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By Clint Waltz
University of Georgia
All plants require nutrients to grow and develop properly.
Various nutrients affect plants differently and are needed in
varying amounts.
Essential nutrients are just that: essential. Plants have to have
them to sustain the vigor they need to resist environmental
stresses, weeds, diseases, insects and other pests.
The 16 essential nutrients are broken into two categories,
primary (nitrogen, phosphorus and potassium) and secondary
(calcium, magnesium, sulfur, iron and others).
The primary nutrients get the most attention because they’re
required in the greatest amounts. Fertilizers are sold based on
their amounts.
The numbers
On a fertilizer bag, the numbers refer to the percentage of
actual nitrogen (N), phosphate (P2O5) and potash (K2O). So 10
percent of the weight of a bag of 10-10-10 is nitrogen, 10
percent is phosphate and 10 percent is potash.
It’s easy to figure the actual weight of nitrogen. The percentage
is listed on the bag. In a 50-pound bag of 10-10-10, the nitrogen
would weigh 5 pounds (0.10 times 50).
It’s not as simple, though, for phosphorus (P) and potassium (K).
To find the amount of phosphorus, multiply the phosphate number
by 0.44. Get the potassium amount by multiplying the potash
weight by 0.83.
How it adds up
So that 50-pound bag of 10-10-10 has 5 pounds of phosphate times
0.44, or 2.2 pounds of phosphorus. It has 5 pounds of potash
times 0.83, or 4.15 pounds of potassium. With the 5 pounds of
nitrogen, then, it has 11.35 pounds of primary nutrients.
Using the same formula, a 50-pound bag of 16-4-8 would have 8
pounds of nitrogen, 0.88 pounds of phosphorus and 3.32 pounds of
potassium. That’s 12.2 pounds of primary nutrients.
So what’s the rest of the weight in the bag? Some of it may be
secondary nutrients. The rest is filler material to make it
easier to apply.
A fertilizer bag containing all three nutrients is considered a
“complete” fertilizer. A product with any nutrient missing is
called “incomplete.”
(Clint Waltz is an Extension Service turf scientist with the
University of Georgia College of Agricultural and Environmental
Sciences.)
