Fabian Fernandez Shares Tips for Making Every Fertilizer Dollar Count

He discusses conserving N, applying enough P and K, VRT, and testing for N.

Published on: Oct 2, 2008

Fabian Fernandez, University of Illinois Extension assistant professor soil fertility, recently discussed places farmers can save on fertilizer.

In the October issue of Prairie Farmer, Fernandez showed readers how important soil sampling is. Here he shares his opinions on maximizing soil fertility, without breaking the bank.

1. With N prices where they are, how can farmers ensure they get the most out of it next season?

I would start by using this nitrogen rate calculator, which is hosted on Iowa State University's Extension Web site. (http://extension.agron.iastate.edu/soilfertility/nrate.aspx)

By selecting your area, the cost of N, the price of corn, and whether the crop is following corn or soybeans, this calculator will help you determine the N rate at which maximum returns can be expected.

However, the calculator does not account for carryover nitrogen. This year, it's unlikely that a lot of N will carryover, even if yields were lower than expected. However, if the field was alfalfa the previous year, or has been fertilized with manure, it is important to account for that N. The nitrogen calculator already accounts for soybean N credits.

A way to account for leftover N is to conduct a pre-sidedress nitrate test. This test is especially useful for corn after beans, alfalfa, or manure applications.

Using SPAD readings or canopy sensing technology can help us manage the last 40-50 lb of N. For this technology to work well, you must calibrate readings by having a reference strip with a non-limiting rate of N. This is necessary because environmental conditions and different hybrids will result in different leaf color.

Minimize losses

if you do a fall application, do not apply N before the 3rd week of October in central Illinois, or the 2nd week of October in northern Illinois and wait until soil temperature is below 50 degrees F at a four-inch depth, or 55 degrees F if using a nitrification inhibitor.

The temperature is important because this is a bacteria-mediated process. Do not use urea- or nitrate-containing fertilizers. Two nitrification inhibitors currently on the market are nitrapyrin (Nserve) and dicyandiamide.

Regardless of the nitrogen product used, the further away from the time the crop will use it, there is a greater risk the nitrogen will be unavailable to the plant either because of immobilization (inorganic N into organic), denitrification (NO3 to N2 or N2O), or leaching (NO2 and NO3 can leach out).

When using urea products in spring, make sure it is incorporated by tillage or at least ½ inch of rain within two or three days of application. Use a urease inhibitor such as (N-(butyl) thiophosphoric triamide or NBPT (Agrotain), especially if the soil has a lot of crop residue cover. Do not apply in frozen soils.

For anhydrous ammonia, make sure the soils are not too wet or too dry. Also, make sure the application depth is enough to prevent N from escaping into the atmosphere. Application depth should be 8-10 inches for coarse-textured soils and 6-8 inches for fine-textured soils

Where feasible, using cover crops that result in a residue with C:N ratio of less than 25:1, such as clover, hairy vetch and rye with legume. This can be an effective way to scavenge unused N.

Also, remember to account for all N applications including those in MAP, DAP or manure applications.

2. What advice do you have for farmers who are thinking of skimping on P and K?

If test levels are above the critical level, you'll be o.k. cutting rates for one crop. For critical levels, I use 30, 40 and 45 lbs/acre of phosphorous for high, medium and low supplying power, respectively. I use 260 and 300 lbs/acre of potash for low and high CEC soils.

If levels are at 60, 65, 70 lbs/acre of P for high, medium, low or 360, 400 lbs/acre of K for low and high, do not apply. Corn removes 0.43 lb P2O5/bu and 0.28 lb K2O/bu. Soybeans remove 0.85 P2O5 and 1.3 K2O. If nothing else, apply removal rates when fertility is adequate.

If it is below the critical level, then applying some extra beyond removal is highly recommended. Typically applying in a band below and to the side of the seed can be beneficial especially when the full rate is not applied.

Also, think of the crop. If you plant wheat, you can probably get by without K application because wheat is not very responsive to K, unless test levels are very low. On the other hand, P is required in large amounts, and having adequate supply while the small plant is getting established is very important.

3. What rewards can farmers with a VRT program expect?

The prolonged wet- and cool-conditions in the spring made for a challenging planting season. Acres planted early that didn’t drown with the later rains are doing well. Other areas had to be re-planted or planted late.

Another challenge was the crusting of soil surfaces in some areas and the large variability in crop emergence. Many acres were "mudded in." This created compaction and variability in the growth of the crop.
Finally, topographic position in the field also made a large difference for crops this year. Crops planted in uplands and side slopes are generally looking better than those in low-lying areas. All these factors will undoubtedly contribute to significant yield variability and equally variable nutrient removal.

As a result of the large in-field variability, we can expect areas of the field where removal of P, K, and other nutrients is fairly typical. In other areas, where yields were lower, less nutrient removal will likely take place.

As for N, regardless of the yield, with the high rainfall amounts we had this year, it is unlikely that there will be much if any N carryover for the following crop. This is especially true for low areas of the field that were flooded and N was more susceptible to be leached or denitrified. Likewise, in coarse-textured (sandy) soils, there is a possibility that some K and sulfur may have been lost through leaching during the wetter-than-normal spring. Except for those very specific soils though, K (as well as P) that was not used by the crop in 2008 can be expected to be available next year.

Traditionally, fertilizer rates are established based on soil test information and the average yield across a field. This year, due to the variability in nutrient removal, the use of variable rate technology along with yield data and updated site specific soil sample information may be one of the best options to guide fertilizer applications and minimize overall fertilization costs for the 2009 cropping season.

An easy and practical way to calculate P and K removal from yield by the crop in a given area is by multiplying the yield monitor data (in bushels) by the amount of nutrient removed per bushel. Removal per bushel will be consistent regardless of how high or low the yield of the crop might have been.

Things to remember

In a variable rate situation that has been done over the years, you should still conduct soil tests to check nutrient levels and make sure you're applying an adequate rate. For instance, during application if something went wrong with the rate and went unnoticed, you won’t be able to correct soil test levels unless you know what those levels are.

Also, remember that stover contains nutrients. Even though the residue is typically spread evenly across the soil surface, displacement of stover by wind, or management can create unevenness in the way P and K are being re-distributed in the soil.

Also, there are processes in the soil that can affect nutrient availability (including pH levels that will affect nutrient availability). It is important to know about these changes to guide the variable rate application.

4. Is it a waste of money to test for N?

It depends. Currently, there isn't a soil test that accurately predicts the need for N fertilization. The reason is soils typically have a large reserve of N. Every year, part of that N becomes available through the process of mineralization. Mineralization is affected by decomposition of organic matter by soil bacteria and microorganisms. Their activity is very much linked to soil water content and temperature.

These factors are unpredictable and make it difficult to predict how much N will become available in a given year. To add to the uncertainty, we don’t know when that N will become available. If it becomes available too early in the spring due to favorable conditions, and then we get a very wet early season, we can expect N losses that wouldn't have occurred during a drier year.

Soil testing for N can be useful to manage the last portion of the total N application. Doing a pre-sidedress soil nitrate test in the spring to determine how much N is available can be useful, especially under conditions where substantial N carryover is suspected.