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August is the time to fertilize lawns

Posted: August 11, 2012 - 11:13pm

Many homeowners are wondering if lawns need to be fertilized again, and the simple answer is yes. The last fertilizer application for the year should be during the month of August.

With all the rain that the area has received, it would be a great time to fertilize if the grass is a warm season turfgrass. Our warm season turfgrasses are Bermuda, zoysia, centipede and St. Augustine. Cool season turfgrass, such as fescue, don’t need to be fertilized until late September. This is called winterizing the lawn.

We don’t winterize warm season grasses. By late September, the warm season grasses are slowing their growth and are getting ready to become dormant; therefore, they are not using the fertilizer put out then.

The best way to determine which fertilizer to use is to have the soil tested. A soil sample will tell what the pH of the soil is, and what the nutrient levels are in the soil. This can save money by not putting out nutrients that are not needed. Also, the pH of the soil is important to plant growth because most of the major plant nutrients are more available to the plant at a pH of 6 to 6.5. So, if the pH is where it should be, no lime needs to be applied. In centipede grass, iron chlorosis can be a problem if the pH is too high.

To choose the proper fertilizer, it is important to know what the three numbers on the fertilizer bag mean. The most common is 10-10-10. The first number on a fertilizer bag is nitrogen, the second number is phosphorus, and the third number 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 amount by multiplying the potash weight by 0.83. 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.

So what’s the rest of the weight in the bag? Some of it might be secondary nutrients. The rest is filler to make it easier to apply.

These three elements have different roles in the growth of the plant. 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 color of plants after fertilization is due to the nitrogen increasing the amount of chlorophyll in the leaves.

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

Potash or potassium is important in photosynthesis and plant respiration. If the level of potassium in the plant falls, then photosynthesis declines and the respiration of the plant increases. Both of these will deplete carbohydrate supply. If the plant has high levels of potassium, the plant will have better winter-hardiness, more drought tolerance and will be more disease-resistant.

In the past, we have managed our turfgrass on nitrogen levels, but now we are beginning to look closely at the potash levels. Most of the lawns that I look at that have disease problems also have low potash levels. On the other hand, lawns that have no or very few disease problems have high levels of potash. Therefore, use a fertilizer that has higher amount of potash for this last fertilization. A fertilizer such as 16-4-8 or 15-0-15 will do the trick.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. Reach him at cphillipshort@comcast.net or at (706) 836-2152.

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