To every type of turfgrass there is a fertilizer, and a good time to use it

Do I need to fertilize my lawn again? This is a question that I am getting from numerous callers.

//

The answer is yes. Your last fertilizer application for the year should be made in August if you have a warm season turfgrass, such as Bermuda, zoysia, centipede or St. Augustine.

If you have a cool season turfgrass, such as fescue, you need to wait until late September to fertilize. This is called winterizing your lawn.

Don't winterize warm season grasses. The warm season grasses are slowing their growth and are getting ready to become dormant. They are not taking up or utilizing the fertilizer that you are putting out.

In late September, the cool season grasses are beginning to grow so they need to be fertilized so they will have the nutrients to sustain their growth.

What is the best fertilizer for the lawn? The best way to determine this is to have your soil tested. County agents are always stressing the importance of soil testing. If you don't know what nutrients you have in the soil, you could be wasting your money by putting out nutrients that you don't need.

Also, the pH of the soil is important to plant growth. The reason for this is that most of the major plant nutrients are more available to the plant at a pH of 6 to 6.5. So, if your pH is where it should be, you don't need to put out lime.

A pH that is too high on some turfgrass can cause problems. In centipede, you can get iron chlorosis if the pH is too high. Take-all disease also is associated with soil pH, occurring more in soils that have a pH higher than 6.4.

When you go to the store to buy fertilizer, it can be overwhelming with all the different fertilizers that are on the shelves. In order to choose the proper fertilizer, you have to know what the numbers on the fertilizer bag mean, and what is the best one for your grasses.

There are three numbers on a fertilizer bag. Everyone is familiar with 10-10-10. The first number is nitrogen, the second is phosphorus and the third is potash. These numbers are based on percentages. 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.

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 by multiplying the potash weight by 0.83. 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 5 pounds of nitrogen, then, it has 11.35 pounds of primary nutrients.

What's the rest of the weight in the bag? Some of it might be secondary nutrients. The rest is filler material to make it easier to apply.

These three elements have different roles in growth.

Nitrogen is essential for plant growth. It is a part of every living cell and is usually responsible for increasing plant growth more than any other element. The dark green of plants after fertilization is from nitrogen increasing the chlorophyll in the leaves. Large amounts of nitrogen can reduce the food storage parts (roots) and fruiting parts. Nitrogen leaches from the soil and has to be replaced by fertilizers.

Phosphorus promotes root formation and growth, and improves the quality of flowers, fruits and vegetables. It is an element that can build up in our soils because it is tightly bound by soil particles and plants use very little of this element. So, when soil phosphorus levels are high, you don't need to apply a fertilizer with this element in it.

Potassium is important in photosynthesis and plant respiration. If the level of potassium in the plant falls, then photosynthesis goes down and the respiration of the plant increases. Both of these will deplete carbohydrate supply. A plant with high levels of potassium will have better winter hardiness, more drought tolerance and more disease resistance.

Most of the lawns I look at that have disease problems also have low potash levels in the soil. On the other hand, lawns that have no or few disease problems have high levels of potash. Therefore, use a fertilizer that has a higher amount of potash for this last fertilization, such as 16-4-8 or 15-0-15 to get the needed potash.

Reach Columbia County extension agent Charles Phillips at (706) 868-3413 or charlesp@uga.edu, or the extension at www.ugaextension.com/columbia.