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Understanding and Using Home Lawn Fertilizers - Part III: Returning to phosphorus and potassium.

Bob Mugaas, UMN Extension Educator

Photo 1: Steps for properly taking a soil test.
Karen Vidmar, University of Minnesota.

Over the last couple of articles we have explored how to better understand the sometimes bewildering array of lawn fertilizers available and make appropriate choices for one's own lawn as well as the large and diverse role that the nutrient nitrogen plays in the life of lawn grasses. Part III, the final section of this three-part series, will turn attention back to the nutrients phosphorus (P) and potassium (K) and the important roles they play in turfgrass health. Since determining the amount of P and K in a container of lawn fertilizer was thoroughly discussed in the first article of this series, this last article will focus on the importance of P in K in turfgrass health and their application as a component of lawn fertilizers. Before beginning, remember that the best way to determine the amount of P and K in your soil is to have a reliable soil test done such as that performed by the University of Minnesota Soil Testing Lab. Collecting the soil to test properly is important for accurate soil test results. See Picture 1.

Turfgrasses and the important role of phosphorus(P) in plant health.

Phosphorus is an important component of many different plant molecules and compounds responsible for promoting healthy turfgrass growth. In fact, without ample P, normal healthy growth is not even possible. Some of its more important roles include those related to the storage and transfer of energy within the plant, promotion of healthy, well developed root systems, proper formation of plant cell membranes and, it is an important component of the cell's genetic material including its DNA.

In addition to its important roles within the plant, a critical component to understanding the use of phosphorus fertilizer is how it behaves in the soil. That is, phosphorus moves very little in the soil with most of it being bound tightly to soil particles. As a result, phosphorus does not move into the soil water solution as easily as other nutrients (e.g., nitrogen). That's both good news and bad news for the grass plant. The good news is that P is much less likely to move down through the soil via leaching and be lost as a plant nutrient. Hence, its supply in the soil is relatively stable. The bad news is that with such little movement in the soil it can make it more difficult for the plant to obtain sufficient supplies of P to support healthy growth.

Fortunately, healthy, actively growing lawn grasses have an extensive, dense, fibrous root system that makes them relatively efficient as extracting P from the soil compared to many other plants. For this reason, the application of supplemental P via fertilizer to an established, mature stand of turfgrass is usually unnecessary or needed in only very small amounts. Again, this is not because our grass plants require less P for growth than other plants. Rather, they are just more efficient at extracting available P from the existing soil. It should also be apparent that when our lawn grasses have shallow restricted root systems, such as in compacted or waterlogged soils, they do not have as much access to potential soil reserves of phosphorus and hence supplemental applications of P via a fertilizer may be necessary.

Additional phosphorus at establishment is beneficial

Photo 2: Available soil phosphorus is needed for these young developing grass plants.
Bob Mugaas, U of MN Extension.

Application of a phosphorus fertilizer, often purchased as a lawn starter fertilizer, is recommended at the time of establishing a new lawn or completely renovating an existing lawn. In this instance, the additional phosphorus is applied and lightly incorporated into the soil surface where it is more readily available to the very young, developing grass root system. See Photo 2. This aids in the promotion of vigorous seedling growth and quicker establishment. Once established and the grass plants have developed a deeper much more extensive root system, the application of supplemental phosphorus fertilizer can be reduced or even eliminated (at least temporarily) depending on existing soil P levels as determined by a soil test.

Fertilizer phosphorus and water quality issues

While loss of phosphorus from a lawn via leaching is considered to be very low, losses via runoff and soil erosion can occur and can create pollution problems for nearby lakes and rivers receiving stormwater runoff. Once in a lake, the additional P, even in very small amounts, can stimulate rapid and excessive algae growth. In turn, these 'algae blooms' as they are called can significantly reduce water quality for many different uses and negatively impact fish and other lake life.

The Minnesota Phosphorus Fertilizer Law:

Because of this concern for excessive phosphorus in lakes and rivers from fertilizer, the Minnesota legislature passed a statewide law that restricts the application of phosphorus fertilizer to established lawns and other turfgrass areas. The law states that fertilizers used on lawns will be restricted to 0% phosphate (P2O5) content. Exceptions include application at establishment, either by seeding or sodding, and then only during the first year of establishment. Applications can also be done when a soil or tissue test shows a need for P. In those cases, lawn fertilizers with P can be used. More detail pertaining to the law and the penalties than can be applied is found in Chapter 18C.60 of Minnesota Statues. Because of this law, soil testing becomes even more important for managing applications of phosphorus to turfgrass.

Recent U of MN research sheds additional light on the potential for lawn phosphorus losses.

Recently completed research at the U of MN, clearly showed that neglected lawns (i.e., those receiving no additional fertilizer), became thin and weed infested over the course of the study and contributed more runoff volume and more phosphorus loss during the growing season than those receiving sufficient levels of fertilizer to maintain good turfgrass density and active growth. In addition, there was essentially no difference in the amount of runoff volume or phosphorus loss between those lawn plots receiving nitrogen and potassium only and those receiving nitrogen, a low rate of phosphorus (1.0 pound of P annually) and potassium. Hence, where background levels of soil P are high on an established healthy stand of grass, there does not appear to be any significant decline in plant health or vigor, when P is no longer being added via fertilizer. In general, that result would support one of the law's assumptions that additional P is not necessary for healthy turfgrass when ample levels already exist in the soil. Likewise, neglecting a lawn entirely by applying no additional fertilizer to maintain plant health and density would not be a good practice.

For those of you interested in more details and results from this study, there is a link to the complete final report of this study in the Phosphorus section in the Understanding and Using Lawn Fertilizers chapter of the Home Lawn Care section of the Sustainable Urban Landscape and Information Series (SULIS). The U of MN Extension publication entitled Preventing Pollution from Lawn and Garden Fertilizers provides additional information on the responsible use of fertilizers to protect water quality.

Turfgrasses and the important role of potassium (K) in plant health.

Even though grass plants contain rather large amounts of the nutrient potassium there is still much we don't know about the biological role and activity of potassium in the plant. Potassium is important in the synthesis of many plant components and in the regulation of many physiological processes including the efficient use of nitrogen. While potassium is involved in many of these activities, it is usually not an integral part of the final product produced. Some of those plant processes where potassium is involved but not part of the end product include the formation of carbohydrates and the photosynthetic process, activation of various enzymes that in turn control other plant processes and, aids in the formation of plant proteins.

Where soil potassium levels are low and plant deficiencies exist there is often a reported increase in the incidence of turfgrass diseases and reduced tolerance to environmental stress. However, the visible, field detection of a potassium deficiency can be difficult at best. While research continues, our understanding of the biology and specific roles that potassium plays in the plant remains rather elusive.

Potassium is held on the surfaces of soil particles and moves little in most soils, though it can gradually move out of the root zone in very sandy soils. Its movement in soil is often considered to be between that of nitrogen, which is very mobile, and that of phosphorus, which is very immobile. Where soils are high in native potassium, supplemental potassium fertilization may be unnecessary; however, where soils are low in native potassium, supplemental applications can be very important. However, an application of additional potassium does not necessarily create obvious visible symptoms as one might associate with the nutrient nitrogen. Again, soil tests are essential to determine the potassium level of a soil and to develop a potassium fertility program.

Photo 3: Healthy, dense lawns can help protect water resources.
University of MN Extension.

Concluding remarks

It is hoped that this series of three articles beginning with the March 1, Yard and Garden News issue followed by the April 1 issue and concluding with this one has provided at least some basic insight into the importance of the three major nutrients contained in most lawn fertilizers to a healthy, vigorous lawn. Selecting an appropriate fertilizer and applying it correctly may just be one of the more important things you can do for your lawn and protect the environment at the same time.

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