Hello this is John Pennington, county extension
agent for Agriculture and Water Quality in Washington County, and today I’m going to
be talking about understanding the numbers on your fertilizer product so that you can
protect water quality whenever you use these fertilizer products in your lawn, garden or
landscape. Calculating the amount of fertilizer needed
to achieve a desired result in the lawn garden or landscape is an essential skill for home
gardeners, lawn care professionals, and do-it-yourselfers. It is hard to underestimate its importance.
Too much fertilizer, and you kill your plants and harm water quality, but applying too little
fertilizer can reduce plant health, vigor and yield. To make sure that you apply the
right amount of fertilizer, be sure to first soil test and measure your intended application
area. Many sources will give you a recommendation
for and amount of some nutrient, like nitrogen, to apply, but neglect to tell you how much
fertilizer you will need to apply to achieve this.
Luckily, the math involved to figure this out is fairly simple and easily mastered,
but first you will need to understand that when you buy a bag or box of fertilizer that
it is not just a bunch of one nutrient. There are either other nutrients in the fertilizer
product or filler material. For example consider this diagram which shows
the chemical formula of Urea, a common form of nitrogen fertilizer. It shows that 46 percent
of the Urea compound is actually nitrogen, while 54 percent is carbon, hydrogen and oxygen.
So, if I took a 10-pound bag of urea home to fertilize my lawn, it would only have 4.6
pounds of nitrogen in it. Another example is this bag of 13-13-13, otherwise
known as triple-13 fertilizer. It is 13 percent nitrogen compound, 13 percent phosphate and
13 percent potash. The way you can tell what kind of nutrients
are in a fertilizer product is to look at the fertilizer grade or the three-number combination
located on the front of almost all fertilizer products. This number combination always starts
with the percentage of nitrogen, phosphate and potash.
Also the fertilizer analysis should tell you what is in the product, like what the nitrogen
compound is. An analysis can cover every nutrient, but we are primarily concerned with the three
most important nutrients: nitrogen, phosphate and potash.
Store-bought fertilizers can come in all sorts of combinations or nutrient ratios, so be
sure to select a product that most closely fits your soil test recommendations. Since
fertilizer nutrient analysis area always expressed by a percent by weight, you can figure out
how much to apply per unit area of your lawn, garden or landscaped area to meet soil test
recommendations. To do this, simply multiply a weight of fertilizer
product by the percent of fertilizer nutrients within the product as listed on the nutrient
analysis For example, your soil test results read to apply 1 pound of nitrogen per 1,000
square feet. To meet this application rate, you could put
out 8 pounds of 13-13-13, but then you would be applying 1 pound of phosphate and 1 pound
of potash, also; both of which were not recommended, were not needed by the plants in this case,
and can result in degraded water quality of surface waters and less money in your pocket.
Or you could have bought 22-0-4, put out only 5 pounds of product over 1,000 square feet
and reached your 1 pound nitrogen per 1,000 square feet recommendation, not put out any
phosphate and only a negligible amount of potash.
This would result in an appropriately fertilized lawn, savings in your pocket and protected
water quality. Sometimes home-generated compost or animal
manure is a preferred fertilizer source, and you can easily have these types of fertilizer
sources tested through your local county extension office to find out their nutrient contents.
To find out more information on how to understand fertilizers, so that you can protect water
quality, then check out the fertilizing your lawn fact sheet that can be bound online or
at your local county extension office. For more information on protecting water quality,
contact your local county extension office or watershed partnership. This podcast was
funded by the Arkansas Natural Resource Commission and Environmental Protection Agency.