Answer: According to Washington State University, Blueberries have a shallow, fibrous root system, so they're susceptible to drought injury. A uniform and adequate supply of water is essential for optimum growth. If this isn't supplied by natural soil water or rainfall, then you must irrigate. On the average, plants need 1 inch of water per week. Check the soil frequently for adequate moisture and irrigate if necessary.
Apply 10-10-10 fertilizer annually in the spring at the rate of 2 ounces (1/4 cup, or equivalent rate of another well-balanced fertilizer) per plant the second year after planting, increasing by 1 ounce each year until you reach a total of 6 to 8 ounces (3/4 to 1 cup) per plant. If the soil is quite fertile, an application of 5 ounces of ammonium sulfate (21-0-0) per mature plant is sufficient.
Apply this fertilizer at the time the buds are swelling. Spread it evenly around the plant, over an area approximately equal to that of the maximum spread of the bush, without touching the base of the canes.
The first 2 or 3 years after you start mulching, plants may become pale green, because much of the soil nitrogen is used by organisms that decompose the mulch. Therefore, it's often necessary to fertilize each plant with 1 to 2 ounces of ammonium sulfate in early May and again in late June to avoid nitrogen deficiency. In general, you should avoid fertilizing after July 1.
Check the soil pH every year or two, especially if growth is poor. If the pH is above 6, you can apply sulfur to the surface of the soil or mulch at the rate of about 1 ounce of elemental sulfur or 6 ounces of aluminum sulfate per plant. Water or lightly rake the sulfur into the soil or mulch. (It's best to delay this sulfur application for about a month after applying fertilizer, to avoid possible burning of blueberry roots.)
Ammonium sulfate fertilizers used over a period of years gradually will lower the pH. However, if you use nitrate fertilizers, you tend to raise the pH.
Remember that your visual assessment of plant growth and fruiting is extremely useful in a good fertilizer program. If the plants are growing well (10 to 12 inches of new growth each year) and if you obtain average yields, there's no need to worry about whether plants are getting an adequate amount of nutrients.
Hope this answers all your questions about growing blueberries!
Whenever you fertilize, use either ammonium sulfate (21 percent nitrogen) or a well-balanced fertilizer containing potassium sulfate. These fertilizers gradually will lower the pH of the soil. Blueberries grow best if the soil pH (a measure of acidity) is between 4.5 and 5.5.
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