Fertility in the Modern Crop Growing Environment

North Middlesex Seed Solutions – Ryan Kennes – CCA-ON

Over the first few years of my career in agronomy I have attended many presentations and discussions on soil fertility in Ontario, and a specific topic that usually comes up at these talks is the downward trend of soil fertility.  This is due to the fact that today’s modern crops are pushed to yield more and more, and in effect mines the soil for nutrients much more than crops of the past.  Unfortunately, as the soil fertility test results from various labs trend downwards through the years, some farmers are still fertilizing the way they did 10 or 20 years ago – which for the most part is not enough to maintain good soil levels anymore, resulting in this downward trend. When you get soil test results back, what do you look for?  The basics I look at first are pH, Phosphorus (P) and Potassium (K) levels, second I look at Magnesium (Mg), and third I might look at Zinc (Zn) and Manganese (Mn).   What to the test numbers mean for these nutrients, what is good and what is bad?  Below I list general guidelines for the most common macro and micronutrient soil test analysis: pH – Once you get down below 5.5 pH I start to recommend liming.  Usually sandy soils only. Phosphorus – In Ontario, most labs test soil P using the Sodium Bicarb method.  For this method, I like to see a minimum 20 ppm, but ideally 25 ppm.     Potassium – I like to see 150 ppm, but minimum you should have 120 ppm. Magnesium – Same as Potassium (120-150 ppm).  However, also want to see it higher than the potassium level because potassium and magnesium interact in the soil solution in that a higher Potassium:Magnesium ratio can cause improper uptake of these two nutrients.  Usually only a problem in high sand content soils. Zinc – Over 1 ppm. Manganese – Over 10-15 ppm.  Usually low in sand soils or high organic matter soils (mucks).  Plan to amend this with a foliar application; it is not usually economical to amend with dry fertilizer. There are many other nutrients essential to crop development, however the above nutrients are most commonly found in Ontario.  What Macronutrients a Crop Will Remove: Crop Corn Soybean Wheat Total 3 yr rotation Yield 200 bu 60 bu 100 bu Phosphorus-P removal 75 lb/ac 50 lb/ac 50 lb/ac 175 lb/ac Potassium-K removal 50 lb/ac 75 lb/ac 30 lb/ac 155 lb/ac   In the past, a lot of farmers might have been able to get all of their fertilizer needs on in one year with, say, the corn crop, saving passes over the field.  These days, however, the ability to get it all on in one shot is difficult for a variety of reasons, from a logistical/equipment standpoint to a crop safety standpoint.   The challenges of high fertilization Because of higher nutrient removal rates, some farmers have responded with higher fertilization rates.  They are fertilizing and trying to re-build their soil, but sometimes over-applying the more toxic fertilizers, urea in combination with potash, can pose the possibility of causing crop injury.  Morris Sagriff, our agronomist at Dupont Pioneer, has seen an increased occurrence of seedling burn and injury on sand-type soils as a result of this phenomenon.  He explains that in some cases it simply stunts the plant for a period of time, but never fully recovers, shaving bushels off the final yield.  There are safe limits to what you can apply in one shot, depending on the crop type, method of application, soil type, and the blend of fertilizer.  In particular, too much urea and potash together are more toxic than other fertilizers.  Corn: On sands and sand loams you must watch how much total Urea + Potash you might be broadcasting at one time prior to planting the corn crop, particularly if you do not get a rain after broadcast to dilute the nutrients into the soil profile.  In some cases, it might not be safe to broadcast over 230-250 lb/ac total N + K for risk of seed burn on sandy soils.  Loams and Clay Loams on the other hand have better nutrient absorptive capability, thus can handle it better. As you are probably more familiar with, there are also limits to how much you can put down in a 2x2 starter band, and this goes for all soil types, not just sand!  I would be hesitant to put more than 55 lb/ac actual N + K (if the N source is urea, there are higher limits if it is a different source).  In order to avoid these issues, my best recommendation would be to apply the majority of potash either in the fall or ahead of any other crop – soys, wheat, or edible bean crop.  Or, better yet, split-apply your corn N needs with a side-dress application. Soys, Edibles, and Wheat: Soybean and edibles are safe in terms of broadcast because you are not spreading urea, or in the case of edibles, not a lot of urea, and wheat you are not spreading the majority of urea at the time in which it could pose a threat to germinating seed.  You would need to spread an immense amount of potash alone to burn a seed in these scenarios.