The vast majority of producers understand the need/importance of sulfur for optimal corn yields, and most are targeting a ratio of 10-12:1 for N:S. This means that for every 10-12 lb/ac of N applied to their corn crop they will aim to have 1 lb/ac of sulfur – so an N rate of 180 lb/ac would have 15-20 lb/ac of sulfur.
This seems to be the standard recommendation in Ontario, and while some data suggest the need for more or less, I think this rate should be adequate for most situations. For fields with very low (<2%) organic matter (OM) there may be a positive economic response to slightly higher sulfur rates (maybe an 8:1 ratio).
A rough rule of thumb is that for every 1% OM there is 100 lb/ac of S. The activity of soil microbes will release some of this sulfur (much the same as with N) but when the starting pool is low higher rates from ‘outside’ sources may be required (either fertilizer or organic sources – i.e. manure).
And while there is a good understanding around the need and rate of sulfur, there is some confusion (including my own) as when best to apply this nutrient.
As mentioned above, there is a tremendous pool of sulfur in the soil OM, but it typically isn’t released until later when the soil microbes are more active. This suggests the need for some sulfur fertility early. However, the plant-available form of sulfur is sulfate (SO4-), which is an anion and prone to leaching, which means early applications may be lost to heavy spring rains.
To address the above ‘riddle’ I did some reading with the goal of a better understanding when the corn crop needs sulfur.
An article from CropLife, which provided the sulfur uptake graph below, helped provide some clarity. The sulfur uptake graph shows that only a small amount of sulfur is needed when the crop is small and likely this demand can easily be met with just about any method of starter fertilizer – a bit of sulfur in a liquid pop- up, some sulfur in the mix for a 2x2 band (MESZ, ammonium sulfate), etc.
At about the V7-8 stage the demand for sulfur rapidly increases and continues until maturity. As mentioned above, soil OM can release sulfur later in the growing season, which suggests that the ‘crucial’ period to ensure adequate fertilizer-based sulfur is most likely during the rapid vegetative growth stage, after V7-8.
With this knowledge, below are a few of my thoughts on how sulfur could be incorporated into different fertilizer application strategies.
For growers who apply all of their fertilizer up-front, my suggestion would be to work at the higher end of the sulfur application range, to try and offset potential leaching losses. The sulfur can be fully applied in the starter band or with the bulk nitrogen (liquid or dry) or some combination. I wouldn’t recommend this strategy on low OM soils.
For growers who split their N applications and side-dress ‘early’ a small amount of sulfur will be needed in the starter program and likely most of the sulfur can be applied with the side-dress. I don’t see any need to push the sulfur rates in this scenario unless on low OM soils.
For growers who split their N and aim for ‘later’ N applications (Y-drops or late top-dress) there
is a good chance that the later N application will be after the crop has entered the ‘critical’ period for sulfur. So, in these situations, I think the majority of the sulfur should be applied upfront. If some additional sulfur will be applied with the later N application there likely isn’t any need to push the sulfur rate, but if the later N application won’t include any sulfur then I think the upfront rate should be on the higher side to offset any potential leaching losses.
As with all fertility recommendations, there are always many thoughts/ideas/strategies.
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