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| Department of Agriculture and Aquaculture |
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D.2.0 INTRODUCTIONWild blueberry plants produce commercial crops from what would be considered some of the poorest fertility agricultural land on the planet. These are acid (or "sour") soils with low levels of the elements which are considered important for plant growth, and high levels of the elements which are considered undesirable for plant growth. While most other agricultural plants would grow very poorly in the soils in which wild blueberry crops are produced, wild blueberries have a number of adaptations which allow them to survive and even thrive in these environments. Despite these adaptations, the crop still seems to benefit from the wise use of synthetic and organic fertilizers.
FACTORS AFFECTING THE BASE FERTILITY OF WILD BLUEBERRY FIELDS Wild blueberry plants are present on a wide number of soil bases in New Brunswick. There are a few pockets of barrens in the province, and many of these are being managed for wild blueberry production. The southern barrens of the Pennfield region have sandy loam soils of various history, with relatively high levels of slowly-decomposing organic matter, and medium levels of the major soil nutrients. By contrast, the barrens of the northeastern part of the province tend to have much lower levels of organic matter and lower base soil fertility. The wild blueberry plants share these barrens with a few other plant species, including jack pine. There are a few other barrens present in other areas of the province, primarily in the central and eastern counties, but they are less widespread. Barrens tend to require greater quantities and more tailored input of fertilizers. In general, fields originating from hardwood or mixed forests are richer than those originating from softwood forests. This is due to the nature of the decomposing material and due to the fact that the base soil of the hardwood forest is richer. By the same token, wild blueberry fields originating from abandoned hay fields tend to be richer than those abandoned from pastures, since the hay fields tend to have been richer fields to begin with, and because they are more likely to have benefited from better field management. In addition, hay fields also benefit from having been plowed more often, thereby distributing the organic pad throughout the soil profile. This is probably also one of the explanations underlying the superior yielding capacity of wild blueberry fields which originate from abandoned agricultural fields, versus those originating from cleared forests. As a rule, the yield potential of wild blueberry fields with more fertile soils is higher, though the ability to improve these fields through fertilizer use is made a little more complex by the fact that weed control must be complete. This is because many of the weed species are very responsive to fertilizers. COMPONENTS OF SOIL FERTILITY There are several components associated with soil fertility and knowledge of these components is useful in measuring the fertility status of a wild blueberry stand and in developing fertilizer recommendations. Four of the most useful measurable components are: Soil organic matter, soil acidity (pH), macronutrients, and minor nutrients. 1. Soil organic matter. This refers to that component of the soil of which the origin is not mineral, rather it consists of material of biological origin which has achieved a certain level of decomposition. Because the mineral base of wild blueberry soils is of relatively low fertility, organic matter can be an important long-term nutrient store for the crop. A 1992 survey in New Brunswick revealed organic matter levels between 2.5 and 12%. The lower levels are associated with many soils of the Acadian Peninsula, Kent County and central New Brunswick. Because burning wild blueberry fields generally has a negative long-term impact on organic matter levels, it is desirable to favor mowing over burning on fields with organic matter levels below 3. Burning every second or third cycle should help to moderate any increase in disease or insect incidence created by mowing, as explained in fact sheet C1.5.0. 2. Soil acidity (pH). Soil acidity, or the "sourness" of a soil is often expressed as a number called the pH. The pH scale runs from 1 to 14, with 1 being very sour and 14 being the least. As an example, pure water generally has a pH of about 7. The pH found in New Brunswick wild blueberry soils ranges from 3.9 to 5.3. Most plants other than wild blueberries have trouble tolerating such acid soils, because the acidity results in poor nutrient availability and the accumulation of elements toxic to most other plants. Older literature from the State of Maine recommended raising the pH of such soils so that the wild blueberry plant could grow better and yield more. Research in New Brunswick has failed to demonstrate this effect, but this may need further validation. 3. Macronutrients. Macronutrients are those which are found in relatively large quantities in plants. These include the "big three" often used in common fertilizer formulations (e.g.17-17-17 or 18-46-0), which are: Nitrogen, Phosphorus, and Potassium (also referred to as N-P-K). Macronutrients also include Calcium (Ca), Magnesium (Mg) and Sulfur (S). The role of some of these are thought to be very important in wild blueberry nutrition.
b) Phosphorus: Phosphorus is also an important plant nutrient, especially in the most active growing plant parts like stem tips, flowers, seeds and young roots. It is probably the only deficiency that can be visually diagnosed in wild blueberry fields, as it is characterized by premature leaf reddening, purple blotches, and what is thought to be a predisposition to powdery mildew (refer to fact sheet C3.3.0). Phosphorus availability should be very low in wild blueberry fields, since this element tends to be tied up in acid environments. However, it is thought that phosphorous nutrition in wild blueberry plants is assisted by the plant's association with mycorrhiza, which is a fungus living on the roots of the blueberry plant. The mycorrhiza provides the plant with elements like phosphorus in "exchange" for the energy provided by the plant's photosynthesis products. Not a great deal is known about the mycorrhizal relationship between wild blueberry plants and mycorrhiza, though it seems fairly safe to assume from studies on this plant and others, that : the lower the nutrient status of the field, the more important is the mycorrhizal association; that the mycorrhiza are encouraged by a good nitrogen status in the soil; that the mycorrhiza are less active when a lot of phosphorous is available; and that mycorrhiza may also be important in providing the plants with other nutrients and with conferring some degree of disease resistance. Phosphorous is a common component of fertilizer recommendations, and has shown to be useful as a "pickup" fertilizer in "burned out" fields in New Brunswick. It may also prove to be useful when plants have undergone a lot of stress like land leveling or winter damage to roots. c) Potassium. Relatively little research work has been done on potassium nutrition of wild blueberries, though it might be assumed from work on other crops that the nutrient plays some role in winter hardiness and storage organs like the rhizomes. The presence of this plant nutrient is always lower in sandy soils, so it can also be assumed that potassium may sometimes be low in wild blueberry fields, and leaf diagnosis often bears this out. OTHER DISTINCTIVE FEATURES OF WILD BLUEBERRY NUTRITION Pruning provides some unique features to the wild blueberry nutrient cycle. Pruning by burning has been shown to release nutrients to the plants, particularly when burning takes place in the spring. However, it is also clear that repeated burning removes nutrients from the plant-soil system because some nutrients may escape through the air or by leaching. Mowing, by contrast, conserves more nutrients, and may even contribute to organic matter. Nonetheless, it is quite common to fertilize a stand after mowing to avoid having short plants. Another pronounced feature of wild blueberry nutrition is the residual impact of fertilizers. Because wild blueberry soils tend to have low rates of nutrient conversion, the impact of fertilizers have been measured in research plots for years after their application. DIAGNOSING THE NUTRIENT STATUS OF WILD BLUEBERRY FIELDS Some fields reveal poor nutrient status by the short stature of the plants and the smallness of the leaves, but sometimes this can only be appreciated by someone who has seen several other fields which are in better shape. While premature reddening can also reveal nutrient deficiencies, there are a few other conditions like winter damage to roots that can cause plants to look unhealthy. A much more reliable tool for assessing the nutrient status of a field is leaf tissue analysis. When leaf tissues are sampled at "tip dieback" in the sprout year (refer to fact sheet D1.0), the nutrient content of the leaves are compared to the standards reported in Table 1. This allows the agrologist to recommend appropriate fertilizers. If leaf nutrient levels are monitored over several cycles, it can also allow an agrologist to judge the impact of the fertilizer regime, and develop appropriate long-term strategies. This is desirable from both an economic and an environmental perspective.
STUDIES ON THE FERTILIZATION OF WILD BLUEBERRY FIELDS
Prepared by John Argall, P. Ag., Blueberry Specialist, Gaétan Chiasson, P. Ag., Horticultural Specialist, Yves Martineau, P.Ag., Soil Specialist, N.B. Dept. of Agriculture & Rural Development. | |||||||||||||||||||||||||||||||||||||||||||||||
