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Updates to the corn fertilizer guidelines

Daniel Kaiser and Fabian Fernandez
Extension Soil and Nutrient Management Specialists

A new update has been posted for the corn fertilizer guidelines in Minnesota. Throughout the fall we will be posting information regarding changes made to the guidelines as they pertain to nitrogen, phosphorus, potassium, and sulfur management for corn. For the first installment we will take the time to outline changes made to the nitrogen guidelines for the state of Minnesota.

A previous e-news was set out this spring outlining changes to the nitrogen rate guidelines for non-irrigated corn production. The revised publication was delayed due to questions that arose based on the suggested application rates of N for corn on corn. The new suggested rates are listed in Table 1 of the publication.

There is one noticeable difference in the new guidelines versus what was released this spring: the Maximum Return to Nitrogen (MRTN) suggested rate for corn following corn at the 0.05 price ratio. When we first updated the N rates this spring, the MRTN rate at the 0.05 price ratio for corn following corn was near 200 lbs N per acre. However, there were some issues with the guidelines as the acceptable range was greater than expected and did not make sense compared to the range for the other price ratios.

Why did this occur? That question can be answered if one understands how the MRTN guidelines are developed. The guidelines are developed using data collected from field trials across Minnesota. Our plan is to continue to add new sites over time to enlarge the database used to calculate the MRTN. When the database is small, data from a few sites can significantly affect the guidelines. In the case of this spring values there were two locations in the database that were affecting the size of the acceptable range. These were locations where the highest rate of N did not increase yield. When the two sites in question were omitted it significantly changed the results to what is most commonly observed or expected by our collective experience.

Would adding more data affect future guidelines? The advantage of the MRTN approach is that the guidelines can be adjusted to account for changes in N requirement as hybrids or other conditions evolve. For example, the most noticeable increase we have seen recently has been the increase in N requirement for corn following corn where the average MRTN value has slowly increased over time since around the year 2000. We will continue to update the guidelines as new data are collected, which may result in a gradual change in the suggested rate of N. While the recent change this spring increased N rates, a large increase is not expected to occur from new data. The large change from the last update of the corn fertilizer guidelines was simply a result of adding multiple years’ worth of data into the database all at once. The fact is that once a critical mass is achieved large changes in the values from new data tend to be attenuated, unless truly there is a large change in the amount of N required across many locations.

Should we be worried about increased N loss with higher rates of N applied? There always is the concern that increased application of N may increase the amount of N potentially available for loss. However, past research data have shown that the potential for N loss increases substantially once the optimal rate of N is exceeded. A recent four-year study from the long term drainage plots at the Southern Research and Outreach Center in Waseca showed no difference in nitrate concentration or load between 160 and 200 lb N/ac as spring-applied urea. The evidence available does not support the fact that an increase in the N rate to those suggested for the new guidelines will result in greater potential for N loss.

Are there any differences in the guidelines based on soil type? Included in the update are the guidelines for corn grown on irrigated sand and for non-irrigated sandy soils with low yield potential. The past publication also included guidelines for soils considered to be of medium productivity. The guidelines for medium productivity resulted in suggestions similar to the lower end of the acceptable MRTN range in Table 1. With the new update there were locations included in the N rate database that would be considered medium productivity. As in the past, these locations fell towards the low end of the MRTN range.

Are there soils that require more or less N than the MRTN range? The answer to that question is likely no. There may be soils out there that may be marginal or highly productive that may need more or less N. The guidelines themselves serve as a starting point and represent a rate that should provide a profitable return to N most of the time. The Corn N Rate Calculator has an option to display a distribution of percent of sites in the entire Minnesota database within a given economic optimum N rate.

If I farm in Northern Minnesota are these rates accurate for me? There is an ongoing question as to the validity of the rates listed in Table 1 for northern Minnesota. Research on optimum N rate for corn in northern Minnesota was conducted in the late 2000’s. The data shows that there may be slightly less N needed, particularly in the Red River Valley, when corn follows similar crops in the southern Minnesota corn growing regions. The rate used in Northern Minnesota should not exceed what is suggested in Table 1. Our most current data for northern Minnesota would place the suggested rates towards the lower end of the acceptable range in Table 1.

Are there any distinctions between source, rate, and timing in the new guidelines? The guidelines themselves assume that best management practices are being followed. The rates themselves are independent of the aforementioned factors but should be taken into consideration to prevent the loss of N.

Will there be any print copies available of the new fertilizer guidelines for corn? At this time the guidelines will be on-line only. They can be found at:

The University of Minnesota Nutrient Management website - Fertilizing corn in Minnesota

With new data coming in this fall we are not anticipating printing copies until we incorporate the new data into the N rate database and determine what –if any changes—may be necessary to the suggestions in Table 1. By maintaining an on-line set of guidelines we can easily change and adapt the guidelines as new data is included.

A second resource is available for use, the corn N rate calculator

This calculator contains the most up to date information regarding suggested N guidelines for corn and can be adapted for various cost of N/value of corn price ratios.

Are there any other changes to be aware of? The other change made to the new guidelines is for when corn follows alfalfa. Past guidelines have suggested a N credit be taken for corn when it follows alfalfa, but new research indicates that this credit is highly variable and depends on several factors. A new table is included in the guidelines which outlines some of these scenarios and suggested rates of N to apply. This information was drawn from the publication:

Managing the rotation from alfalfa to corn

As with any N guidelines the amount of N needed can vary by year so in years with adverse weather conditions, utilizing all information available to adjust the rate of N is warranted weather corn is grown following alfalfa, corn, or soybean. In the next e-news release the data regarding P application will highlight changes in suggested P fertilizer rates for corn.

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  1. Could you explain the N Price to crop value ratio - Do you mean the price of N total to be applied to the total value of the crop , or a pound of N to a bushel price of corn ? Thanks - sorry - I just need help !

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  3. The price ratio is calculated by taking the cost per lb of N divided by the estimated value of the corn in $/bu. So if corn is $4.00 per bushel and N cost is $0.40/lb N then the ratio would be $4.00/$0.40 which gives a price ratio of 0.10.


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