Modern blueberries (Vaccinium spp.), which grow in U.S. Department of Agriculture hardiness zones 3 through 7, are hybrids of the wild blueberry, native to North America. They have a somewhat shallow root system that lacks the fine root hairs of most plants, limiting the plant’s ability to absorb the required nutrients to help it grow. A beneficial relationship with mycorrhizal fungus, a soil organism, helps solve that problem. But overuse of fertilizer may deplete the fungus population, diminishing the plant’s ability to absorb important minerals, such as phosphorous. If your soil has a healthy population of mycorrhizal fungus, but it is phosphorous-deficient, using bone meal will help your blueberries grow.
About Bone Meal
Bone meal is made by steaming animal bones and then grinding them into a powder. At one time, bone meal was considered a complete fertilizer source. Dick Bir, retired horticulture specialist with North Carolina State University, tells "Fine Gardening" magazine that the manner in which it is processed today results in the removal of most of the nutrients. It is still a good source of phosphorous for depleted soil.
Linda Chalker-Scott, extension horticulturist with the University of Washington, claims that most non-agricultural soils have sufficient phosphorous and that too much of the element can cause problems. One example of this is that too much phosphorous blocks the beneficial relationship between the plant’s roots and mycorrhizal fungus. Not all plants require this relationship, but it’s an important one for blueberries. The plant then has to expend more energy into root growth, at the expense of the rest of the plant. For these reasons, it is imperative to test your soil before applying bone meal.
Soil test results are your best guide as to how much phosphorous to add to the soil. A general rule of thumb calls for 2 pounds, or 6 cups, per 100 square feet of soil. To keep the bone meal from burning the blueberry plant’s roots, mix it thoroughly into the top 6 to 8 inches of soil and then water the soil to a depth of 12 inches. Because bone meal is a slow-release nutrient, add it to the soil six months before planting blueberries.
Although they aren’t considered organic, superphosphates -- which have a 0-20-0 analysis -- and triple super phosphates -- which have a 0-46-0 analysis -- supply phosphorous quicker than bone meal. These products are manufactured by subjecting the minerals to acid. Use 1 1/4 pounds, around 2 1/2 cups, of superphosphate or 1/2 pound, around 1 cup, of triple superphosphate for 100 square feet of soil. Always re-test the soil before planting blueberry bushes, whether you use bone meal or superphosphate.