A well-planned fertilization program begins with a soil test and proper interpretation of the analysis. The following provides general guidelines for obtaining soil samples, a summary of soil test results and information for evaluating those results to develop an efficient Christmas tree fertilization program. This discussion is based upon analysis and results provided by the North Carolina Department of Agriculture & Consumer Services (NCDA&CS) Agronomic Division, although other testing services may provide the same type of information.
Because of soil variability, it is important to take samples from several locations in each field. For each field or operating area, at least 20 soil samples should be taken with a soil core sampler and mixed thoroughly in a plastic bucket (not a galvanized bucket as zinc contamination occurs). Samples should be taken to a depth of 4 to 8 inches, depending on soil type and tree species. Soil boxes, obtained from the County Cooperative Extension centers, NCDA&CS Regional Agronomists or fertilizer dealers, should be labeled carefully and filled to the level indicated on the box. A soil sample information sheet should also be completed and submitted along with the samples to the address shown on the box or sheet. If samples are to be sent by U.S. mail, write "Soil Sample" on the outside of the container in which they are shipped.
Results of analyses performed on the sample or samples will be shown under "Test Results." These results are given in the following order:
North Carolina soils are grouped into three classes: mineral (MIN), mineral-organic (M-O), and organic (ORG). The classification is based on the humic matter content of the soil and geographic location. Soils on which Christmas trees are grown are almost all classified as mineral and designated as MIN.
Percent humic matter is a measure of the soluble organic constituents of the soil. The absolute value is not critical but generally the higher the value, the better. It generally runs three percent or lower and cannot be used as a guide for herbicide application based upon organic matter.
Weight/volume refers to the weight per unit volume of the soil and varies with soil texture and organic matter content. A clay loam will have a value of approximately 1.0, whereas a sandy loam may be 1.15 or higher. Also, as the organic matter content goes up, the W/V declines.
Cation exchange capacity is a measure of the soil's capacity to hold elements such as calcium, magnesium, potassium, hydrogen, aluminum, iron, manganese, zinc and copper. A high CEC is desirable because leaching of fertilizer nutrients is less likely, and higher reserve quantities can be maintained, thus assuring an adequate supply throughout the growing season. Christmas tree soils have a range in CEC from 3.5 and 15.0.
The base saturation percent is an expression of the portion of the cation exchange capacity (CEC) that is occupied by nutrient cations, principally calcium, magnesium and potassium. Generally, the higher the base saturation percent, the higher the plant nutrient supply and the less acidity present to interfere with plant growth. A well-limed and fertilized soil will have a base saturation of 80 percent or higher.
Extractable acidity (Ac) is the portion of cation exchange capacity that is occupied by the acidic cations aluminum and hydrogen. This value is one of the factors used to calculate the lime requirement of the soil. Extractable acidity will be relatively low when the soil is properly limed.
pH is a logarithmic expression representing the concentration of hydrogen ions in soil solutions. pH measures how acid or basic a soil is. As pH goes lower, the more acid the soil. Values above 7.0 indicate a basic soil. The pH measurement is important in that it affects the availability of several plant nutrients. For Fraser fir, the value should be 5.3 to 5.8. The acceptable range for Virginia pine and white pine is 5.0 to 5.5.
P-I and K-I
These are index values representing the nutrient availability of phosphorus and potassium to plants. These values are interpreted to be low if the index is below 25; medium if it is 26 to 50; and high if it is 51 to 100. Values above 100 are considered very high. Optimum values vary for individual crops, with index values in the range of 50 to 70 desired for Christmas trees.
Ca% and Mg%
These values refer to the percent of cation exchange capacity that is occupied by calcium and magnesium. On well-limed Fraser fir and white pine soils, Ca% should be 50-60, and Mg% should be 10 or higher.
Mn-I, Zn-I and Cu-I
Manganese, zinc and copper are the remaining three elements that will be routinely determined on soil samples. The values expressed are indices with 25 and above considered adequate for normal plant growth. On many soils, the Mn-I may be over 100 and frequently results in manganese toxicity symptoms, especially on lower pH soils.
Lime and fertilizer recommendations are shown under "Recommendations" on the report. These suggested treatments are based upon test results and have been developed through many years of research and experience in North Carolina. Under the suggested treatment the following will appear:
Any lime suggested on your report is designed to raise and maintain the soil pH to an appropriate value for the crop. In addition to supplying the essential calcium and magnesium, lime neutralizes aluminum, which becomes toxic to plant roots when the soil pH is too low. The efficiency of plant uptake and use of phosphorus is also enhanced when soils are properly limed.
The value presented is the suggested amount of nitrogen to be applied annually per acre. Nitrogen can be applied using several types of material. However, each source has both advantages and disadvantages, and careful planning is necessary for proper application rates and timing. Additionally, nitrogen applied above the recommended rates increases the risk of ground water contamination or soluble salt injury (particularly if applied near the tree).
This is the suggested rate of phosphate P2O5 to be applied annually per acre. The rate suggested is based upon the level present in the soil. Although low phosphorus may severely stunt tree growth, there is no advantage in exceeding the recommended rates. Any phosphorus source may be used and should be thoroughly incorporated when possible. This is especially important if the soil test level is low.
This is the suggested rate of potash K2O to be applied annually per acre. When soil test results recommend potash rates in excess of 100 pounds/acre, half should be applied in the spring and half in the fall.
A $ will appear in this column when magnesium is low. A blank or 0 indicates no special need for magnesium, and any lime source may be used. If the $ appears, then lime should be of the dolomitic type.
Cu, Zn, B and Mn
These values will normally be blank as they are micronutrients and most soils do not show deficiencies.
Note: Accompanying each soil test report is NCDA Note 5, "Christmas Trees" which provides additional information on lime and fertility requirements for Christmas trees.
This discussion mentions only a few of the many aspects of Christmas tree fertilization. Should questions arise regarding North Carolina soil testing or recommended fertilization practices, please contact your local Cooperative Extension Center, or the NCDA&CS Agronomic Division at (919) 733-2656.
Publication date: April 23, 2014
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