Ben Hill County Ag

Cotton Fertility

Fertility (Glenn Harris)

Replacing Nutrients Leached by May Rains
It seems like every time we get a lot of rain I hear people say “well, I guess I lost all my
fertilizer”. While nutrient leaching (nutrients dissolved in water moving downward out of
the root zone of plants) is a legitimate concern, especially on our sandy Coastal Plain soils
of South Georgia, this statement is not exactly true. First, not all fertilizer nutrients are
mobile in soil. Phosphorous for example is usually considered immobile and most
positively charged elements or cations, like calcium and magnesium adsorb to the cation
exchange capacity of the soil and do not leach readily. Most micronutrients are held by
organic matter and/or pH and do not move. Therefore, nitrogen, sulfur and boron are the
most “mobile” in soil. Even then, they have to be in the right form, namely the negatively
charged nitrate, sulfate or borate forms. By the way, this is why most soil testing labs like
UGA do not routinely test for nitrogen, sulfur and boron in soils. They are considered
“transient”, i.e. they can be there one day and (after a big rain) not be there the next. Oh,
and what about potassium? Potassium is more mobile than phosphorous but contrary to
some current thinking, it is NOT as mobile as nitrogen, sulfur and boron.

So the next question is “how much fertilizer do I need to put back”? There is no easy
answer to this question because it depends on which nutrient, which form of nutrient, how
much you put out, what soil type (i.e. how sandy) how much rain you got etc. But let’s for
example take the case of cotton fertilized in South Georgia before the heavy rains in May
this year. Hopefully most growers followed soil test recommendations and put about 30
pounds of nitrogen, 10 pounds of sulfur and the recommended P and K at planting. The P
didn’t move much at all, the K may have moved some but it is likely still where roots will
get it eventually. So that leaves N and S. Even if you lost half of your N and S you would
only have to replace 15 pounds of N and 5 pounds of S. This can easily be done at N
sidedressing time between first square and first bloom. Boron can be foliar fed any time
before first bloom. Our recommendation is for 0.5 lb B/a and can be tank mixed with
herbicide or growth regulator sprays. Bottom line is look to maybe sidedress on the earlier
side, replace about 10-20 lbs N/a and include S with your sidedress N.

Fertilizing Late or June Planted Cotton – Reduce N Rates
Due to the heavy late-May rains, a higher percentage of Georgia cotton is going to be
planted late, in June, this year. The tendency is to think “hey its late, I need to rush this
cotton so I am going to put higher rates of N out at planting”. This is actually the opposite
of what you should do! While it is always important to get off to a good start, if you get off
to TOO good of a start with extra N at planting, you could interfere with the
“vegetative/reproductive” balance and reduce yields. In other words, you want the plant to
shift from vegetative (“growing stalk”) to reproductive (flowering/fruiting) as quickly as
possible (as early as 5 nodes) since there is not as much time to flower and put on fruit
before frost.

So how much do I reduce my N rate by and when? On page 76 of the UGA Cotton Production
guide, it is recommended to reduce your total N rate by 25-30% . It is not stated, but I would
recommend taking some off of both preplant and sidedress applications if possible. So
instead of roughly 30 lb N/a at planting and 70 at sidedress for May planted cotton for a total
of 100 lb N/a….consider 20 lb N/a at planting and 55 lb N/a sidedress for June planted cotton.
If you put out 30 lb N/a in early May before the rains and don’t plant until June, you still
should have about 10-20 lb N/a available so could just plan on an early N sidedress.