Blueberry Fertilization Practices

Soil-building practices prior to establishment can go a long way toward providing the fertility necessary for a healthy blueberry planting. High levels of soil organic matter are especially important in blueberry culture, contributing to the soil's ability to retain and supply moisture to the crop, buffering pH, and releasing nutrients through decay. Soils rich in organic matter are also a desirable environment for symbiotic mycorrhizal fungi that assist blueberry roots in absorbing water, nitrogen, phosphorus, and other minerals.(Yang et al., 2002) Green manures in advance of planting can play an important part in cycling organic matter into the soil system, as can applications of composts and livestock manures. ATTRA has several publications that can be useful in these areas, including Overview of Cover Crops and Green Manures, Manures for Organic Crop Production, and Farm-Scale Composting Resource List.

Once a blueberry planting is established, supplemental fertilization can be applied in a number of forms and by several means. Generally, supplemental nitrogen is the greatest concern, followed by potassium. Blueberries have a low phosphorus requirement and typically require little, if any, phosphorus fertilization. In fact, excessive phosphorus has been one of the factors linked to iron chlorosis in blueberries. High calcium levels are also undesirable.

Nitrogen fertilizer recommendations vary somewhat from region to region. As a general guideline, 100 to 120 pounds of nitrogen per acre are commonly recommended on mulched berries; a reduced rate of 50 to 60 pounds per acre is advised where little or no mulch is used.(Clark, 1987) In conventional production, nitrogen is often applied in three split applications — one at bud break, followed by two more at six-week intervals. Adjustments may be necessary for less-soluble organic fertilizers. One rule of thumb suggests that these fertilizers be applied from one to four weeks ahead of the recommended schedule for soluble fertilizers. This allows additional time for the decomposition processes to make nutrients available. Applications after mid-July are discouraged, as they tend to promote late growth that is particularly sensitive to freeze damage. Table 2 shows natural materials used by organic growers for supplementary fertilization.

Table 2. Natural materials for supplementary fertilization.(Penhallegon, 1992, and Nitron, no date)

Material

Estimated N-P-K

Characteristics

Alfalfa meal

3-1-2

Slow to medium N release Good micronutrient source

Blood meal

12-1.5-0.6

Medium N release, 6-8 weeks

Cottonseed meal

6-2.5-1.7

Slow N release, 4-6 months

Feather meal

13-0-0

Slow N release, 4-6 months

Fish meal

10-4-0

Slow N release, 4-6 months

Soybean meal

7-1.6-2.3

Slow N release, 4-6 months

Compost

Variable

Analysis depends on feed stock

Fortified compost

Variable

Analysis depends on materials added

Current fertilization practices among organic growers vary considerably. In one example (Moore et al., 1994), an organic blueberry grower in the Missouri Ozarks applied % pound of feather meal per mulched plant in late May of the establishment year, followed by a similar application four to six weeks later. In subsequent years, an additional (third) application of % pound of feather meal was made earlier, in mid- to late-March. As the feather meal products available in this region contain roughly 13% nitrogen, this grower was applying approximately 141 pounds of actual N per acre in the establishment year, and an annual total of 212 pounds per acre thereafter.

Using the same schedule of split applications, another organic grower in the Arkansas Ozarks, also growing mulched berries, applies cottonseed meal (estimated at 7% N) at 1 pound per plant each time — that's two times in the establishment year and three times in subsequent years. At these rates, this grower is applying roughly 152 pounds per acre in the establishment year and about 229 pounds per acre in subsequent years.(Watkins, 1988) However, it should be noted that many sources of cottonseed meal are contaminated and will not be allowed in organic production. Contact your certifier first.

Associate professor John Clark (1987) at the University of Arkansas believes the fertilization rates used by many organic growers are probably excessive. Despite the slower release of organic-based nitrogen, the carry-over from previous seasons probably results in roughly the same amount of nitrogen released each season as is being applied.

Clark suggests that the best way to determine whether fertilization rates are "on target" is to test foliar nitrogen levels annually. This testing is done in late July or early August (in Arkansas) by sampling leaves from the mid-shoot area on fruiting canes and sending them to an analytical laboratory. Lab results showing nitrogen levels below 1.6% indicate a nitrogen deficiency; a level above of 2.2% indicates excess nitrogen. This service is available through Cooperative Extension in Arkansas and other states. Several commercial laboratories also provide foliar analysis. ATTRA identifies laboratories that offer various soil and plant tissue-testing services in its publication Alternative Soil Testing Laboratories.

Potassium for blueberries is often adequately provided through decaying mulches. The need for further supplementation should be determined by soil and/or tissue testing. Where additional potassium is needed, it can be applied in a number of mineral forms—including sulfate-of-pot-ash-magnesia or K-Mag,™ granite meal, and greensand. Some forms of potassium sulfate are also allowed in organic production. See your certifier before buying fertilizer.

High-quality compost is an all-around good blueberry fertilizer. Depending on the humus condition and biological activity in the soil, compost may provide all the fertility needs of the crop. Where compost is of average quality, it may still function as a good soil conditioner. Using aged animal manures in blueberry production also is possible, but less common.

Fertigation — the practice of injecting soluble fertilizers through drip irrigation lines—is a common practice in conventional blueberry production. Since fertigation is based on the complete solubility of fertilizers in water, there are limited options among organic fertilizers. Early attempts at fertigation with blood meal by Arkansas blueberry growers resulted in clogged emitters and algae growth. In the 1990s, however, researchers in California successfully demonstrated the use of spray-dried fish protein and poultry protein in drip systems.(Schwankl and McGourty, 1992) In addition, several organic liquid fertilizers—derived from fish emulsion, seeds, kelp, or seaweed—are available.

Unlike the roots of grapes and bramble fruits, which grow well into the inter-row area, blueberry roots are not very extensive. As a result, all fertilizers and acid-forming amendments must be applied under the plant canopy to assure that they reach the roots.

Foliar feeding of blueberries is practiced by some organic growers and is especially helpful when plants are stressed. Foliar fertilization programs usually employ seaweed and fish emulsion. The Ozark Organic Growers Association has recommended a seaweed-fish mix applied three times per growing season—at bud break, just prior to harvest, and just after harvest. More detailed information is available in ATTRA's Foliar Fertilization publication.

Organic Gardeners Composting

Organic Gardeners Composting

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