–22), sidedress S increased corn yield in 26 of 55; where it responded the gain ranged 3–34 bu/ac and averaged 11. Starter-only S had no effect in 8 of 9 trials, and a 2% organic matter loamy sand responded in 5 of 6 years, averaging 20 bu/ac.

Of about 150 Iowa corn trials, approximately 50% showed a statistically significant yield increase from S. Iowa research supports about 15 lb S/ac on fine-textured soils and 25 lb S/ac on coarse soils.

Wet sulfate deposition in the Midwest fell 72% between 2000–02 and 2020–22, from 5.3 to 1.5 kg S/ha per year: the reason free atmospheric sulfur no longer covers crop removal on many Corn Belt fields.