I have about 120 blueberry bushes that I am still trying to lower the soil acidity. I would like to apply ammonium sulfate this year and need to know the best time(s) to apply. Thank-you, Dean
Alpena County Michigan
Blueberry Fertilizer Recommendations for New Growers
Mark Longstroth, Small Fruit Extension Educator
Generally, in small plantings of blueberries the soil pH is too high for blueberries, which prefer a soil pH of 4.5 to 5.5. Often the soil nutrient content is good for all nutrients but nitrogen, so you should provide nitrogen alone, the only nutrient the plants really need.
Blueberries only use the ammonium form of nitrogen found in urea (46-0-0), ammonium sulfate (21-0-0) or the ammonium phosphates (MAP, 11-48-0; DAP, 18-48-0 or APP, 15-62-0). Michigan State recommends fertilizing with ammonium sulfate (21-0-0) when the soil pH is 5 or above. Ammonium sulfate is more acidifying than urea (46-0-0). The ammonium phosphate fertilizers also work well when the soil pH is high, especially if the soil phosphorus is low. Do not use the ammonium phosphates if the soil phosphorus is above 300 ppm. Urea is recommended when the soil pH is below 5.
In commercial plantings fertilizers are spread across the acreage using a fertilizer spreader. The nitrogen recommendation for commercial blueberries is for 50 to 70 pounds of actual nitrogen per acre (238 to 333 pounds of 21-0-0). For small plantings it makes sense to apply the fertilizer on an individual plant basis. Since blueberries are generally planted 3 feet apart in rows 10 feet wide there are 1,452 plants in a commercial field. These plants would receive 0.5 to 0.8 ounces of actual nitrogen. This works out to 2.5 to 4 oz. of 21-0-0 per plant.
For example, if I were to apply 100lb/acre of a fertilizer mix I would apply one oz/plant.
100#/acre 0.068 lb. X 16 oz equals 1.01 oz
1452 plants/acre plant 1 lb. plant
My nitrogen recommendation is for one ounce of 21-0-0 for each year age of the plant. Apply the fertilizer in a loose ring around the plants about 6”to 8” away from the crown of the plant. Too close will burn the roots and kill the plant. Next year apply 2 oz. (2 ounces); One ounce for each year age of the plant.
1 year =1 oz, 2 year = 2 oz, 3 year = 3 oz, 4 year = 4 oz.
Most growers apply fertilizer in the early spring. We recommend applying the fertilizer two time, half as early as possible in the spring and the second half before bloom as the leaves are emerging.
A healthy well-pruned blueberry plant should be growing new shoots that average 8 to 16 inches long each year. Bushes that are not pruned will grow a lot less, since the growth is diluted among many shoots. Plants where most of the shoots are over 12” long are vigorous and the fertilizer rate should be reduced. Half the fertilizer should be applied in the early spring when the buds begin to grow and half at bloom.
Other Cultural Recommendations
Breakup the root ball when planting! Many times, I see plants with poor growth where the roots are still in the peat ball as the plant came from the nursery. Growers are amazed when I reach down and easily pull the plant out of the ground to show them that there are no roots out into the soil. Break up the root ball so the plants roots have to grow out into the soil. Removing and replanting stunted root bound plants is worthwhile if they come out of the ground easily.
Dead or diseased wood should be pruned off at planting and early each spring. Phomopsis twig blight is a common disease in Michigan and removing infected wood helps keep the planting clean.
Prune off weak spindly growth and remove flower buds in the spring until the plants are about 4 years old. Early in the life of a blueberry plant we want to grow a large bush and not harvest any fruit. Fruit will stunt the plant’s growth. In the fourth year, the plant should be large enough to grow a crop worth the effort to spray and pick.
Try to encourage new growth from the base of the plant. Vigorous pruning of old canes that show little growth should cause new shoots to grow from the base of the plant.