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Fertility 101

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A healthy blackberry planting will produce higher yields, be more competitive for water and nutrients, and be more resistant to pests and diseases. One way to guarantee the health of plants is to make sure the nutritional needs of the plant are met. Nitrogen is the main nutrient needed by the plant, but it is not the only nutrient needed (Table 2-1). The optimum time for fertilizer applications depends on factors such as the soil type, the crop, the nutrient, and the climate. For example, nitrogen fertilizers are often a brief and limited source of nutrients because of the mobility of nitrates, which mostly dissolve in soil water. The same happens with the nitrogen provided by microorganism feeding after applying high levels of organic matter. Also, climate and temperature influence nitrogen availability because microorganism populations increase nitrification in spring. Therefore, crops respond positively after nitrogen applications in spring.

Perform a soil test at least one year before planting to allow pH adjustment to 6.5 and nutrient amendment. Biennial soil and foliar testing can provide valuable information by identifying nutrient deficiencies and pH imbalances. Base all fertilizer and soil amendment applications in an established planting on foliar and soil analysis results. This practice will eliminate problems with nutrient imbalances and save money because fertilizer is only applied when needed. Table 2-2 gives the sufficiency ranges for the various elements in blackberry leaves. For foliar analysis, collect 50 to 100 mature leaves from primocanes in the section six to ten nodes from the terminal in mid-to-late July. Contact your local Extension agent or fruit specialist for more information on how to collect samples and where to send them.

Nutrient availability in the soil is dependent on pH, soil type, moisture content, nutrient mobility, and nutrient concentration. Most nutrients are available between a pH of 6.0 to 7.0. Iron, boron, copper, manganese, and zinc are unavailable in soils with a high pH. Calcium, potassium, magnesium, and molybdenum deficiencies, as well as aluminum, manganese, and iron toxicity, are common in acidic soil. Heavy nitrogen fertilization will lower a soil’s pH over time, increasing its acidity. Certain soil types have a greater ability to hold and supply nutrients because of a high cation exchange capacity (CEC). Soils high in clay and organic matter have a high CEC, while sandy soils have a low CEC. Sandy soils require higher fertilization rates because of the lower CEC. Many nutrients rely on water to move them towards and into the roots, so adequate soil moisture is necessary for uptake.

The interaction among elements is also important. For example, high levels of phosphorus can lead to zinc deficiencies, so take care when using poultry litter, which is high in phosphorus, as a fertilizer. High potassium concentrations will limit magnesium uptake, resulting in deficiency. Excess nitrogen can induce calcium, potassium, and magnesium deficiencies.


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Nitrogen (N)

Nitrogen is one of the most important elements for growth of most plants. It has a dynamic cycle in the soil, water, and air and is usually the element in the lowest levels in the soil compared to crops’ needs. Generally, canberries need more N than soil can provide to support plant growth and fruit production, and nitrate is preferred instead of the ammonium N form. Because it is the nutrient element most demanded for blackberry cultivation, N is the focus of any fertilization program. Erect blackberries require about 25 pounds per acre in the first year of the planting. In the second year, they require about 40 pounds per acre, and they require about 50 pounds per acre in year three and all following years.

Manures and Other Organic Materials for Nitrogen

Manures and composts are good sources of N and organic matter but have varying concentrations of N, P, and K. Because of the varying numbers, an accurate recommendation rate is difficult to make. Only half of the N in manure or compost will be available to the plant in the year it is applied. The remaining N will be released in subsequent years. Account for this holdover when figuring fertilization rates. Ideally, apply manure during late fall or winter to allow time for adequate decomposition. Fresh manure cannot be applied within 120 days of harvest if fruit will come into contact with the ground or is likely to be splashed with soil. Apply fully composted manure at anytime and at higher rates than uncomposted manure. Another advantage of using composted manure rather than fresh is that the composting process breaks down many of the weed seeds present in fresh manure.

Cottonseed meal (7-2-2) is a predictable source of N that is easy to spread within a row. Blood meal is a more expensive alternative, but it has higher N levels (12-1.3-0.7) and is readily available to the plants. In order to apply approximately 60 pounds of N, use 860 pounds of cottonseed meal or 500 pounds of blood meal per acre. Feather meal (10% N) can be used at 500 pounds per acre. If the aisle cover crop is contributing N to the soil, this should be taken into consideration before applying supplemental N.

Beyond Nitrogen

All nutrients other than N should only be added as needed, based on soil and foliar analysis:

Phosphorus (P) is an important macronutrient for caneberry commercial production because it is required in relatively large amounts, and it often becomes deficient in commercial orchards. However, compared with other crops, caneberries require low levels of P. Excessive P can obstruct micronutrients uptake. Thus, base accurate P2O5 applications on leaf analysis, soil pH and nutrient content, CEC, the desired yield, and other factors. Soil pH directly impacts P availability. Phosphate ions react with carbon and magnesium in alkaline soils and with aluminum and iron in acidic soils, generating few soluble substances. Phosphorous does not have good mobility in soil, so surface banding of P is not as effective as subsurface banding. The development of roots is necessary for P fertilization in caneberries because roots absorb this nutrient from soil solution. Increased plant root mass will aid P uptake. Due to its immobility in the soil, preplant application and incorporation of P, if needed, may satisfy the needs for this element for the life of the planting.

Potassium (K) is utilized to transport nitrates from roots to leaves and to regulate stomata for proper gas exchange (carbon dioxide, water vapor, and oxygen) with the atmosphere. In caneberries, base proper K2O applications on leaf and soil analysis and soil parameters because excessive amounts of banded K may burn new roots, especially in sandy soils. Uptake of K occurs essentially through diffusion, so root mass is needed to improve K plant uptake. Soil chemistry affects the availability of K. Soil supplies K due to its cation exchange capacity (while clay particles are negatively charged, K cations are positively charged). Potassium becomes more effective when it is broadcast into soils before plants are established. Potassium is mostly required during fruit development and affects fruit quality. Adequate K content in the plant is usually reflected in appropriate fruit firmness. In caneberries, no relationship has been found between K content in soil and K levels in leaves. Potassium content fluctuates in leaves during the growing season, and it decreases as fruiting increases.

Calcium (Ca) is an essential nutrient for cell wall membrane structure, permeability, and for several physiological processes. Calcium is usually present in sufficient amounts in both soils and in plant tissue, and it is rarely applied to blackberries. Calcium levels in blackberries are between 0.2% and 1.0%. Calcium deficiency is not commonly observed. Foliar Ca applications work well when corrections are needed. In ‘Cheyenne’ blackberries, Ca fertilization increased plant growth after two growing seasons.

Magnesium (Mg) is essential for chlorophyll production and N metabolism. Plant concentration of Mg ranges from 0.1% to 0.4%. Studies have demonstrated that leaf Mg concentration is positively correlated with Mg content in the soil. Through leaf analysis, various types of relationships have been observed between Mg and other nutrients. For example, leaves with a high Mg foliar content show lower Ca levels.

Sulfur (S) and N are both key components of proteins. Sulfur in the sulfate form (SO4) is moderately mobile in soil. Deficiency symptoms are similar to those of N deficiency. Sulfur applications are usually not required; however, if it is needed, 20 to 50 lb S/acre could be sufficient for making nutritional corrections. Overall, proper amounts of plant S concentration are between 0.1% and 0.5% with a common N:S ratio of 15:1.

Boron (B) iis important for auxin activity. In caneberries, B is necessary for bud break and fruit. Boron promotes plant growing tips and roots. When it is deficient in soils, roots do not grow properly thus limiting other nutrient uptake. Boron, which is present in very small amounts, has high mobility in soils. Boron deficiencies can promote plant abnormalities such as reduced yields, small berries, and in extreme deficiencies, cane dieback. For predicting B needs in fruit crops, soil test is less accurate than tissue test. Postplant application of B and other micronutrients should be based on tissue analysis, not soil tests. To correct nutritional problems in caneberries, either broadcast or foliar spray may be used. B rates between 1.0 and 1.5 lb B/acre broadcast and 0.1 and 0.15 lb B/acre foliar may be used. Foliar applications are preferred over broadcast. Annual growth applications of B should not be used. Take care not to over apply B because toxicity can occur rapidly.

Copper (Cu) is required for carbohydrate and protein synthesis: it activates numerous enzymes and enhances respiration. Plant tissue concentration of Cu typically varies between 5 and 20 ppm. Copper soil content is often sufficient for caneberry growth. However, if deficiencies occur, foliar applications may be used, but only if necessary. Constant applications of Cu increase soil content to excessive amounts.

Manganese (Mn) is necessary for P and Mg uptake. Manganese deficiency in caneberries is rarely observed; however, in soils where pH is greater than 7.0, Mn deficiency is probably present. Instead of soil applications, several foliar sprays during the growing season at rates between 1.0 and 2.0 lb Mn/acre will be effective.

Zinc (Zn) is regularly present in small quantities in fruit plants. It is a component of organic substances and complexes such as proteins and auxins. Zinc concentrations in caneberries should be between 20 and 50 ppm-1. A common observable symptom of Zn deficiency is a terminal leaf with a rosette shape and light green, yellow, or white interveins, mainly in older leaves. Foliar applications are frequently used, but soil applications of this nutrient, either broadcast or banded, can be effective. Rates between 5 and 20 lb Zn/acre, applied broadcast, are recommended if deficiencies of this element occur.

Iron (Fe) is a component of several organic substances, such as enzymes. It is involved in chlorophyll synthesis; thus, chlorosis is a typical symptom of Fe deficiency. In plant tissue analysis, proper content of Fe varies from 50 to 250 ppm. When it is needed, foliar sprays are the best method to apply Fe. Foliar rates of 1.0 lb Fe/acre are effective. Similar to Mn, Fe is both strongly and easily tied-up or fixed by the soil.

Table 2-1. Nutrients Needed for Proper Blackberry Growth.
Nutrient Function Soil Mobility Limiting pH Plant Mobility Defenciency Symptoms
Nitrogen Basic plant growth Very mobile None Very mobile Yellowing of foliage; stunting (cane height and diameter); lower leaves turn red or fall off
Phosphorus Metabolism; stimulate root growth Immobile Very high Mobile Stunted, dark green foliage; purple hue in older leaves
Potassium Stomatal opening and closing; movement of nitrates Very mobile Low Very mobile Small necrotic spots on older leaves; interveinal chlorosis on young leaves
Calcium Cell wall formation; cell division and elongation Very immobile Low Very immobile Tip burn in unfolding leaves; chlorotic young leaves; blossom end rot
Magnesium Chlorophyll production; nitrogen metabolism Immobile Low Mobile Interveinal chlorosis starting at leaf tips, margins of older leaves
Boron Auxin activity Mobile None Immobile Deformed fruit; delayed bud break
Zinc Auxin production Immobile High Immobile Rosette of terminal leaves; reduced leaf size; short internodes
Iron Chlorophyll production Very immobile High Very immobile Chlorotic young leaves; brown leaf margins; apical bud growth suppressed
Sulfur Hardening off for cold/drought tolerance Mobile None Immobile Thin stems; stunting; yellow leaves (similar to nitrogen deficiency)
Manganese Phosphorus and magnesium uptake Immobile High Immobile Dull, interveinal chlorosis in older leaves; spotting or gray specks
Molybdenum Nitrogen metabolism Mobile None Immobile Interveinal chlorosis; leaf dieback; deficiencies uncommon

Table 2-2. Macro- and Micronutrient Foliar Sufficiency Ranges for Blackberries.
Nutrient Range
Nitrogen 2.00–3.00
Potassium 1.50–2.50
Phosphorus 0.25–0.40
Calcium 0.60–2.50
Magnesium 0.30–0.90
Sulfur 0.10–0.50
Manganese 50–200
Iron 50–200
Boron 30–50
Zinc 20–50
Copper 5–20


Extension Specialist (Small Fruits)
Horticultural Science
University of Arkansas
University of Tennesee

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Publication date: Nov. 2, 2015

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