Most questions I am getting on cotton right now are about side dressing nitrogen. The two most common are:
- How much do I apply?
- What source of nitrogen do I use?
Let’s just say that an analysis of N-P-K of 30-40-60 was applied at planting (because the soil test called for 40 lbs of P and 60 lbs of K).
Since it’s most prevalent in our area, the Wiregrass, let’s also use a soil group 1 for this example.
A farmer wants to put out a total of 90 lbs of N for a soil group 1. If he applied 30 lbs of N at planting, then he are needs to add an additional 60 lbs. I am getting this question with the concern of having lost the 30 lbs of N put out at planting due to the excessive rainfall early.
Our cotton specialist for the Wiregrass, William Birdsong, has recommended adding 15-20 lbs of nitrogen to the 60 lbs if the cotton is looking yellowish or the leaf samples show a deficiency of nitrogen. In that case go with a total of 75-80 lbs of N at side dress.
Using poultry litter at planting will probably eliminate the need for the extra 15-20 lbs of N. I have been seeing a lot of acreage needing the side dress nitrogen sooner than years past, indicating that we have lost some of the nitrogen from planting.
This leaves the question of what source of nitrogen to use? We have several sources, including ammonium nitrate, ammonium sulfate, urea, 18-0-0-3, 28-0-0-5, 32% and so on.
Be careful, though, that you’re comparing apples to apples. Don’t think in terms of dollars per ton. Instead, dollars per pound of nitrogen.
No matter if its liquid or dry, you can compare it in dollars per pound of nitrogen. For example: 30-0-0-5 is costing $450 per ton ($450/2000/0.3) or $0.75 per lb of nitrogen with sulfur and 18-0-0-3 is costing $220 per ton (220/[0.18*2000]) or $0.61 per lb of N with sulfur. The wise choice would be to go with the liquid if the farmer is set up to apply liquid.
Confusing? Don’t worry, there’s an app for that. A nitrogen price calculator from the University of Wisconsin. It will do the work for you so you can save money.