Many plant growth experts consider potassium to be the plant nutrient that has been passed over and not given credit for the role it plays in plant growth. The way potassium functions in the plant has not been well understood, while functions of other nutrients such as nitrogen and phosphorus have been more clearly defined. Potassium seems to be involved in many growth processes, but one of its most important roles has to do with water relations within the plant.

Just as high nitrogen levels produce a plant with thin cell walls and high water content, the absence of adequate amounts of potassium has the same effect. As the amount of potassium supplied to the plant is increased in relationship to the nitrogen level, cell walls become thicker and the water content of the plant decreases. This makes the plant less susceptible to the potential invasion of a disease or an insect attack as well as more stress-tolerant.

Potassium has a great deal to do with the balance in the plant between leaf and root growth. As the level of potassium supplied to the plant increases in relationship to the nitrogen level, the rate of leaf growth is reduced. With this reduced demand for food material by the leaves, more becomes available for stolon, rhizome and root growth.

Potassium is considered to be the most leachable of the plant nutrients and must be supplied at a constant rate. It may even be lost from the plant through its leaves during rain or irrigation.

Use of fertilizers with relatively high potassium levels has been hard to "sell" because unlike other nutrients, its application to a turf does not necessarily result in a change that is easy to see or measure. However, research indicates that when potassium is supplied in optimum levels, the turf plant is less susceptible to other factors such as drought, heat, cold and disease.

From time to time, depending on the local soil and its pH level, nutrients such as sulfur, magnesium and iron may be required. A soil test and/or a tissue test identifies nutrient problems. Most minor nutrient problems are the result of soil with too high or too low pH. When a minor nutrient problem is identified by a soil or tissue test, the missing minor nutrient available to the plant should be applied as efficiently as possible. The best way to do this is to use a "chelated" form of that nutrient. The chelation process helps move the nutrient through the soil solution without becoming unavailable.

When the environment cannot supply the turfgrass plant with the nutrients it needs to perform its assigned function, the needed nutrients should be supplied as a fertilizer.

Turf fertilizers typically contain only nitrogen (N), phosphorus (P) and potassium (K), but other macronutrients and some micronutrients may be included, depending on local soil conditions. Fertilizer packages or bags have three numbers which represent the percentages of N, P and K. On the back of most bags the guaranteed analysis is provided (Table 4.9). As shown in Table 4.9, the fertilizer is 15 percent nitrogen.

It is important to note that 7.5 percent, or one-half, of the nitrogen is in soluble form and the other half is one of the slowly-soluble nitrogen forms (Tables 4.9 and 4.10). Soluble nitrogen materials result in a fast increase in growth rates, especially leaf growth rates and a fast green-up (Figure 4.9). However, this rapid increase is fairly short-lived.

Slowly-soluble nitrogen materials result in a relatively slow increase in growth rates and color, but their effect lasts longer. Depending on the material and the environment, some may supply nitrogen from 3 to 4 months up to an entire growing season.

Fast growth requires higher amounts of water. However, the plant has thinner cell walls which may make it easier for insect or disease invasion. The plants may also be more susceptible to heat and cold damage.

Fertilizer is a salt. Fertilizing materials are salts, just like table or rock salt. If fertilizers are over applied, they can cause plant death, known as "fertilizer burn." Water moves from areas of low osmotic pressure to areas of high osmotic pressure. Normally, solutions inside the plant are higher than the soil solution, so water runs into the plant. If a fertilizer is over applied, water can be pulled out of the plant, thus killing the plant from lack of water.

The possibility of fertilizer burn is countered by watering fertilizer into the soil. Any salt effect is thus diluted. The possibility of fertilizer burn is greater on hot days than on cool days because plants use more water on hot days.

Fertilizing materials vary in their ability to produce a burn (Table 4.11). The salt index number has no units and is only used to compare materials. For example, if a fertilizer has urea as its nitrogen source, there is only half the chance for fertilizer burn as when ammonium nitrate is used. Note that all the slowly soluble nitrogen materials have a very low salt index, and thus a very low burn potential. If a soil or water test indicates the presence of a potential salt problem, then it makes sense to use low salt index fertilizer materials.

Cultivation of turfgrass includes aeration, topdressing and vertical mowing. These practices:

improve the aeration of the soil and the infiltration of water,

promote the growth of the root system and

reduce the compaction of the soil surface and the accumulation of thatch.

All of these benefits are essential to producing a vigorous, healthy lawn.

Compaction is a physical process that slowly reduces the amount of oxygen contained in soil (Table 4.12). Roots of the turfgrass plant need oxygen and give off carbon dioxide as a product of their growth process. Oxygen from the atmosphere moves into the soil through very small pore spaces to the roots, and carbon dioxide escapes through the soil into the atmosphere. As the soil is trafficked, soil particles in the top 1 to 2 inches are compacted into a layer so that less and less oxygen can enter the soil and less and less carbon dioxide can escape. The net result is a thinner and thinner turf, until ultimately the soil can no longer support any turf growth at all. Only a few weeds can grow in these oxygen-deficient, compacted soils where grasses cannot grow.

Since compaction is the result of a physical process, it takes another physical process to reduce or prevent its effects. Aerifiers have a number of hollow or open metal tubes called tines which are used to relieve compaction (Figure 4.10). Some aerifiers have solid metal tines. The tines are 3/8 to 1 inch in diameter and penetrate the soil to a depth of 2 or 3 inches, or as deep as 12 inches. As the tine is pulled out of the soil, a soil core is removed and is deposited on the turf surface. The hole left in the turf becomes an avenue for oxygen to penetrate into the soil and for carbon dioxide to escape. Root growth around the hole is greatly increased, and the vigor of all plants around the hole is enhanced. The greater the number of holes poked in the turf, the greater the increase in that turf’s vigor.

The frequency at which any turf area may need aerification is solely dependent on the amount of traffic it receives and to some extent the texture of the soil under the turf. Under a normal maintenance program, areas with foot traffic only may never need aerification. When traffic becomes heavy enough to thin the turf, it is time to aerify. Heavy traffic areas may need to be aerified at least two or three times a year during periods of active turf growth.

The best time to aerify is when the turfgrass is actively growing. Roots will rapidly fill the holes and the area will recover quickly. If the turf is aerified during its dormant period, the open holes may allow excessive loss of soil moisture.

Even with good management, problems occasionally arise as a result of thatch accumulation, weeds, insects, diseases or excess shade.

The spongy turf that results from accumulated organic residues between the soil and the green leaves is referred to as "thatch" (Figure 4.11). A certain amount of thatch is desirable, as it adds resilience to the turf, reduces compaction of the soil surface, and prevents soil erosion. However, excess thatch reduces water infiltration, creates shallow-rooted turf, encourages insect and disease infestations and makes mowing difficult.

Management practices such as thorough and infrequent watering, close and frequent mowing, proper fertilization and occasional cultivation (aeration, vertical mowing and top-dressing) will prevent excess thatch accumulation. If lawns are mowed frequently, grass clippings will not promote thatch accumulation, and it is beneficial to leave them on the lawn. However, if a lawn is mowed so infrequently that the turf is covered with clippings, then the clippings should be removed. Excess tree leaves should also be removed rather than shredded with a mower. Scalping the lawn by close mowing in several directions at the first sign of spring green-up will aid in thatch prevention. Grass clippings and tree leaves collected after scalping can be disposed of through composting and used in flower and shrub beds or gardens.

Excess fertilization with nitrogen leads to thatch accumulation. Lawns should be fertilized in early spring and fall when the grass is not growing vigorously. During the summer months, keep fertilization to a minimum and do not exceed 1 pound of nitrogen per 1,000 square feet every 45 days.

If thatch accumulation becomes a problem, lawns should be dethatched in early spring by vertical mowing with a special dethatching tool in two or more directions. For bermudagrass or bluegrass lawns, the vertical blades may be spaced only ½ to 1 inch apart, but for St. Augustine or centipedegrass lawns the blades should be 2 to 3 inches apart. After dethatching, rake or sweep the lawn to remove the organic debris brought to the surface. Aeration should follow the vertical mowing operation. Run a hollow spoon aerator over the lawn two to three times when the soil is moist so that the spoons penetrate 3 to 4 inches into the soil. After aeration, fertilize the lawn to encourage recovery of the grass.

Such a renovation may never be necessary, or it may be needed every 2 to 3 years, depending on the turfgrass species and your maintenance program,

Shade from tree canopies creates several problems for turfgrasses, including reduced light and competition for water and nutrients. The result is usually shallow-rooted turf that is more susceptible to drought stress, winterkill, wear and disease infestations. Of the turfgrasses, St. Augustine and tall fescue are the most shade-tolerant, zoysia and centipede grasses are intermediate in shade tolerance, and bermudagrass and buffalograss are the least shade-tolerant. However, even St. Augustinegrass requires some sunlight to produce an acceptable lawn in shade.

Increase the mowing height for turf growing in heavy shade (St. Augustinegrass and tall fescue should be mowed at 3 inches). Also, reduce nitrogen fertilization of turfgrasses in dense shade to prevent succulent growth that is more susceptible to diseases.

If tree canopies are dense and structures or low-growing shrubs screen the lawn, it may be necessary to selectively prune tree limbs to increase light penetration. Root pruning or removal of shallow-rooted trees also will improve conditions for turfgrasses in shade. If dense, low growing trees and shrubs are an essential part of your landscape plan, a shade-tolerant ground cover such as Vinca (Vinca major) can be used in place of turfgrasses.

Because of their variation in texture, growth, habit and color, weeds create an unsightly appearance in lawns. Proper management is the best means of controlling weeds. Thorough and infrequent watering, judicious fertilization and proper mowing are important steps to weed control. However, lawns damaged by insects or disease, wear (compaction) or other problems are highly susceptible to weed invasions.

Crabgrass, goosegrass, sandbur and dallisgrass are the major grassy weeds which cause problems in lawns during the summer. The first three are annuals that emerge from seed each spring. Dallisgrass is a perennial grass that recovers from rhizomes in the spring, but also produces seed that spread throughout the lawn. With the exception of dallisgrass, these weeds can be controlled with preemergence herbicides applied in early spring. Dallisgrass can be controlled by spot-treating the plants with a contact herbicide. Cool-season weedy grasses, such as rescuegrass, annual bluegrass and ryegrass, can be controlled by preemergence herbicides applied in early fall, or by contact herbicides applied when the permanent turfgrass is dormant.

Broadleaved weeds such as dandelion, chickweed, henbit, clover, dock and mustards can be controlled with selective post emergence herbicides. Only materials recommended for St. Augustinegrass should be used on St. Augustine lawns, as the turf might be damaged by some materials that are safe for bermudagrass.

Nutgrass creates special problems in lawns because it grows very rapidly and spreads to ornamental beds and gardens. St. Augustine grass effectively competes with nutgrass and crowds it out; however, when the grass is damaged by insects or disease, nutgrass can become a problem.

For complete weed control recommendations, contact your county Extension office.

Caution: Follow label recommendations for all herbicides and use them only on the grasses specified on the label.

White grubs, larvae of the May or June beetles, also cause extensive damage to lawns. The grubs feed on roots of lawns 1 to 2 inches below the surface. If the infestation is heavy, grubs consume the entire root system and the sod can easily be lifted or rolled up. To check for the presence of grubs, dig 1-square-foot sections of sod at several locations in the lawn and examine the roots and soil to a depth of 4 inches. (After examination, the soil and sod can be put back in place.) Treatment is justified when more than four grubs per square foot are found. Treatment should be made during the last 2 weeks of July and the first 2 weeks of August.

Other insects such as ants, ticks, chiggers and earwigs commonly infest lawns but cause little damage to turf. For complete insect control recommendations, contact your county Extension office.

Properly managed lawns may be attacked by diseases, but they recover much faster than poorly managed lawns. Identification of the disease attacking a lawn is essential to successful control. Some turf diseases have characteristic symptoms that can be readily identified by homeowners.

Brownpatch is a fungus disease that damages grasses in spring and early fall. Brownpatch is characterized by circular patches of yellow or brown grass that may vary from less than 1 foot to several feet in diameter. The outside of the circle has a "smoke ring" appearance caused by the actively spreading fungus. In this area, the leaves of the grass may be easily pulled from the stem because of the deterioration caused by the fungus. The grass in the center of the circular patch may recover within weeks, giving the diseased area a doughnut-shaped appearance. The fungus is most active when humidity is high and the air temperature is between 75 and 85 degrees; fungus activity stops when the air temperature reaches 90 degrees. Preventative fungicides should be applied in early fall.