Farmers and gardeners learn much by daily tending soils and plants. But the winter “off-season” affords us time to dig deeper into topics of interest and learn from co-cultivators and experts in the field. I spent some time in a variety of grower meetings, conferences, and webinars in January and February where research findings were shared. Many gardeners are interested in growing blueberry so in this article I’ll share tips for success including some insights I picked up from presentations by Oregon blueberry researchers David Bryla, Ph.D. (USDA) and Bernadine Strik, Ph.D. (Oregon State U.)
Highbush blueberry plants evolved to grow in low pH, high organic matter sandy soils with high water tables. These soils contain more ammonium nitrogen than nitrate nitrogen, hence blueberry’s preference for the ammonium form of plant-available nitrogen. The shallow, fibrous root system grows almost entirely in the top 12 inches of soil. Most of the roots are very fine, the width of a human hair, and can’t penetrate or thrive in clayey, compacted soils. The key to success is create garden conditions that mimic those in blueberry’s natural environment.
Blueberry thrives in well-drained, porous soils, high in organic matter (4% – 20%). The soil pH should be in the 4.5-5.5 range.
Soil preparation starts in fall
- Begin by testing the soil in the late summer or fall prior to spring planting. For gardeners, soil testing labs provide the most accurate pH measurement of your soil, as well as baseline information on organic matter and nutrient levels. pH probes sold to gardeners are generally inaccurate and pH color kits using litmus paper are only accurate to ½ of a pH unit (5.5, 6.0, 6.5, 7.0, etc.
Add organic matter
- The top 12 inches of soil should be one-third to one-half organic matter by volume. Peat moss (3.0-4.5 pH), plant-based compost (7.0-7.5 pH), and lightweight potting soil, a.k.a. soilless growing media (5.5-6.5 pH) are the materials most often mixed into the soil. Research has shown that adding compost (especially animal manure compost) can increase soil pH.
- Some Oregon growers incorporate 2-3 inches of aged softwood sawdust into topsoil prior to planting. The benefit is that sawdust has a low pH, decomposes slowly, and increases organic matter levels. For Maryland gardeners, large amounts of sawdust are difficult to come by, but bark fines are readily available. You would need to apply 1.0 lb. of additional ammonium sulfate (21-0-0) per 100 sq. ft. nitrogen for the soil microbes that slowly decompose the bark fines.
Lower soil pH
- Elemental sulfur is applied (based on soil test results) in the fall prior to spring planting, and incorporated to a 6-8 inch depth.
- Pelletized and prilled forms of sulfur are easier to apply than powdered sulfur but take longer to lower soil pH.
- An oxidation process, driven by special soil bacteria, converts the sulfur to sulfuric acid, releasing hydrogen ions that lower soil pH. The bacteria are most active in warm, moist soils. The process takes 6-12 months. Iron sulfate can also be used to lower soil pH but 6 times as much is required, increasing the cost.
- Re-test soil pH to monitor pH levels and apply sulfur as needed to maintain the 4.5-5.5 range.
- For container blueberry plants, mix 3 TBS. of sulfur into the top few inches of growing media, for a 15-gallon container, to reduce the pH by one unit (e.g. from 7.0 to 6.0).
- Ammonium sulfate fertilizer is recommended because it supplies nitrogen in the ammonium form and helps acidify the soil.
- Fertilize at full bloom and again three weeks later.
- Urea is another good nitrogen source, recommended when soil pH is below 5.0 because it is only one-half as acidifying as ammonium sulfate. The nitrogen in urea is converted to ammonia and then to ammonium.
- Oregon research studies show that feathermeal (12-1-0.5) and soluble fish fertilizers (4-1-5) work well in organic blueberry production. Organic growers prefer to inject fertilizers into irrigation water, known as “fertigation.” Another interesting finding was that there were no significant yield differences between the lowest (20 lbs./acre) and highest (240 lbs./acre) nitrogen fertilization rates.
- Organic matter and organic fertilizers release ammonium ions with relatively little oxidized to the nitrate form as long as soil pH is in the 4.5-5.5 range. When soil pH is >6.0 most of the nitrogen from decomposing organic matter will be converted to the nitrate form with negative effects on plant growth.
- Oregon research indicates that organic acids (humic and fulvic) applied in liquid form, increase blueberry root growth while lowering soil pH.
- Blueberry root systems need to be kept moist. Plants can tolerate hot weather but not drought. Water your blueberry bed thoroughly and consistently when rainfall is lacking. Soaker hoses and drip irrigation work well.
- Blueberry grows and produces best when the pH of irrigation water is <7.0. Commercial growers often acidify irrigation water to maintain low soil pH. The pH of municipal water in our region is typically 7.5-7.8 and has a high salts and bicarbonate content. Just be aware that your irrigation water can drive up soil pH.
- Blueberry roots cannot compete very well with weeds for nutrients and water. Mulch is essential to keep soil cool, improve water infiltration, conserve soil moisture, reduce weeds, and increase organic matter.
- Use aged wood chips (never fresh), shredded bark, pine needles, or sawdust as a mulch. These materials are low in pH (4.5-5.2) and salts, and decompose slowly.
- Interestingly, a recommended growing system in Oregon uses strips of heavy-duty weed barrier to cover beds after they have been mulched to further reduce weed growth and moisture loss.
A well-planned and maintained blueberry bed can produce well for 20+ years. Start yours in 2021!
Lowering Soil pH for Horticulture Crops. Purdue Extension
Organic Blueberry Research– eorganic.org