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Nitrogen (N) fertilizer is one of the most costly and important inputs in corn production. If not enough N is applied, yield can be reduced, but if too much is added, the excess can be lost to groundwater or to the atmosphere. Therefore, accurate N rate recommendations are economically and environmentally important.
For many years, N recommendations in the Midwest were based on yield goals; however, several Corn Belt universities have recently adopted a substantially different recommendation system developed from recent N trial results and the relative cost of grain and N. The new approach is detailed in Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn (PM 2015-April 2006). Even as we accumulate the needed data to consider a similar change in Indiana, it is worth examining the Purdue Agronomy’s history of N recommendations.
AY-50 a based its recommendations on soil color and moisture holding capacity, stand, growth, quality of the preceding legume, years after plowing down the legume, and manure application (Table A). The highest N recommendation was up to 120 pounds of N per acre on light-colored soils after a grass sod or 3 or more years after growing a legume. Recommended N rates decreased the more recent and better performing the legume, falling to 0 pounds of N per acre for the 1st year of corn after alfalfa or clover. The document did not mention soybean as a rotation crop for corn.
Nitrogen recommendations in AY-50 a for light colored soils were slightly higher than those for dark colored soils, presumably in recognition of the inherently higher N supplying capacity of the darker soils. Droughty soils had even lower recommended N rates than light colored soils, perhaps due to lower anticipated yields. The importance of applying N fertilizer in the row, particularly on poorly drained soils, was notable. Recommended ‘starter’ rates were not clearly stated, however.
In 1962, S.A. Barber and R.K. Stiver’s Fertilizing Farm Fields in Indiana (Extension Circular 474), had recommendations structured similar to those in AY-50 a, but averaged about 20 pounds of N per acre higher (Table B). The heading on a picture depicting the results of a N rate trial reads, “Nitrogen stimulates corn yields greatly where corn is grown following corn”.
In 1962, a bushel of corn bought 20 pounds of N. Nitrogen from anhydrous was 5¢ a pound, corn was $1.08 a bushel, and the average Indiana corn yield was 82 bushels per acre.
In 1968, the first recommendations specifically based on yield were published in Marvin W. Phillips’ and Gary M. Lessman’s Corn Fertilization (Purdue Extension publication AY-171). The authors said that long-term continuous corn experiments on several Corn Belt soils found that these soils supplied about 40 pounds of N per acre per year.
“Since corn requires 150 to 300 lb. N/acre to produce a yield of 120 to 200 bu./acre, most of the nitrogen has to be supplied as fertilizer unless a previous legume crop has built up the soil’s nitrogen supply,” they wrote. Their N recommendations were based on the difference between crop N requirements at different yields and an estimate of N derived from soil and residue based on the previous crop (Table C). Previous crop scenarios affecting the recommendations were similar to those from 1953, although corn following soybeans was added. Surprisingly, the N rate recommended for corn after soybeans was 20 pounds per acre greater than that for continuous corn when the desired corn yield had been attained.
Some emphasis was given to N fertilizer sources, timing, and placement in 1968 even though N costs were relatively inexpensive at this time, just 5¢ per pound of N from anhydrous compared to $1.05 per bushel of corn. The 1968 average corn yield in Indiana was 88 bushels per acre, somewhat below the lowest category of recommendation, 100 bushels per acre. An estimated 112 lbs. of N per acre was applied to corn in Indiana at this time.
The admonition, “Do not apply nitrogen in excess of what the corn crop can utilize,” and reduced N recommendations for yield goals greater than 150 bushels per acre were the most significant additions to Clifford D. Spies’ 1972 version of Corn Fertilization (Purdue Extension publication AY-171). The most significant change in nitrogen recommendations was a 10 to 30 lb N/acre decrease in recommended rates for yield goals >150 bu/acre and the removal of the ‘+’ from the highest category (Table D).
Nitrogen was still 5¢ per pound so there was little compelling economic incentive to manage N more efficiently. A bushel of grain, which sold for $1.56, could buy 30 pounds of N. The average 1972 corn yield in Indiana was 104 bushels per acre, and an average acre of Indiana corn received 126 pounds of N per acre.
In 1981, Spies and David B. Mengel’s Corn Fertilization in Indiana (Purdue Extension publication AY-171), reveals a rising awareness of nitrate-N’s adverse impact on water resources. Recommended rates were nearly identical to the 1972 version and based on the idea that Corn Belt soils only provided 40 pounds of N per acre, enough to produce only 30-40 bushels of grain per acre. More emphasis was placed on the comparative efficiencies among fall, spring, and sidedress N applications than in earlier recommendations. Fall-applied N was still considered appropriate, but it was suggested that application rates for fall-applied N be 5-10% more than spring-applied N. Adding an inhibitor to fall-applied anhydrous was recommended to enable 10-14 days earlier application - when soil temperatures were <55 °F. Spring preplant N application was considered suitable for all but the sandiest Indiana soils when conventionally tilled.
By this time, a bushel of corn paid for only 16 pounds of N, as anhydrous-N increased sharply to 15¢ per pound and corn sold for $2.47 a bushel. An average acre of corn in Indiana received 146 pounds of N per acre and the average yield was108 bushels per acre.
The most recent N recommendations for Indiana, Ohio, and Michigan were published in 1995 in Purdue Extension Publication AY-9-32-W, Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat & Alfalfa, edited by M.L. Vitosh (Michigan State University), J.W. Johnson (The Ohio State University), and D.B. Mengel (Purdue) (Table E). In this tri-state publication specific N recommendations were made for different N fertilizers and application placements and timings, as well as for distinct soil types and tillage systems. Many of the recommendations were illustrated with results from research trials as explanation and justification. Concerns for profitability, groundwater quality, and conservation of energy were cited as reasons for the detail. The biology and chemistry of nitrogen loss mechanisms were presented as background for understanding the differences in nitrogen efficiency resulting from differences in placement, timing, and form.
Considerable importance was placed on urea hydrolysis and volatilization of ammonia from surface-applied urea-containing fertilizers. This emphasis likely arose because the application of urea-containing fertilizers had recently become more prevalent and were expected to become the dominant nitrogen sources in the future. Also, contemporary measurements of ammonia volatilization found losses could be substantial and therefore greatly reduce corn yield.
Perhaps the most notable contribution of the tri-state publication to Indiana N rate recommendations was the use of grain yield potential and N credits to calculate a continuous N rate recommendation. The equation was:
N recommendation (pounds per acre) = -27 + [1.36 x yield potential (bushels per acre)] – N credit.
Nitrogen credits of 30 pounds of N per acre for corn following soybeans and 0 pounds of N per acre for corn after corn are the most recognized and utilized credits. Other credits were based on plowing down a perennial legume and were much like those used in the 1953 N recommendations.
In 1995, a bushel of corn bought more than 15 pounds of N, with anhydrous-N at around 20¢ a pound and corn selling for $3.38 a bushel. The average Indiana corn yield was 113 bushels per acre. An average acre of corn in Indiana received 132 pounds of N per acre.
Between 2000 and 2006, a bushel of corn grain bought about 10 pounds of N, a return to relative prices last seen in 1960 (Figure 1). Farmers have become more interested in using N efficiently, especially since early 2006, when forecast grain prices of $2 a bushel meant that a bushel would only buy 7 pounds of N as anhydrous ammonia or 4 pounds of N as 28% urea-ammonium nitrate solution. Although grain prices rebounded in 2006 and 2007 to more than $3.00 a bushel, N is still relatively expensive by historical standards.
Average yield from 2000-2006 was 150 bushels per acre with an average N application rate of 148 pounds of N per acre.
In 2006, more than 30 N rate trials were conducted with cooperation of the Purdue farms, farmers, and the agriculture industry. Many more are under way this year. After next season, this new database of trial data will be used to evaluate N rate recommendations, perhaps resulting in yet another system for making N rate recommendations for corn in Indiana.
Thanks to Kevin Smith, Kelly Delp, and Brad Joern for editing and formatting assistance.