When I first started working in the agriculture industry I spent my entire month of May blending fertilizer. At this point in time when a grower came in and asked for a “34-17-0” blend, I was a pretty happy guy. It meant effort and less chance of screwing up, win-win for me as an 18 year old trying to make a few bucks in my first summer job. Now a days when someone asks for that blend, I cringe.
Plants need numerous nutrients for proper health and development, some being required, others being beneficial. All of these nutrients are important, but macro nutrients tend to be the ones needed in the largest quantity. These nutrients include nitrogen, phosphorous, potassium and sulfur (and calcium and magnesium depending who you talk to). I want to focus on the importance of sulfur today.
Having a proper fertility package is one of the most important things when starting off your crop in the spring, but it is very often overlooked. I like to put it into terms of us as people, we have 3 essential macro nutrients being protein, carbohydrates and fat. If we were only to eat carbohydrates and proteins, we would begin to run into a whole world of problems due to malnutrition from lack of fat. This occurs with plants as well, even if we don’t see obvious deficiency symptoms, the plants are still weaker and losing yield because of it. Leipigs Law of the minimum shows us that a plant/crop can only produce as much its most limiting factor will allow it. If we only have sulfur in the soil for 30 bushels of wheat, you will be hard pressed to get any more out of that crop due to the lack of sulfur. Sulfur has some key benefits in plants, including being a component of amino acids and enzymes.
Canola is the first crop I would like to touch on why it is important. Many are aware of this due to its promotions by groups such as the Canola Council and many agronomists. What is often left out is why is canola such a heavy user of sulfur? Many growers never ask, and a lot of individuals that promote sulfur don’t really know the specifics behind why canola needs so much. This isnt going to get technical, just some basics and the basics come down to the plant family canola is a part of, Brassicaceae. This family of plant also includes mustards and even vegetables like cabbage and cauliflower. This family of plants has secondary metabolism compounds known as glucosinolates. These compounds are made up mainly of sulfur. The main reason these compounds are important is due to their ability to increase a plants defense system/stress tolerance against pathogens or insects. If we under fertilize sulfur on a canola crop, these compounds begin to be broken down by an enzyme so the plant can use the sulfur else where in the plant, meaning the plants defense system is taking a back seat to other basic physiological processes. This can open your crop up to increase disease or insect risk and increased susceptibility to stresses such as heat. Sulfur is also a component of phytoalexins which are part of the systemic acquired resistance of a plant, or part of its natural defense system There is more that sulfur does in canola as well, but this is the quick and dirty version. Remember, ratio’s of nitrogen to sulfur are important as well. 5:1 (N:S) being the range you want to be in simply because for every 5 lbs of N your crop is going to use 1 pound of S under ideal conditions to achieve the yield you want.
Wheat is often overlooked even more so than canola when it comes to sulfur fertilization. Again, understanding the importance of sulfur in wheat is important. I stated earlier that sulfur is important in amino acids within plants, amino acids are constitutes of proteins and usually a higher protein in wheat means a higher premium. The most common practice to increase protein in wheat is with a late N application, which is fine and consistently works. But since we know protein in a plant isnt made up of only nitrogen, but sulfur as well we can see that sulfur may be the magic trick to bumping yield a bit along with protein. Ratio’s for wheat of N:S are ideally in the 7:1 range.
Lastly, I want to note the importance of sulfur in pulses. All to often I run into growers who don’t fertilize their pulses at all, then there are a few that put down phosphorous, but consistently the few I deal with who put down a full, balanced nutrient package with their pulses have the highest pulse yields. Pulses are high in protein, and as just said with the cereal section, sulfur is a key component of proteins within the plant. The reason many individuals aren’t aware of is its role in activating enzymes that are important for nodulation and nitrogen fixation. Being able to more efficiently fix nitrogen, especially once plants get to flowering when pulses are really needing N and the plant isnt focusing as much on rooting or nodulation can be that extra few bushels of yield you are looking for.
To calculate a range of sulfur your crop will need here are some uptake numbers:
0.25lbs of sulfur/bushel of wheat
0.6lbs of sulfur/bushel of canola
0.25lbs of sulfur/bushel of peas
0.13lbs of sulfur/bushel of lentils
Simply put if you want a 100 bushel wheat crop, your crop is going to need about 25lbs of sulfur per acre (100 bushels * 0.25lbs/bushel/ac)
Remember to keep sulfur in mind when doing your fertility planning for next year.
Going to try and focus a little more on some nutrient importance over the winter when writing as I feel this is the area where many growers could get major yield bumps from. Properly balancing your macro's and then going in and touching on your micro's is a tactic that I think has the most influence on yield, especially in comparison to some like which seed treatment to use. Nutrients each have many roles in the plant, but I want to try and focus on key factors that a nutrient might have in a specific crop.
Note: Try and do a soil test to see where sulfur levels are in your fields and remember sulfur is very mobile in the soil so you may have 7lbs in one spot and 32lbs in another. Also, I am sure you have heard that sulfur comes down with rain and irrigation water, but it is still great practice to monitor levels and try sulfur on your farm if you havent already.
Source: Marschners Mineral Nutrition of Higher Plants