Learning the art of sand-green fertility
Recent advances have put more science into sand-green fertility management. However, there's still no "cookbook" that works for all greens.
Sand-based putting greens are less forgiving than native soil greens when it comes to nutrient management. Sand greens inherently have lower nutrient-holding and -buffering capacities and allow large amounts of water to flow through them. As a result, they are prone to multiple, sudden and perhaps serious nutrient deficiencies.
Speak with several golf-course superintendents and you're likely to come away with an equal number of different nutrient-management strategies. Variations in climate, root-zone-mix composition, golfer expectations and budget account for some of the differences in fertilizing strategies. The differences also reflect the fact that there is as much art as science in managing the fertility of sand greens. However, recent advances in our understanding of nutrient behavior in sand greens have put more science into the process. Let's review some of what we now know about sand-green fertility.
Nitrogen management Nitrogen (N) stands apart from all other nutrients with regard to its impact on turfgrass growth. Turfgrass actually exhibits shoot-growth responses all the way up to 15 pounds of N per thousand square feet or more (see Figure 1, below). However, for several reasons, superintendents in the temperate regions of the United States commonly use just 2 to 5 pounds. In reality, most superintendents culture turfgrass in a perpetual state of N deficiency. Thus, any N they apply induces a surge in shoot growth. The result is a strong and ever-present link between N supply and shoot growth rate.
Rates and frequency of nitrogen application Many factors affect the rate and frequency of N you apply to sand greens. Many superintendents follow the USGA Green Section recommendation of light, frequent N applications - generally in the range of 0.1 to 0.25 pound per thousand square feet (lb./M) every 7 to 14 days. The idea is to promote slow, steady shoot growth that favors the health of the turfgrass and stable putting conditions. Superintendents following this approach make extensive use of water-soluble fertilizers.
The appropriate interval between light N applications varies with factors such as weather as well as the criteria adopted by the superintendent. The two most commonly employed criteria are optimizing turfgrass color and minimizing the amount of clippings produced.
A significant number of superintendents simply do not have a budget that allows for frequent N applications. Their alternative is less-frequent applications at higher rates, for example, 0.5 to 0.75 lb./M every 4 to 6 weeks. Superintendents following this strategy often use only granular products.
Both approaches to N application have their place. Light, frequent applications are the norm on courses where golfers demand fast putting greens. By contrast, mowing at heights that provide slower speeds results in conditions more compatible with the use of granular fertilizers and less frequent applications of higher N rates. To say that one N application strategy is better than the other ignores differences in the expected outcomes. Both can result in high-quality playing surfaces.
Annual N rates on bentgrass greens in the Northern states generally range between 2 and 5 lb./M. Rates in the 2-pound range reflect a desire to increase putting-green speed and favor root growth over shoot growth. While it is true that high N rates stimulate shoot growth at the expense of root growth, it also is true that low N rates can lead to reductions in root growth. The annual N rate below which root growth declines is not clearly defined. Some evidence suggests that extensive foliar feeding of N at low rates adversely affects root growth compared to granular fertilizer applications.
Superintendents that apply 4 to 5 lb./M annually aim for high-density turf that rapidly heals from injury. This group of superintendents may also include those who have elected to live with Poa annua rather than wage an expensive and unending battle with the pest. Applying 4 to 5 pounds of N per season provides greater color uniformity of bentgrass/Poa annua putting greens (see photo, page Golf 6).
Nitrogen carriers A great deal of attention is devoted to the virtues of different N carriers. At issue is the N-release pattern of the slow-release N material and the balance between the water-soluble and slow-release components. Choice of N carrier comes down to personal preference and choosing a product that fits the rate and frequency of application you desire. Light, frequent applications are most convenient to apply in water-soluble form with a sprayer or through the irrigation system.
All slow-release N fertilizers contain some water-soluble N. The question often arises as to what is the best balance between water-soluble and slow-release N. The water-soluble component provides quick turfgrass greenup, and the slow-release component promotes uniform long-term color. My research with bentgrass suggests that a one-to-one ratio of water-soluble to slow-release N best meets both of these criteria.
Golfer obsession with green speed has impacted the type of fertilizer superintendents apply. Mowing at 0.125 inch or lower to gain Stimpmeter speeds of 10 or more results in substantial mower pickup of granular fertilizers. Superintendents have responded by shifting to spray applications of water-soluble fertilizers.
Fertilizer suppliers are addressing the issue by manufacturing smaller fertilizer particles, and many have adopted the practice of reporting the SGN of their products. SGN is the average particle diameter, expressed in millimeters and multiplied by 100. For example, if the average particle size is 1.5 millimeters, the SGN of the fertilizer is 150.
Mowing greens at 0.156 (5/32) inch or less calls for an SGN of no greater than 100 to keep mower pickup of the granules to acceptable levels. Now that fertilizers meeting this requirement are being manufactured, the choices of slow-release N carriers suitable for closely mowed greens have expanded. An important additional benefit of smaller fertilizer particle size is greater application uniformity at low rates.
Phosphorus and potassium management We culture turfgrass in a N-starved state. Therefore, applying N triggers shoot growth. However, for this growth to occur, the plant also must take up additional phosphorus (P) and potassium (K). This gives rise to what is known as "nutrient demand."
Several consequences of nutrient demand exist. First, what constitutes an adequate supply of P and K in sand greens varies with the rate of N application. The greater the amount of N you apply, the greater the demand for P and K. We've documented this by noting the rate of "drawdown" of soil P and K when increasing annual rates of N were applied to creeping bentgrass (Figure 2, above).
Another consequence of nutrient demand is that when soil levels of P and K already are at levels that satisfy this demand, applying amounts serves no purpose. You can see this in Table 1, where applying P and K to bentgrass growing on soil well-supplied with these nutrients had no influence on their concentrations in the clippings. In short, once turfgrass demand for P and K is met, there is no further uptake, even when supplies are increased through fertilization.
Nutrient demand gives rise to a strong linkage among the concentrations of N, P and K in turfgrass shoots. In other words, the N:P:K ratio in the plant tissue is remarkably constant. It will vary somewhat with the time of year and weather, but on an annual basis, it stays close to 9:1:7 for a high-quality creeping-bentgrass putting green. In fertilizer terms, this equates to an N:P subscript 2O subscript 5:K subscript 2O ratio of 4:1:3.
Adjustment of soil P The first step in managing P on sand greens is to adjust soil levels so they satisfy turfgrass demand. We've found that the optimum soil-P level and the amount of fertilizer-P required to attain this level varies substantially with the composition of the root-zone mix. Of particular importance is the carbonate content of the sand with which the mix is blended. Higher carbonate levels reduce P availability. Thus, higher soil-P levels are required to satisfy turf's demand when carbonates are present. The results of our research on this are shown in Figure 2 (above). As you can see, the amount of soil P necessary to meet plant demand increases dramatically with increasing carbonate content in the root-zone mix. We conducted this research at an annual N rate of 5.0 lb./M. The optimum soil-test levels would have been lower at lower annual N rates.
Based on the data in Figure 2, I consider 35 ppm soil-test P to be a good target level for all sand greens. This avoids having to take into account the carbonate content of the sand. Of course, using this target level for sand greens with no or little carbonate will result in P levels higher than what the turf actually requires. In spite of the belief that high soil-P levels encourage invasion of bentgrass greens by Poa annua, we've found no relationship between the two.
It is crucial to understand that the 35 ppm optimum P level only applies when the soil is analyzed with the so-called Bray P-1 method. Other soil-test methods are being used around the United States, and each is unique regarding what test value represents an optimum level.
By far the best time for adjusting sand greens to the optimum soil-test P level is during grow-in. If your sand has significant levels of carbonates, this may require monthly applications of starter fertilizer at an N rate approaching 1.0 lb./M. For established greens with suboptimal soil-test P levels, you should consider using starter fertilizer for your first application of the year. If soil tests are very low, starter-fertilizer application should be repeated late in the season as well. Whatever you choose to do, make sure that you keep close track of your soil-test P until you've reached optimum levels.
Maintaining soil P levels After you've adjusted soil-test P to its optimum level, the task is to keep it there. How much P you need to apply annually to accomplish this goal depends on how much P you're removing in the clippings. This is where the concept of nutrient demand comes back into the picture. How much P you're removing depends on your annual N rate. Adjusting the annual P-replenishment rate according to your N rate is a simple matter. Employ the "magic" N:P subscript 2O subscript 5 ratio of 4:1. In other words, for every pound of N applied annually, apply 0.25 pound of P subscript 2O subscript 5 per thousand square feet.
Over time, this program will result in slowly declining soil-test P levels and an adjustment application of P will periodically be necessary. Occasional soil testing will tell you when this is necessary.
Adjusting soil K levels Sand putting greens pose special challenges in the management of K levels. Compared to native-soil greens, sand greens have a low capacity to bond K for two reasons. First, sand greens, even with organic amendments, have low cation-exchange capacity (CEC). What little CEC exists largely comes from organic amendments, but these typically compose just a small fraction of the mix. The second problem with K is that cation exchange sites in organic matter have a stronger preference for Ca and Mg than for K.
With the low CEC and preferential bonding of Ca and Mg in sand greens, there is a limit to the amount of K that you can load onto the cation exchange sites before leaching losses become excessive. Our research as well as others' indicates that when soil-test K levels rise much above 100 ppm, the concentration of K in the soil solution increases rapidly, and large amounts may leach from greens.
However, 100 ppm of exchangeable K is not enough to meet the full-season demand of turfgrass for K. This tells us that management of K in sand greens requires multiple applications of the nutrient during the season, which means you have the task of determining frequency and rates of K application.
To gain some idea of how often you may need to apply K, look at Table 2. In this study, we periodically applied K at the rate of 0.3 lb./M to greens with a variety of 80/20 root-zone mixes and recorded the effects on clipping-K concentration. If we assume that clipping-K levels should remain at or above 2.0 percent, these data tell us that at the 0.3 pound rate, you'd need to apply K subscript 2O monthly. Increasing the rate would likely allow you to stretch the application intervals, such that you might be able to get by with three or four applications per season.
Table 2 shows that Isolite (an inorganic amendment) in the root-zone mix resulted in higher concentrations of K in the clippings over a longer time. This reflects the fact that cation exchange sites on inorganic materials have stronger K-holding power than organic amendments and reduce K leaching loss correspondingly.
The ratio of N:K subscript 2O in bentgrass clippings is close to 4:3. This suggests that plant demand for K is about 0.75 pound K subscript 2O for every pound of N you apply. In addition to meeting plant demand, you must compensate for K leaching. We've researched this and have never found it necessary or useful to use a N:K subscript 2O ratio greater than 1:1 in sand greens with 80/20 root-zone mixes. However, if I were a superintendent with pure sand greens, I'd apply N and K subscript 2O in a ratio of as high as 1:2 to compensate for what would undoubtedly be a higher K leaching rate.
Considerable research exists on the effects of high clipping-K concentrations on turfgrass drought tolerance and susceptibility to winter injury. Some of this research shows increased drought and cold tolerance with increasing clipping-K concentrations. However, an equal or greater number of studies have shown little or no response to K applied for these purposes. The principle of nutrient demand suggests a good explanation for these conflicting results. Applying K to turfgrass in which K demand is already being satisfied does not significantly alter tissue concentrations of the nutrient.
To increase turfgrass tissue-K concentrations, an unsatisfied demand for K must exist, or you must create new demand through N application. Applying K alone or applying N and K late in the season (when turfgrass growth is suppressed by low temperatures) offers little chance of increasing shoot-K concentrations. To be effective, you must accompany late-season K applications by N and apply them when the turfgrass shoots are still growing.
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