Daniel Kaiser and Fabian Fernandez
University of Minnesota Soil Fertility Specialists
Over the winter we have done intensive data compilation and analysis and have a few updates to the corn guidelines publication. The primary update is on nitrogen application rates for corn following corn and corn following soybean. The updated publication is not finished yet, so this article will serve as the current rate guidelines starting spring of 2016.
The current guidelines still use the maximum return to nitrogen approach. The updated nitrogen guidelines reflect additional sites of nitrogen response data collected starting in 2011 through 2015. At this time, the data have not been uploaded into the online corn N rate calculator as the site is currently undergoing a scheduled maintenance. A future article will be released when the new data are available. The guidelines in Table 1 are for corn grown on non-irrigated soils in southern Minnesota. For growers with irrigation, please refer to the publication:
Fertilizing Corn on Irrigated Soils
The primary adjustment in the new guidelines is an increase in nitrogen rates. For corn following soybean, the old rate for the 0.05 price ratio is now the rate for the 0.10. A major change in the guideline for corn following corn at the 0.05 price ratio which is greater than was used for past guidelines.
One question this winter has been why there is a 40 lb difference in the nitrogen rate between the 0.05 and 0.10 price ratio. That difference manifested itself with the addition of the most recent data. Many of our recent studies have shown that corn following corn will respond to nitrogen well up to 200 lbs of N or more but the efficiency of the N is very low. This means that the increase in yield produced per pound of N applied is very small. To put this in perspective, there is only a 1% difference in yield between the rates for the 0.05 and 0.10 price ratios for the corn following corn rates.
What about corn grown in Northwest Minnesota?
Right now we have a small database for corn in the Northwestern part of the state. The suggestions given in Table 1 consist of sites primarily south of I-94. At this time the suggestions given for the 0.10 price ratio should be adequate for corn grown in Northwest Minnesota. Most of our current data have shown that rates of 120 lb of N or less are adequate when corn follows soybean and 160 lb of N or less are adequate when corn follows wheat.
Is there anything new on the horizon for P and K?
At this time the primary adjustments to the upcoming guidelines revisions is the addition of new expected yield levels. Similar adjustments were already made in the publication mentioned earlier “Fertilizing Corn Grown on Irrigated soils ”
For those utilizing expected removal, a summary of nutrient removal values for corn in the upcoming revision of the publication are given in the above table. These values are a summary from thousands of values collected since 2008. There are some cautions on using removal values which will be highlighted in a future news release. In particular, what should be done if removal rates are used in years following record yield.
The sulfur guidelines for corn will be updated as well
There are two important considerations in the upcoming revision, which are included in the footnote of the table below.
First, no sulfur is currently suggested when corn follows soybean and when soil organic matter in the top six inches is greater than 4.0%. We are currently focusing on research trials in Western Minnesota on poorly drained soils with high pH where corn has shown sulfur deficiencies even after sulfur is applied. A low rate of sulfur may be warranted in isolated field areas that have shown the typical striping of sulfur deficiency in the past. In these cases 10-15 lbs of broadcast sulfate-sulfur should be sufficient.
Second, a low rate of sulfur may be warranted for corn on corn situations with high crop residue regardless of the soil organic matter level. Under certain conditions sulfur has been shown to increase greenness of plants and reduce variability in corn emergence. A rate of 10 lbs of broadcast S per acre is suggested. This rate may be reduced if the sulfur is band applied away from the corn seed.
The suggestions are for broadcast sulfate-sulfur sources. Source of sulfur fertilizer can make a difference when correcting a deficiency. Sources like ammonium-, potassium-, or calcium sulfate contain more readily available sulfur that products that contain elemental sulfur. Elemental sulfur must be oxidized before it can be taken up by the plant. The oxidation process will likely not occur until the soil is sufficiently warm, typically in July and August. Thiosulfate liquids such as ammonium- and potassium thiosulfate should be used with caution as thiosulfate is half sulfate and half elemental sulfur. It is important to ensure that enough sulfate-sulfur is applied with ATS to satisfy the immediate needs of a corn crop early in the growing season.
Is there any new information on micro-nutrients?
Our recent research suggests that the main target for micro-nutrient application in corn is zinc. This is nothing new as we have known for many years that some soils test low in zinc and can benefit from a broadcast application of the fertilizer. Targeting low testing areas is important to achieve a positive return on investment with zinc. The previous statement also applies to chelated zinc in the starter fertilizer. Targeting fields with zinc soil tests less than 0.75 ppm (DTPA test) is critical to maximize the potential for a positive return.
There has been talk about other micro-nutrients such as boron and manganese for corn in Minnesota. The need for these nutrients, particularly on non-irrigated soils, is very low. There have been no confirmed manganese responses in corn in Minnesota. There have been reports of low soil and plant tissue tests for boron over the last few years. Similar to manganese, we have not seen a response to boron in any of the field trials over the past five year.
We are looking for on-farm testing sites focused on corn response to boron for 2016 where we will apply a single rate of boron in three strips across fields and use combine yield-monitor data to determine if there is any benefit of applying boron in corn and whether soil or plant tissue tests can be used to predict a response, should there be one in the field.
When money is tight focusing on the basics such as N, P, K, or S is important as these nutrients are more likely to be deficient than micro-nutrients. Knowing what nutrient is needed where is important to stretch the most out of your fertilizer dollar.
For any additional information please refer to the University of Minnesota Extension Nutrient Management website . Much of the work that went into modification of the current guidelines was funded through checkoff dollars from the Minnesota Corn and Minnesota Soybean Growers and the Minnesota Agricultural Fertilizer Research and Education Council.