Feature

Turfgrass Nutrition – What’s Old is New Again

By Richard Schmidt, Ph.D., Professor Emeritus; Xunzhong Zhang, Ph.D., Research Associate Professor; and Mike Goatley Jr., Ph.D., Professor and Extension Turfgrass Specialist, Virginia Tech

(Note from Mike Goatley: Professor Emeritus Dick Schmidt remains very much engaged in keeping up with the world of turfgrass research even now in his 10th decade on this planet! I asked him to please put his unique perspectives on the evolution of turfgrass nutrition programs over the past 60+ years and the ways that the research and application programs have evolved in print. Dr. Zhang and I chimed in with some of our perspectives, but Dick led the development of what is a nice review of nutrition and strategies that allow a turfgrass manager to get the most out of a fertility program and produce a healthy, functional turfgrass plant.)

Recently a USGA agronomist indicated that it is desirable to keep bentgrass golf putting greens a bit yellow during hot weather. For Dr. Schmidt, this brought back memories of when he first started studying turfgrass ecology under Professor Burt Musser at Penn State and frequently accompanied him when he toured some of the golf courses in Pennsylvania. It was during some of these trips that Dick recalls hearing senior golf course superintendents state, “Keep the bentgrass on the yellow side during the hot season.” Dr. Goatley notes that during his career, a constant phrase uttered by turfgrass managers that he equates with Dick’s recollection of the value of off-color bentgrass during summer stress is to keep the grass “lean and mean.” Studies regularly show that these mid-20th century superintendents had valid reasoning for avoiding aggressive nitrogen fertility programs on bentgrass during summer stress, and that the recent comment by the USGA agronomist was most observant. Data show that high nitrogen fertility programs on bentgrass greens did enhance green color prior to and during hot weather; however, top growth was exacerbated causing a reduction in carbohydrate content. High nitrogen applications consequently reduced root production. Additional environmental stresses (heat and drought stress primarily) increased accumulation of metabolic toxic oxygen species (free radicals, more on this later in this article) which may damage plant lipids, proteins and other organic components, the literal building blocks of your turfgrass tissues. These internal stress responses reduce photosynthetic efficiency and eventually lead to cell death (and visible loss of turf). Although turfgrasses possess their own antioxidant defensive systems the levels may not be adequate to reduce damage that is exacerbated with the high nitrogen applied under these additional stresses.

Photo 1: Virginia Tech has led research that verifies both the presence and the concentrations of hormones in seaweed extracts and how those compounds can promote turfgrass root development.

Photo 2: The April greening response of trafficked bermudagrass where 0.25 lb nitrogen per 1000 square feet rates of soluble nitrogen were applied monthly in the fall through mid-October (foreground block), mid-September (middle), and no fertilization past mid-August (background).

Considering the color, the density, and the height and frequency of clipping of bentgrass that the golfer demands, how does one effectively manage quality turf? First, one needs to assure all the requirements of a healthy soil are met: pH, aeration, appropriate levels of organic matter, and sufficiency levels of all required nutrients other than nitrogen. Then the answer for nitrogen management is to follow nature’s lead in carbohydrate accumulation. Research at Virginia Tech in the late 1960s changed industry recommendations regarding fall nitrogen fertilization on cool-season turf. This work demonstrated that fall fertilization during active growing periods of fall and early winter were very positive for root development and carbohydrate storage, and ultimately led to healthier plants the following spring when active growth resumed. Until this point in time the standard nitrogen fertilization programs for cool-season turf still primarily emphasized spring fertilization. During the fall when temperatures are lower and light intensity is decreased, top growth is naturally going to be reduced by the plant growth response to the season, and carbohydrate accumulation stimulates root production. There obviously is some leaf and shoot development for cool-season grasses, but the roots are the focus of growth responses during a period of shorter days and cooling temperatures of fall and early winter. And to get the benefits of desirable fall nitrogen fertilization responses, one must make the applications while the cool-season grass is still actively growing because nitrogen uptake potential is greatly reduced once average daily high temperatures are at or below freezing. Emphasizing nitrogen fertilization at this time will not stimulate the leaf production, thus reducing plant respiration and enhancing carbohydrate reserves and root development. This practice enhances chlorophyll and photosynthate production both in the fall and the following spring, thus enabling the grass to better tolerate subsequent spring and summer stresses by reducing the levels of free radicals.

There are still benefits to spring and even some summer nitrogen fertility programs, but remember what the plant is programmed to do as a response at that time of year: the focus is on leaf development and not roots and carbohydrate storage. Hence, it is very important to pay attention to the levels and frequency of the applications in order to achieve the usual goals of color and desirable growth/recovery rates. Keeping cool season plants active during more stressful environmental periods has been shown to improve recovery rates from spring/summer disease complexes. Of course, being too aggressive with nitrogen fertilization almost always increases disease susceptibility. Research in a controlled environment at Virginia Tech showed that applying N at 0.15 lb per 1000 square feet biweekly improved turf quality, photochemical efficiency, chlorophyll content, N and amino acid content, and antioxidant activity in shoot and roots of creeping bentgrass subjected to heat stress. Such spoon-feeding strategies are now the norm in most creeping bentgrass summer management programs, a practice that was almost always discouraged a few decades ago.

Warm season grasses such as bermudagrass have a very different set of cycles in how they conduct photosynthesis than cool-season grasses. Carbohydrate accumulation and root and top growth are all maximized during the warmer part of the year. Therefore, it is desirable to fertilize with nitrogen during the warmer seasons of the year. However, in Virginia we have found advantages with early fall nitrogen fertilization on bermudagrass. Dr. Goatley refers to this as ‘responsible’ N fertilization of the bermudagrass with the intent being to maintain activity as long as possible so that the plant continues to photosynthesize and store carbohydrates. He recommends flexible N fertility programs applying up to 0.25 lb N per 1000 square feet every couple of weeks prior to killing frost, but during the period when bermudagrass growth is still active but slowing due to the shorter days and cooler temperatures. Bermudagrass fertilized in such a manner maintained a better color during fall and continued to conduct photosynthesis and accumulate carbohydrates than bermudagrass not fertilized in early fall. The desirable responses were further enhanced with the addition of iron and biostimulants treatments to sustain bermudagrass activity as long as possible without sacrificing winter hardening.

Let us consider some more Virginia Tech research findings regarding iron fertilization. Iron is a micronutrient that has long been used by turfgrass professionals for its ability to trigger a rapid color response without a flush of shoot growth. There were many early research trials that showed how effective iron fertilization was for maintaining desirable color on bentgrass greens during warm weather without the undesirable effects of excessive nitrogen fertilization. Another interesting observation in the field that led to more extensive research occurred during an extremely dry winter in the 1970s that resulted in the loss of many bentgrass plots in fertility trials at Virginia Tech. However, a pattern was observed in that all plots that survived that winter drought had fertilizer applied that contained iron. The following year samples were taken from plots fertilized with and without iron and placed in a rainout shelter during the winter. Again, the samples taken from plots that were fertilized with iron best survived the lack of moisture during the winter. Subsequently more research was initiated using iron fertilization, and additional work with iron fertilization in sod production systems showed that iron fertilization enhanced sod maturity as well as helped reduce the impact of drought.

Iron is the fourth most abundant element in the earth’s surface and yet it is very often deficient in our turfgrasses, even though it is required in very small amounts. The main reason iron deficiencies occur is because the earth’s atmosphere is conducive to oxidation. Most plants cannot readily take up oxidized forms of iron. Soils high in alkalinity, phosphorus or calcium may also contribute to a plant’s iron deficiency. Grasses have evolved a very unique strategy to absorb oxidized iron by way of a metabolic process that involves the synthesis and release of a nonprotein amino acid from the roots that is called a phytosiderophore. These compounds chelate the oxidized iron, enabling it to be more readily absorbed into the roots and transported to the leaves where it is reduced and is utilized by the plant as a precursor to chlorophyll production and in energy transfer reactions. Many of you likely apply chelated micronutrient sources, so you are mimicking the chelation strategy of nature that occurs by way of the phytosiderophores. However, when the grass is under stress, the rate of phytosiderophore production can become limiting such that iron is not being sufficiently chelated and absorbed by the roots. Under these conditions grasses readily respond to iron fertilization and it is not uncommon to see cool season grasses instantly green while liquid iron is being applied during hot weather. Don’t forget the possible color, growth, and overall turfgrass health responses associated with supplemental iron applications as part of your seasonal fertility programming.

Finally, another research area briefly mentioned above that Virginia Tech has been a leader in for many years that fits well in the development of fertility programs is the use of biostimulants. Much of our earliest work was done in sod production systems where we observed these organic materials, when used in small quantities, enhance plant growth and development in ways that cannot be attributed to traditional plant nutrient responses. These compounds don’t replace plant nutrition requirements, but they have been shown to improve nutrient use efficiency, provide hormonal growth responses, or stimulate a desirable plant stress response. And that final point is key – biostimulants are most valuable when coupled with stress management programs for your turf, be it cool- or warm-season grasses.

Biostimulants have also been called positive plant growth regulators or metabolic enhancers. A lot of our research has evaluated biostimulant function in conditioning plants to better tolerate environmental stresses. Biostimulant-treated tall fescue sod produced better initial rooting than non-treated sod after being subjected to a controlled heated chamber designed to simulate conditions conducive to warm weather field environments. This treatment could lessen loss of installed sod during hot weather. In another study, biostimulant-treated turf irrigated with saline water showed less sodium uptake. This is an indication that brackish water may have use for turfgrass irrigation when the plant is properly conditioned with biostimulants. At Virginia Tech we have demonstrated that seaweed extracts, humic acid, triazole fungicide, amino acid, protein hydrolysates, potassium silicate, 24-Epibrassinolide, methyl jasmonate, salicylic acid, and the microorganism Bacillus subtillis all have biostimulant properties. These responses are hormonal in nature. For example, seaweed contains cytokinin and auxin, humic acid is high in auxins and chelated organic nutrients. Amino acids can be absorbed by plants and directly incorporated into nitrogen metabolism pathways. Biostimulants utilize multiple modes of action to maximize plant health in an integrated, balanced fashion.

Another way that biostimulants benefit plants is their ability to lessen the potential for excessive energy captured by the plant to cause damage. This sounds somewhat counter-intuitive when we know that sunlight is what drives photosynthesis, but what if the plant simply can’t use all of that solar energy it is being bombarded with? Under favorable conditions molecular oxygen inside the plants accepts electrons (energy) during metabolic processes. Under unfavorable conditions the oxygen-accepting electrons produce toxic reactive oxygen species, referred to as free radicals; this form of reactive oxygen species is detrimental to a variety of plant membranes and other compounds etc. Free radicals cause pigment breakdown and loss of photosynthetic efficiency and eventually can lead to plant death. Biostimulants enhance development of compounds that serve as ‘antioxidants’, compounds that scrub the free radicals (i.e. mitigate all of that excessive energy that the plant cells cannot utilize) to produce nontoxic metabolic activity conditioning and plants that better tolerate stress. Plant stress reductions to drought, heat, salinity, UV radiation, and nematode infestation have been associated with antioxidant enhancement. Certain herbicide effectiveness has been enhanced with applications of endogenous biostimulant application.

In summary, many of the turfgrass nutrition principles that we knew to be true in the middle of the last century remain pertinent today. Where we have grown is in better understanding of how and why these fertilization sources and strategies have worked and how they have been improved by further refinement from research in the lab and the field, especially with the strategies where programs are enhanced by iron and biostimulants. By incorporating the proper timing and rates of nitrogen fertilization to the season, the plant, and the plant’s particular use and needs at that time, along with the use of supplemental iron and biostimulant applications, turfgrass managers may obtain and sustain healthy turf, even during sub-optimal growing conditions.