How to safely mine that extra fertilizer

Friday, December 5, 2014

You do not want to mine so much that you reduce yields. On the other hand, you don't want your fertilizer ending up in the Great Lakes. Some guidelines to help you strike the right balance

by PAT LYNCH

During the last three years of good commodity prices, some producers put on extra fertilizer because they knew that high commodity prices wouldn't last. If you were one of these, read on as to how to safely mine some of that fertilizer. If you did not put on extra fertilizer when commodity prices were high, there are things you can do to manage fertilizer dollars for 2015.

A three-year crop rotation of beans, wheat and corn removes about 150 pounds per acre of phosphorous (P2O5)and potassium (K2O). That is the average and there are differences. There is nothing magic about fertilizer products or sources making nutrients more available. You remove that amount of "P" and "K" over three years.

The rules for mining soils are a bit foggy. It is safe to mine soils if "P" levels are in the high 20s and K levels are over 140. There are some who disagree with these numbers, but they are the ones I use. At the other end, if P levels are in the single digits or K levels are below 100, the minimum you must apply is removal rates or you will sacrifice yield.

So that leaves you with the decision as to what to do if P levels are in the low to mid 20s and K levels are between 100 and 140. These levels represent a lot of Ontario's cash crop acres that do not get manure and have not been in tobacco for years.

Mining soils is an art. You do not want to mine so much that you reduce yields. It takes 25-50 pounds per acre of P2O5 to move the soil test by one point. (Lower numbers apply to lighter soils and higher numbers to heavier soils.) So if your soil test is 22, a three-year crop rotation will drop it five to 10 points unless you add more P. Thus, it gets into the teens pretty quickly.

With K, it takes eight to 12 pounds per acre of K2O to move the soil test one point. Thus a three-year crop rotation without the addition of K can drop a field from 120 to 105 quickly.

If you plan to mine a soil, you have to have a soil test. And really you need to have soil tests from the same field three and six years ago. A one-time test is good, but not as good as a series over several years, which will allow you to develop a trend line.

And now we come to the "sticky" part of mining soils. What do you do if you rent? There are agronomic issues as well as moral issues when it comes to mining rented land. You wouldn't remove trees or topsoil from rented land. Should you be removing nutrients even if the landlord will never know?

A couple of other points about fertility. You get the biggest return for the first dollar you spend on fertilizer. Each dollar after that returns less. You want to make sure the last dollar you spend is still giving you a return. Secondly, seed-placed fertilizer for corn and wheat is the best choice. And, third, all P fertilizer should be incorporated. Even if you are no-tilling, you should incorporate P fertilizer.

You spend money for P fertilizer. It is no good to you if it ends up in the Great Lakes. The P fertilizer entering the Great Lakes from Ontario farms cannot be ignored. If you want to look at the consequences of P fertilizer entering Lake Erie, go to http://coastwatch.glerl.noaa.gov/modis/modis.php?region=e&page=1  BF

Consulting agronomist Pat Lynch, CCA (ON), formerly worked with the Ontario agriculture ministry and with Cargill.