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

Penn A-1 creeping bentgrass grown in soils with decreasing levels of potassium from left to right; keep- ing soils at or above the MLSN guideline provides a level of safety that such defciency symptoms will not occur. This creeping bentgrass green at Takarazuka GC near Osaka, Japan, has calcium, magnesium and potassium levels not only below the conventional guidelines, but also below the MLSN guidelines, yet still has produced excellent turfgrass conditions since the soil was frst tested in 2009. To answer the second question, regarding how much of an element to apply, simply add enough of that element to keep the soil at or above the MLSN guideline at the end of the growing season. To calculate that, compare the soil test result to the MLSN guideline and to an estimate of how much of that element the grass will use. To try the MLSN approach, you will need some recent soil test results from tests done using the Mehlich 3 extractant. You will also need an estimate of how much nitrogen will be applied to your turf in the upcoming year. Because nitrogen controls the uptake of other nutrients (5), we can use the nitrogen estimate to predict the grass's use of other elements. Before making the calculations, we will make some assumptions about grass growth and the relationship between fertilizer applied to the two-dimensional soil surface and the soil test levels within the three-dimensional root zone. These include: • The grass cannot use more of an element than it harvests. • The growth and consequently the nutrient uptake are determined by the amount of nitrogen applied. • The concentrations of macronutrients and secondary nutrients in the leaves will be estimated as in Table 2. Minimum Levels for Sustainable Nutrition guidelines Nutrient Analytical test Conventional guideline (ppm) MLSN guideline (ppm) Phosphorus Mehlich 3 >50 18 Potassium Mehlich 3 >110 35 Calcium Mehlich 3 >750 360 Magnesium Mehlich 3 >140 54 Sulfur Mehlich 3 15-40 13 Table 1. Minimum Levels for Sustainable Nutrition (MLSN) soil guidelines for macronutrients and secondary nutrients. A full copy of the current MLSN guidelines is available at www.paceturf.org/PTRI/Documents/1202_ref.pdf • One gram of an element spread over 1 square meter on the surface is equivalent to 4.4 ppm of that element in the root zone of 1 square meter to a 15-centimeter depth, and vice versa. • One pound of an element spread over 1,000 square feet on the surface is equivalent to 22 ppm of that element in the root zone of 1,000 square feet to a 6-inch depth, and vice versa. Example 1: Potassium Let's say the potassium soil test level is 52 ppm, and we plan to apply 3 pounds of nitrogen/1,000 square feet in the upcoming year. How do we determine the potassium requirement to ensure we stay above the MLSN guideline for potassium of 35 ppm? As shown in Table 2, the grass is expected to use half (0.5) as much potassium as it does nitrogen. That is, we predict the grass will use 1.5 pounds of potassium/1,000 square feet, which is equivalent to a depletion of 1.5 * 22 = 33 ppm from the soil. Because we want to keep the soil at or above the MLSN guideline, the total amount of potassium required is the plant use (33 ppm or 1.5 pounds) added to the amount we want to ensure remains in the soil (35 ppm or 1.6 pounds). In our example, this is 68 ppm or 3.1 pounds. The amount of potassium in the soil 01.14 GOLF COURSE MANAGEMENT 133

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