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Issue link: http://gcmdigital.gcsaa.org/i/234582

Expected leaf nutrient content Nutrient Expected % in leaf dry matter Amount in proportion to nitrogen Nitrogen 4 1 Potassium 2 0.5 Phosphorus 0.5 0.125 Calcium 0.5 0.125 Magnesium 0.2 0.05 Sulfur 0.2 0.05 Table 2. Expected leaf nutrient content and proportion relative to nitrogen. These values are suitable as a starting point for most turfgrass species. If site-specifc data are available, those values can be substituted to further refne these calculations for a particular site. From 2006 to 2009, more than 50 varieties of warm-season grasses were grown at the Asian Turfgrass Center research facility north of Bangkok in soils with nutrient levels below the conventional soil guidelines, yet the turf still met all performance goals. test is 52 ppm (2.4 pounds). The amount required as fertilizer is the difference between the amount required (68 ppm or 3.1 pounds) and the amount actually present (52 ppm or 2.4 pounds), which comes to 16 ppm or 0.7 pound. Thus, the fertilizer requirement for potassium in this situation using the MLSN guidelines is 1 pound potassium/1,000 square feet. 134 GOLF COURSE MANAGEMENT 01.14 Example 2: Magnesium If the soil test level for magnesium is 75 ppm and we plan to apply 3 pounds of nitrogen/1,000 square feet in the upcoming year, how do we determine the magnesium requirement to ensure we stay above the MLSN guideline for magnesium of 54 ppm? As shown in Table 2, we expect the grass to use 20 times more nitrogen than magnesium. That is, we predict the grass will use 0.15 pound of magnesium/1,000 square feet (3 pounds nitrogen * 0.05 = 0.15), which is equivalent to a depletion of 0.15 * 22 = 3.3 ppm from the soil. We want to keep the soil at or above the MLSN guideline, so the total amount of magnesium required is the plant use (3.3 ppm or 0.15 pound) added to the amount we want to ensure remains in the soil (54 ppm or 2.5 pounds). In our example, this is 57.3 ppm or 2.6 pounds. The amount on the soil test is 75 ppm (3.4 pounds). The amount required as fertilizer is the difference between the amount required (57.3 ppm or 2.6 pounds) and the amount actually present (75 ppm or 3.4 pounds). Because the amount present is more than the amount required, we do not need to apply any magnesium to keep the soil above the MLSN guideline. How the guidelines were developed We started with soil test data from the PACE Turf database. This consisted of data from more than 17,000 individual soil samples, each drawn from a stand of turf that was performing well. Because the data in those samples were from sites where turf performance was good, we could expect that whatever the nutrient levels were at those sites, those levels would be suffcient to produce turf that performed well. Then we fltered the data, selecting only the data from sites with a cation exchange capacity (CEC) less than 6 cmolc /kilogram. This flter removed all the soils with high nutrient-holding capacity from the working data set. We wanted to look at only the soils that had a relatively low nutrient-holding capacity, yet still produced good turf conditions, to investigate and identify the individual nutrient levels in those soils. For the MLSN guidelines, we assume that if there is enough of an element to produce good turfgrass in a low-nutrient-holding soil (such as a sand root zone from a golf course putting green), then the same amount of that element will be suffcient to produce good turfgrass conditions in a more nutrient-rich soil that has a higher CEC. We think that if there is enough of an element to produce good turfgrass in a sand root zone on a golf course putting green, then the same level of that element in a soil-based green or on a golf course fairway will produce good turfgrass as well. We added one more flter to the data. This was for pH. We selected only those samples

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