Clues to Boron ' s Purpose

As we examine the role of micronutrients in turfgrass nutrition, we enter murky waters when we consider the functions of boron. There is a scarcity of specific research on turfgrasses, so there remain questions about its roles. Since this subject has been reviewed recently (Blevins and Lukaszewski 1998), we interpret the current state of knowledge about boron's importance to turfgrass management. Boron is required by turf in extremely small amounts and normally is present in dry plant tissues in a range of 6 parts per million (ppm) to 30 ppm (Table 1). Jones (1980) proposed a sufficiency range for boron in turfgrasses of 10 ppm to 60 ppm. Table 1 demonstrates, however, that boron levels of less than 10 ppm are commonly observed in turf with no deficiency symptoms evident. It's also doubtful whether 60 ppm boron is a critical concentration for any turfgrasses. Plants differ in their boron requirements, with grasses generally having a much lower demand than dicotyledonous plants. Marschner (1995) reports a critical boron concentration range for grasses of 5 ppm to 10 ppm while that for broad-leaved plants ranges between 20 ppm and 70 ppm. Some latex-producing plants have an exceptionally high boron requirement in the 80 ppm to 100 ppm range. Turfgrasses will respond to small amounts of boron (1.5 pounds/acre to 7.5 pounds/acre) with color, root growth and stand density all improving for five weeks following application (Deal and Engel 1965). Because boron deficiency symptoms can be subtle, slowly affecting meristems of roots and shoots preferentially, an insufficiency may be difficult to spot. As is the case for most micronutrients, turf grown on sand-based media may suffer mild boron deficiency without the manager ever noticing or recognizing the problem. Thus, it's prudent to know something about boron and how to tell if this essential element might be lacking. Boron in the soil Soils generally contain relatively little boron. The earth's crust averages about 10 ppm boron with igneous rocks containing 5 ppm to 15 ppm and sedimentary shales as much as 100 ppm. Most boron containing minerals are weakly soluble, so the release of soluble boron through weathering is slow. Boron can form complexes with clays and amorphous soil minerals and also become incorporated into organic matter. Hence, the 7ppm to 80 ppm present in most soils, less than 5 percent is in the soil, where it's available to plants.