Fertilization

Fertilization is a silvicultural practice used for increasing forest productivity, typically in intensively managed plantations of loblolly pine (Pinus taeda) and slash pine (Pinus elliottii) in the southeastern U.S.

For more information on soil, visit our Soils section.

Stand Demand & Nutrient Supply of the Site 

The nutrient requirements of trees are met in 2 ways: 

  1. uptake from the soil and
  2. mechanisms of internal transfer, called remobilization

photo by Mike JacobsonNutrients in young stands may be limiting when nutrient levels in the root zone are insufficient to meet the growth demands of the young trees (assuming all other requirements such as water, suitable temperature, and light, are adequate).

The quantity of nutrients in the soil and the amount taken up by plants depends on the following factors: 

  • concentration of nutrient ions in the soil
  • rates of soil weathering
  • inputs from and losses to organisms and the atmosphere
  • amount and turnover rate of organic matter
  • moisture content
  • soil texture (clay, sand, or loam) and structure
  • extent of the root systems of plants
  • mycorrhizal associations

See our Soils Overview page for more information about soil flora.

The demand for soil nutrients by trees depends on the tree species and stage of stand development.  Demand for nutrients is greatest when the crowns (branches and foliage) of the trees are rapidly expanding.  Relatively large quantities of nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca) are required at this time.

Coastal Plain soils have been grouped into four general soil types.  They are categorized based on drainage condition and subsequent phosphorus retention.  See our Soil Classification page for descriptions of these groups.  This will get you started in the process of determining the types and amounts of fertilizer your land may need for forestry purposes.

Other Differences in Nutrient Demands Due to Stand Age

Nutrient requirements differ between young trees (1-3 years) and older, established trees (12-16 years) because there are changes in both nutrient requirements and soil nutrient reserves as a forest stand matures, particularly on intensively prepared sites.  Mixing of the soil during site preparation accelerates decomposition of debris and soil organic matter and the release of nutrients they contain.  Bedding operations further concentrate these organic materials in ridges and increase the accessibility of released nutrients to the young trees.

photo by Mike JacobsonThis accelerated release of nutrients from these operations may improve tree growth for several years on sites containing large amounts of litter and soil organic matter.  However, on sandy soils, some of the released nutrients may be lost to leaching before the native vegetation and seedlings grow sufficient roots for nutrient uptake.  These losses may increase the need for fertilizers during the latter part of the rotation period.

It is more difficult to predict the nutrient needs in older stands because older trees have deeper root systems.  Over time, deep penetration may allow roots to absorb nutrients from subsoil horizons even though surface horizons are low in available nutrients.  Surface layers of organic debris developed under established stands serve as a nutrient reservoir, releasing nutrients as the material slowly decomposes.

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Influence of Harvesting & Site Preparation on Nutrient Supply

With the exception of the most infertile soils, harvesting and site preparation increase the availability of nutrients, particularly N, to levels exceeding those required by young seedlings.  On many sites this increased nutrient availability can be seen in the rapid early growth of herbaceous or woody plants that may compete with the selected pine species.

This "competing" vegetation may actually help retain nutrients on the site for later recycling and use by the selected timber species.  It has been shown that accelerated leaching (movement deep into the soil and unavailable for tree uptake) of nutrients often follows the control of competing vegetation.

In the South, Phosphorus is usually the most limiting nutrient. This can be overcome by fertilization.

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Fertilizer Materials

Fertilizer materials used in silviculture are less varied than those used in agriculture.  Dry solids predominate.  Course, dry materials that are uniform in particulate size can be evenly spread in wide swaths from ground equipment or aircraft.

Nitrogen Fertilizers

Nitrogen (N) absorbed from the soil solution by trees is in ionic forms of nitrate or ammonium.  Deficiencies of N most frequently occur in older stands, especially on flatwoods soils and old agricultural lands. A gradual reduction in available N apparently results from its immobilization in the forest biomass.  The N in fertilizers is in a form immediately available when in contact with soil moisture.

Urea (converted to ammonium by soil enzymes) is most commonly used when N is needed because:

  • it is competitive in price
  • it is high in N (45%)
  • it can be formulated into large granules adapted to wide spreading swaths

Diammonium phosphate (DAP) is often the product of choice when both N and P are needed because it has favorable physical properties and contains high percentages of both N and P.

Phosphorus Fertilizers

Phosphorus is the nutrient most frequently deficient in coastal plain soils. Pines absorb most of the phosphorus (P) required in the form of phosphate. Therefore, fertilizers containing P must dissolve for the nutrient to be absorbed.  Soil solution levels of phosphate required by southern pines are low.

Ground rock phosphate (GRP) is a slowly soluble powdered material of varying P content.  The material available in the Southeast contains from 30-33% P2O5 (13-14% P).

The "superphosphates" are the P fertilizers commonly used for silvicultural purposes when only P is needed.  Superphosphates or concentrated superphosphates (CSP) are formed by reacting rock phosphate with sulfuric acid or phosphoric acid.

DAP is an exceptional product and it is competitively priced.

Potassium and Other Fertilizers

Levels of Potassium (K) and other micronutrients are generally low in Coastal Plain soils, but growth responses are seldom obtained from the application of these nutrients to pine plantations.  Potassium chloride (potash or muriate potash) is used directly or blended with other materials. This substance and minor amounts of potassium sulfate and potassium nitrate provide all the potassium (K) fertilizer used in forestry and agriculture.

When K is needed, cost and convenience should determine the fertilizer used because all are soluble in water and equally available to plants.

Micronutrients such as boron (B), copper (Cu), zinc (Zn), and Manganese (Mn) can be purchased as simple salts, organic compounds, or silicate compounds.

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Fertilizer Application

Fertilizing newly established pines (less than 5 years of age) on responsive sites produces a faster growing, more vigorous tree.  P and N are the elements most commonly applied to young pines.  Nitrogen alone has not been recommended here. 

Timing 

P and N should be applied after planting in late spring or early summer of the first year.  The more time trees have to absorb the increased supply of nutrients and outgrow competing vegetation, the better the growth response.  If P is applied alone, early application is best. P can be applied before planting if other constraints favor this timing.

Availability of micronutrients is long-lasting and timing is not critical for these metals.

Potassium fertilization is not commonly recommended for southern pines but increasing evidence indicates that K is often limiting when N and P are applied.  If your budget allows, apply K with N and P.

Rates

Application rates of fertilizers for young southern pine stands have not been determined as precisely as those for agricultural crops. The recommendations below are only general guidelines.  These general guidelines are followed by a link to specific recommendations for the four soil drainage conditions on the Coastal Plain.

P:  Where rates have not been tested, apply at 45-55 kg/ha (40-50 lb/ac) of P.  Note that these rates are in terms of P2O5.  Approximately 100 to 125 kg/ha of P2O5 are recommended. 

N:  Likewise, 45-55 kg/ha (40-50 lb/ac) of N is recommended. 

P + N:  DAP is convenient in that it supplies N and P in the ratio (1:1) usually recommended for southern pines.  When trees are fertilized at midrotation, application levels for P are similar to those at establishment, but those for N are higher (150 to 200 kg/ha). 

K:  The current recommendation for K application is 90 kg/ha (80 lb/ac or 100 lb K2O/ac) 

Micronutrients:  Mn, Cu, Zn at 3-6 kg/ha; B at 1 kg/ha.

University of Florida Extension Publication

UF/IFAS Soil and Analytical Services Laboratory
A soil test can help determine soil attributes like pH, major nutrient levels, micronutrient levels, and other infomation. Your County Extension office can provide assistance in sampling and testing your soils.

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Application Methods 

Application method does not affect seedling growth response to fertilization as long as the method provides a uniform application at a controlled rate.

Banded Application

Banded application systems involve selective fertilizer placement, usually 1m wide over the planted row of trees.

Broadcast Application

This method spreads granulated fertilizers in swaths 15 - 30m wide. Once applied, fertilizer can be incorporated into the soil during site preparation using disking or bedding plows.

Where terrain is relatively dry and easily traversed, tractor-mounted spreaders are suitable.  Where terrain is wet or rough, rubber-tired skidders equipped with spreaders are appropriate. 

Tips for Uniform Application

Whatever method you choose, uniformity of application and rate control are important.  Irregular growth patterns may result from unequal distribution of fertilizers.

Here are some ways to make sure fertilizer applications are uniform:

  • Balloons suspended from 3-wheeled all-terrain vehicles that traverse the area to be fertilized can be used as markers to assure uniform distribution of fertilizers.
  • Open-top containers can be placed across the treatment area to measure the uniformity of fertilizers applied.

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Economic Justification for Fertilization

Fertilization is an investment from which we expect to generate a return - increased stand volume and value resulting from accelerated stand development.  Growth projections and economic analyses conducted on data from loblolly pine stands that were bedded, fertilized with N, P, or both indicated that these treatments substantially increase volume production and the proportion of sawtimber-size trees. 

In several past examples, the net present value (NPV) for the combined treatment (bedding and fertilization) was 2 and 3 times higher than that for either treatment applied alone.  As more valuable products are included in the product mix (chip-n-saw, sawtimber) or if markets for short-rotation products improve or develop, returns on fertilization may be even greater. However, as fertilizer costs increase, the NPV of fertilization treatments decreases.

For more about forest fertilization, see presentation notes from the 2007 Forest Stewardship Videoconference on Forest Fertilization.

UF/IFAS Soil and Analytical Services Laboratory
A soil test can help determine soil attributes like pH, major nutrient levels, micronutrient levels, and other infomation. Your County Extension office can provide assistance in sampling and testing your soils.

For more about economic analysis of forestry operations, see our Investing in Forestry page.

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