Fertilizing at the right time and in the right amount helps lawn along

With spring comes many questions about lawns and gardens, and one of the more common questions is when to start fertilizing lawns and plants.

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Before fertilizing, homeowners should know what fertilizer to use and how much to use. The best way to determine this is with a soil test. The results will provide an analysis of which fertilizer to use, how much to use and when to use it. The cost is $8 through the local University of Georgia Extension Service.

Purchasing fertilizer can be a bit confusing. There are many different kinds with different numbers on the bag.

All plants require nutrients to grow and develop properly. Various nutrients affect plants differently and are needed in varying amounts.

Essential nutrients are ones plants have to have to sustain the vigor they need to resist environmental stresses, such as 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.

These elements help the plant in different ways. The first element is nitrogen. Most people think about nitrogen because it causes plants to turn dark green and put on new growth.

Nitrogen turns plants green because it helps with the plant's photosynthesis process. Nitrogen is very mobile in the soil and will leach or move out of the root zone when excessive water is applied. It can slow down root development.

The second element is phosphorus. Phosphorus is important in root development and flower and fruit set and development. Unlike nitrogen, phosphorus is bound tightly to soil particles and stays in the root zone.

Once the soil levels of phosphorus reach high or very high levels, fertilizers that contain phosphorus are no longer needed. High levels of phosphorus can damage the root system of some plants, such as centipede and St. Augustine grass.

The last number on the fertilizer bag is potassium, which strengthens plants' cell walls. With stronger cell walls, plants can handle diseases, cold weather and drought better. In our soils, potassium leaches out of the soil like nitrogen does, so it has to be replaced.

The best time to fertilize lawns will depend on the soil temperature and the type of grass.

Soil temperature plays an important role in turf growth. Our turfgrass does not reach its full growth rate until the soil temperature at 4 inches reaches 65 degrees for three to four days.

In the winter, our turfgrass loses a portion of its root system and has to add to the root system in the spring. If lawns are fertilized too soon in the spring, it can slow down the growth.

This is especially true in centipede and St. Augustine. If these grasses are fertilized before the soil temperature is 65 degrees, yellow patches can occur in the lawn. These grasses are stressed from the fertilizer and cool soils.

In our area, the soil temperatures reach a constant 65 degrees sometime after the first of May. Last year, it was around mid-May before we reached those soil temperatures.

Bermuda and zoysia grass can be fertilized a little earlier than centipede and St. Augustine. These grasses can be fertilized two weeks after they are completely green in the spring.

For lawns with difficult to control disease problems, potassium levels in the soil should be examined. In numerous lawns, we have been able to reduce the disease levels by increasing the amount of potassium.

This has allowed us to reduce the amount of fungicides being applied. Without a soil sample, I would recommend that using a fertilizer such as 16-4-8 or 15-0-15 to increase the amount of potassium in the soil.

Another aspect to look for is a fertilizer that has slow-release nitrogen in it. Slow-release nitrogen will slowly feed lawns and therefore maintain a more constant color. Also, the slow release will reduce the chance of runoff and leaching.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. He can be reached at cphillipshort@comcast.net.