Will this year's weather change next year's fertilizer program?

Monday, December 3, 2012

For the most part the dry weather in 2012 should not affect your fertilizer program for 2013, except for a few cases involving phosphorus and potassium

by KEITH REID


This summer, many parts of Ontario had extended periods of dry weather, which adversely affected crop yields. This led to questions about whether fertilizer programs should change next year. The answer, as with most things agronomic, is that "it depends . . ."  

It depends on several things – if nutrients are mobile or immobile, what the soil fertility levels are and on your fertilizing philosophy.

When crop yields are down, it is obvious that not as many nutrients have been taken out of the field. For next year, however, it will depend on whether those nutrients are still there when the crops need them.  

For example, if you had applied nitrogen fertilizer for a 180-bushel yield and harvested half of that, there would be nitrogen left over in the soil at the end of the season.  However, unless you did something to capture and hold that nitrogen, much of it will probably disappear before next spring.  It might leach down below the rooting zone in sandy soils or be lost to the atmosphere through de-nitrification in finer textured soils.  We seldom have conditions over the winter dry enough to keep nitrogen in the soil where the next crop can get at it.

As a side note, there won't be any increase in nitrogen left over after a legume crop like soybeans. Nitrogen fixation is affected by moisture stress even more than crop growth, so the growth of these crops was limited as much by lack of nitrogen as water.  

The picture is a little different for the immobile nutrients like phosphorus and potassium. Since these elements were not taken up by plants to the same extent as usual, there will be more left in the soil than would otherwise be the case. That may or may not mean that you should reduce your fertilizer rates for next year. Which side of the line you are on will depend on how fertile your soil is.

If you have fields with low levels of phosphorus or potassium, and you were fertilizing to get optimum use of the fertilizer this year, then the amount of additional nutrient left behind in the soil won't be enough to significantly change the soil test. Your fertilizer program should stay the same, although it may be worthwhile to pull another set of soil samples to check.

On the other hand, if you have a soil with high levels of fertility and you are fertilizing to keep the levels high, then you would be justified in reducing next year's rates so that you only replace what you have actually removed. There won't be any negative impact on crop yields, since you were keeping the soil in the range where it didn't respond to fertilizer anyway.

There is one concern with soil testing following a very dry summer, particularly in clay soils. As the soil dries, potassium can get trapped between the clay layers. This potassium will be released eventually once the soil gets wet again, but it may not show up in a soil test. Don't panic if your potassium test seems unusually low on samples taken during the dry weather last summer, but you may want to pull a second set in the spring just to be sure.  

So, to summarize, there will be a few cases where you can justify reducing phosphorus and potassium fertilizer rates, but for the most part the dry weather in 2012 will not affect your fertilizer program for 2013. BF

Keith Reid is manager (Eastern Canada), Soil Nutrient and GHG Management, Agriculture and Agri-Food Canada, Guelph.