A well-planned fertilization program begins with a representative soil test for each field and careful analysis of the soil test report. The following note provides general guidelines for collecting 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, which provides low-cost soil and plant tissue analysis to NC citizens. Soil test reports present a range of soil property and nutrient levels. In addition, they identify those nutrients that are deficient or excessive in comparison to established optimums for the selected Christmas tree species. Recommended applications for major nutrients (nitrogen, phosphorus, and potassium) and secondary nutrients (calcium and magnesium) are provided. Soil test reports are the best way to determine optimum lime and fertilizer applications for a field. When collected after a nutrient application, soil tests can indicate the progress made toward solving a particular soil deficiency or problem. The ability to interpret soil test results is a skill that can lead to better tree growth, greater crop uniformity across a field, and ultimately higher profits.
Deciding on Soil Sample Areas
Typically, Christmas tree growers take at least one soil sample per field or management unit. Sample areas are ideally between one and two acres but might be smaller or larger depending on the management unit. Separate samples should be collected for distinct blocks of trees, different species of tree (if managed differently), different age groups, and noticeably different soil types. At least initially, separate samples should also be taken for different landscape positions that can reflect differences in soil depth and fertility. Distinct landscape positions may include ridges, upper slopes or "shoulders," side slopes, "toe slopes" or lower slopes, and bottomland. Each can have different characteristics according to their loss or accumulation of soils over time. Aspect can also influence soil development with more organic matter accumulation occurring on north or eastern aspects. Previous land use can also justify separate samples such as former annual crop fields, garden plots, or the areas around former lime piles and burn piles.
Sample More Areas Initially, Combine Later
In a large field, a grower might initially collect a separate soil sample from the upper slope and toe slope as well as from the east and west aspects or sides of the field. Distinctly different results between upper and lower parts of the field would justify collecting separate samples in the future. If the east and west sample results came back virtually the same, future sampling would combine the east and west areas into a single sampling area across the field. Multiple sample areas can always be combined in the future if test results are the same. However, one sample will never reveal important differences if it is collected across distinct sites.
Soil Sampling throughout the Christmas Tree Crop Rotation
With an eight to ten year crop rotation, Christmas trees require a very different sampling schedule compared to annual crops. It would be helpful to monitor soil nutrient levels at least every 2-3 years during the rotation. Prior to setting trees, a grower has particular need for information about new ground. A soil test prior to site preparation can provide an indication of soil properties such as the weight to volume ratio and humic matter content. It is also better to identify possible nutrient deficiencies or toxicities before trees are set than after (when fewer management options are available). Once trees are set and growing, a new test every other year would be ideal to track the impact of recent treatments and determine what is best for future lime and fertilizer applications. As trees approach their intended market size, they require more nutrients to maintain optimal growth. Additional soil and plant tissue samples are especially useful the year before harvest to identify problems or help a grower fine-tune fertilizer applications. Thus from site preparation to harvest, a Christmas tree field might require sampling three or four times.
The Best Tools for Soil Sampling
Use a soil probe or tube to collect the most uniform samples. While a shovel or spade can be used to collect small wedges of soil, they will not be as uniform as the soil core provided by a soil probe. A plastic bucket is recommended. Metal buckets can change soil test results, particularly zinc levels, if a galvanized bucket were used.
Considerations about Soil Sample Depth
Typical soil samples collected from plowed fields are eight inches deep. If soils are mixed to that depth during tillage, that depth of soil sample is appropriate. In most Christmas tree fields little or no tillage occurs. Nutrients are top-dressed on the soil surface. While some nutrients like nitrogen and potash will leach through the soil profile to deeper levels, others like phosphorus and calcium tend to move slowly and become stratified in the layer in which they were applied. If recently applied nutrients are largely in the top 4 inches, an 8-inch soil sample would dilute nutrient levels with additional low-nutrient soil. This could result in soil test recommendations to apply more fertilizer than is truly needed. A 0-4 inch sample better reflects both the area enriched through top-dressed fertilizer and the shallow root zone of a Fraser fir. Other Christmas tree species are likely to be more deeply-rooted, but surface applied nutrients will still be mostly in the top four inches unless the soil is very sandy.
Two-depth Soil Sampling
Some growers will occasionally collect 2-depth soil samples to learn more about their fields. This can be accomplished simply by knocking soil from the probe into two adjacent buckets and by keeping the top portion separate from the bottom. As with a normal sample, the surface four inches represent the portion altered be recent top-dressed fertilizer and lime applications. The lower 5-8 inch sample often indicates residual nutrients from past crop history or native forest soil conditions. During a drought, the lower zone may reflect nutrient levels from which trees are feeding because surface soils are too dry. Two depth sampling can often explain gaps between applied fertilizer and tree performance.
Collecting a Reliable Sample
It is important to collect more soil than the lab needs to insure a representative soil sample. Soil nutrients can be quite variable. Collection of multiple sampling points can help to average out any abnormally high or low samples collected. Lime and fertilizer recommendations need to be targeted toward average conditions for the field, not extremes that might represent a spilled fertilizer bag during a previous application or an old burn site. If collecting random samples across a field, a minimum of 20 soil samples should be taken.
Systematic Plot Soil Sampling
If fertilizer and lime have been applied to Christmas tree fields by hand, some effort should be taken to sample across possible patterns to capture both fertilized and unfertilized areas. One soil sampling approach in Christmas trees is to collect sub-samples systematically around a smaller number of randomly selected trees. If 4 soil cores were collected with a soil probe around individual trees and 5 trees were sampled across the sample area, the sample would represent a mix of 20 cores. By sampling above and below the tree and on each side of it in-row, soil is likely to include both fertilized and unfertilized areas. Ten or more trees could be sampled in larger blocks. After thorough mixing in the bucket, the half-pint needed for the soil test is packaged and remaining soil is scattered back onto the field. Some growers have marked and returned to the same trees or plots throughout a rotation in an effort to be more precise.
Disease Management Concerns
Soil sampling should be practiced with consideration of disease management, particularly Phytophthora root rot (PRR). If diseased trees or previously infected areas exist in a field, do not sample those areas at the same time as the rest of the field. Soil-borne diseases can be transmitted on soil moved on tillage equipment or shoes. Even a shovel or soil probe can move contaminated soil. If a soil sample is needed from an area with PRR, sample that area last or at a different time. Be sure to clean tools, buckets, boots, and hands after working with any soil that might be contaminated. After cleaning, a rinse tools with a 10% solution of Clorox (90% water). Sanitation practices are critical to minimizing potential disease spread.
Acquiring Soil Boxes
Soil boxes can be obtained from local N.C. Cooperative Extension center, NCDA&CS Regional Agronomists, or fertilizer dealers. Sample boxes should be labeled carefully. Be sure to keep a map or log of the samples you collected to avoid confusion when the results come back. Only fill soil to the level indicated on the side of the box. A soil sample information sheet should 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.
Interpreting the Soil Test Report
This section provides detailed descriptions of the different test result categories provided in an NCDA&CS soil report. The following headings represent the columns in the second row provided for each soil sample. Particular considerations related to Christmas tree production are provided.
North Carolina soils are grouped into three classes: mineral (MIN), mineral-organic (M-O), and organic (ORG). The classification is based on the percent of humic matter in the soil (HM%) and in some cases, the soil’s weight to volume ratio (W/V). This classification can help to identify very different responses to lime and fertilizer.
Soils on which Christmas trees are grown are almost all classified as mineral and designated as MIN. Soil chemistry in mineral soils is largely driven by the clay component as opposed to the organic component in mineral-organic and organic soils. Mineral soils contain more exchangeable aluminum than the other soil classes. For optimum growth, most crops require a higher target pH in mineral soils and more lime to overcome negative effects of aluminum. For most crops grown on mineral soils the target pH is 6.0 – 6.5, on mineral-organic soils 5.5, and on organic soils 5.0. Fraser fir and Norway spruce are among the few acid-loving crops that have a target pH of 5.0 - 5.5 on mineral soils.
However, about five percent of Christmas tree fields have soils that are high enough in organic matter to be classed as mineral-organic soils rather than mineral. These high-elevation mountain soils usually are dark brown or black in color. They tend to have higher cation exchange capacity (CEC) values, lower pH levels, lower calcium levels, and different behavior of other nutrients as well. Because of their greater buffering capacity, raising the pH can be slow, require additional applications of lime, and might not even be necessary. Target pH for Fraser fir should be 5.0 on such sites. The optimum calcium level for Fraser fir should be reduced from 55% to 45%. By recognizing the difference of these high elevation sites, a grower can avoid making a number of costly mistakes.
Percent humic matter is a measure of the soluble chemically-active constituents of organic matter in the soil (humic and fulvic acids), not total organic matter. There is no direct correlation between the percent humic matter and total organic matter of a soil. The amount of humic matter can vary somewhat with the weather and season. The absolute HM% value is not critical but generally the higher the value, the better. It generally runs three percent or lower. Percent humic matter is used to calculate lime recommendations. Percent humic content should not be used as a guide for herbicide application since those should be based upon total organic matter. In general, the percent humic content increases as a soil’s weight to volume ratio decreases.
Weight/volume refers to the weight per unit volume of the soil and is expressed in grams per cubic centimeter (g/cm3). The ratio will change according to soil texture and organic matter content. As the organic matter content goes up, the W/V ratio declines. It is another value on a soil test report that can be used to classify soil type. Sands, clays, and organic soils will exhibit different weight to volume ratios. Organic soils have the lowest W/V ratios nearing 0.5 g/cm3. Sands have the highest W/V ratios that can approach 1.5 g/cm3. Clays usually fall between those two extremes. A clay loam will typically have a value of around 1.0 g/cm3, whereas a sandy loam may be 1.15 g/gm3 or higher. The W/V ratio is generally inversely related to the cation exchange capacity (CEC) of the soil. Organic soils with a low W/V ratio generally have a high CEC.
Cation exchange capacity (CEC) is a relative measure of the nutrient-holding capacity of a soil. It is expressed in units of meq/100 cm3 and is calculated by adding extractable calcium, magnesium, potassium and exchangeable acidity. CEC levels in Christmas tree soils range between 3.5 for sandy sites to 15 meq/100 cm3 for clayey or organic soils. A value of 8 meq/100 cm3 is considered to be normal.
The CEC will vary with any changes in soil pH, organic matter and clay content. Chemical exchange sites within clay particles and the surface of organic matter provide the locations where nutrients are bound to soil. Without these exchange sites, positively charged nutrients including nitrogen, potassium, calcium, magnesium, and most micronutrients would leach away.
A high CEC is generally considered to be desirable. Nutrients are less likely to leach and adequate nutrient reserves are more likely to be maintained. Higher nutrient reserves translate into more pounds of nutrient per acre. The down-side of a soil with a high CEC is that it will require larger nutrient applications to change nutrient levels and alter balances between nutrients. Once optimums are achieved in a high CEC soil, they will generally hold for a longer period of time. Most organic soils and some clays tend to have high CEC levels. However, if a Christmas tree field has a CEC of 10 meq/100 cm3 or above, it may require special consideration whether it classifies as a mineral-organic soil or not. As discussed in regards to Soil Class, such soils should have a lower target soil pH and reduced calcium and magnesium requirements.
Sandy soils, by nature, have low CEC values and little can be done to change it. Such soils retain smaller nutrient reserves and leach any excess fertilizer not bound to the CEC. A farmer would be wise to apply smaller more frequent applications of fertilizer to not overwhelm the soil’s nutrient holding capacity. Higher applications would simply be lost to leaching and subject crops to greater risk of salt injury. Depending on how your fields test for CEC, you might want to use very different fertilizer application strategies.
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, higher base saturations reflect higher supplies of plant nutrients and less acidity in the soil to interfere with plant growth. As base saturation increases so does the pH of a soil. A well-limed and fertilized soil will have a base saturation of 80 percent or higher.
In fields that have produced several rotations of Christmas trees, have a history of previous row crops, or have been limed to reach a higher target pH, base saturation can sometimes be excessively high. When the base saturation is 95% or greater, readily available nutrients like calcium and potassium may block availability of other cations including micronutrients. Issues associated with high base saturation will usually be corrected in the process of reducing associated high pH. Foliar application of micronutrients have sometimes been used to overcome deficiencies where soil availability is limited.
Extractable acidity (Ac) is the portion of cation exchange capacity that is occupied by the acidic cations aluminum and hydrogen. Like CEC, it is calculated as milli-equivalents per 100 centimeters cubed (meq/100 cm3) . This value is one of the factors along with pH that is used to calculate the lime requirement of the soil. Extractable acidity will be relatively low when the soil is properly limed. If a soil has a high Ac value, more lime will be needed to make an equivalent change in pH than for a soil that had a low Ac value. Thus it could be an important factor to consider when looking at soil reports for a potential new field. High Ac values can be a particular concern in some high elevation soil types especially if they have not been previously farmed.
pH is expressed as a logarithmic 14-point scale representing the concentration of hydrogen ions in soil solutions. The pH value is a measure of how acid or basic a soil is. As pH goes lower, the more acid the soil. A pH of 6.0 is ten times more acidic than a neutral pH of 7.0. A pH of 5.0 is 100 times more acidic than a 7.0 pH. A pH of 2.0 would represent the approximate acidity of lemon juice. Values above 7.0 indicate a basic soil.
Soil pH is used along with the Ac factor to determine liming rates. In addition to supplying 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 pH measurement can reflect acidity levels that severely limit availability of several plant nutrients. For instance, as soil pH values drop below 5.5, they increasingly limit availability of phosphorus. Soil pH’s above 6.0 limit availability of the micronutrient, manganese. Managing soil pH to optimize availability of nutrients is a critical task for any farmer or land manager.
Target soil pH values are listed by conifer species in Table 1. A higher target pH during the establishment of Fraser fir, Hemlock and Norway Spruce allows more lime to be applied to acidic mountain soils early in production. This allows Christmas tree growers to achieve optimum pH levels sooner than would be possible if the lower maintenance soil pH were used. The distinction between establishment and maintenance phases of production is for the field, not the crop. Soil in new fields can have extremely low pH values that are difficult and take time to raise with surface applied lime. Once fields are established to optimum production, the pH seldom will return to initial low levels. As multiple crops are produced, the maintenance target pH of 5.5 provides enough lime without incurring the equally problematic risk of overshooting the 5.8 pH target.
|White Pine, Virginia Pine||pH 5.5|
|Fraser Fir, Hemlock, Norway Spruce||pH 5.8, establishment|
|pH 5.3 - 5.5, maintenance|
|Leyland Cypress||pH 6.0|
|Blue Spruce, Red Cedar||pH 6.5|
P-I and K-I
These are index values representing the nutrient availability of phosphorus and potassium to plants. Generally, these index values are considered 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 most Christmas tree species.
However, for Fraser fir Christmas trees grown on highly acidic mountain soils, the target index values for phosphorus and potassium are usually elevated to a range of 80 to 100. Vigorous Christmas trees have the potential to double their volume every growing season and place a high demand on soil fertility, phosphorus in particular. High levels of phosphorus are needed for good bud set. Further, mountain soils typically have high phosphorus-fixing capacity that limits nutrient availability to plants. By targeting a higher P-index, enough phosphorus can be applied to insure a small portion is available to Christmas trees.
Many Christmas tree growers try to “front-load” a majority of their expected rotational phosphorus needs during site preparation when it can be combined with some form of tillage. This is the best way to overcome a soil’s phosphorus-fixing capacity -- sooner in the rotation rather than later. Even with extra phosphorus applied during site preparation, annual surface applications are needed to insure optimal growth. On new fields, target phosphorus levels can take several years or even the entire rotation to be achieved. However, at some point in the life of a Christmas tree field, very high phosphorus will likely be present in the surface layers of the soil. At that point, further annual surface applications become an unnecessary cost. Regular monitoring with soil and tissue samples insure that a fertilizer program remains on track and phosphorus is applied when needed.
The target index for potassium (K-I) has also been elevated to 80-100 for optimal Christmas tree production in the mountain region. Potassium (K) deficiencies are associated with slow growth, weak color, and spindle-shaped foliage (short needles at the tips and base of shoots with more normal lengths in the middle of individual shoots.) More frequently, growers have induced salt injury to trees by applying too much potassium or allowing workers to improperly place handfuls of fertilizer over tree roots.
Potassium leaches quite readily. Deficiencies tend to occur after extended wet periods when soluble potassium has been washed out of the soil. Potassium deficiency can also be induced by applications of calcium either as lime or gypsum. Monitor potassium status with soil and plant analysis at least every two years to ensure an adequate supply.
Excess potassium is a more widespread problem than potassium deficiency – often a result of over-reliance on balanced fertilizers. Potassium fertilizers are very salty and can result in root injury where carelessly applied. Excess potassium can induce deficiencies of calcium, magnesium and several micronutrients. Excesses of potassium can be corrected with lime or gypsum applications in addition to foliar applications of the deficient nutrients. High levels of potassium will also drop over time as a result of leaching.
Ca% and Mg%
These values refer to the percent of the cation exchange capacity (CEC) that is occupied by calcium and magnesium. These nutrient values also correlate to a soil’s percent base saturation (BS-%). On fertile soils, calcium is usually the predominant cation. However, many non-cropland soils are severely deficient in calcium and magnesium. Large applications of lime may be needed initially, even for an acid-loving crop like Fraser fir. Where calcium is particularly low, additional applications of gypsum (calcium sulfate) may be needed to supplement calcium without raising the pH further. For mineral soils with a CEC of “8” or below, the target Ca% should be 50-55, and the target Mg% should be 10-15. In mineral-organic soils where the CEC is “10” or above, targets should be reduced to 45 Ca% and 10 Mg%.
The Mg% value should be used to determine which kind of lime is applied to a field or if a magnesium fertilizer is needed. If the Mg% is low, Dolomitic lime which contains both calcium and magnesium would be the preferred choice. If Mg% levels are adequate, calcitic lime, containing only calcium, would be a better choice. Non-liming sources of magnesium that do not significantly alter soil pH include ground-applied magnesium sulfate, sulfate of potash magnesia, magnesium oxisulfate, and foliar applications of magnesium sulfate or epsom salts.
Balancing pH, Ca-%, and Mg-% can be the most difficult aspect of managing soil fertility in Christmas trees. Growers can spend a decade or more struggling to get soil test numbers up where they should be and then have to fight to get excess levels on some fields down again. Deficiencies of calcium can affect all aspects of growth but are most noticeable in a pattern of late summer / early fall needle drop. Deficiencies of magnesium can appear as poor color or marginal needle yellowing. High levels of calcium and / or magnesium are not usually expressed directly as toxicities, but appear as severe manganese (Mn) micronutrient deficiencies. Most often Mn deficiency symptoms are associated with lime piles or old burn piles.
Mn-I, Zn-I and Cu-I
Manganese, zinc and copper are three micronutrients that are routinely reported on soil samples. The values expressed are indices. A value of 25 and above for any of these three micronutrients is considered adequate for normal plant growth. Each element is treated somewhat differently. Manganese and zinc each have additional nutrient availability indices to provide a better fit to variable soil conditions.
Because manganese is highly limited at higher pH levels, a manganese availability index (Mn-AI) is used which takes soil pH into account. The Mn-AI decreases as pH increases. On many NC soils, manganese levels may be over 100. In other crops, this can result in manganese toxicity symptoms, especially at lower pH’s. Manganese toxicity symptoms have only been observed on Christmas trees at much higher levels in NC soils. More frequently, Christmas trees exhibit manganese deficiency symptoms in high pH soils. Manganese is still present in higher pH soils, but not readily available for plant uptake. Manganese deficiencies in Fraser fir Christmas trees are visible as stunted, yellow growth in the midsection of trees.
An alternate availability index is also applied to zinc soil test index values. Availability of zinc is indirectly related to pH of the soil. The zinc availability index is calculated differently for each soil class based upon the target pH for that class. The zinc index is multiplied by a factor of “1” for mineral soils, “1.25” for mineral-organic soils, and “1.66” for organic soils. As with the standard indices, a value of 25 is considered to be adequate for most crops. Zinc deficiencies and toxicities are rare in Christmas trees, but do occur. When observed, zinc deficiencies have usually been created by saturation of other cations rather than the absence of zinc. Zinc deficiencies appear as uniformly stunted needles with white tips and margins.
Suggested Lime and Fertilizer Treatments
Lime and fertilizer recommendations are shown under "Recommendations" on the report. These suggested treatments are based upon soil test results listed on the second line of the individual soil sample report. The equations that generate suggested treatments for Christmas trees represent an integration of research and practical experience with different species across North Carolina.
Under the suggested treatment the following categories appear:
Any lime suggested on your report is designed to raise and maintain the soil pH at an appropriate value for the Christmas tree crop being grown. Liming decisions should be made with consideration of calcium and magnesium indexes. The magnesium level determines the choice between dolomitic lime that contains both calcium and magnesium and calcitic lime that only contains calcium. For an acid-loving crop such as Fraser fir, the use of gypsum as a pH neutral calcium source may also be an important soil fertility tool used alone or in conjunction with lime. Generally as a safety factor, single applications of lime are limited to no more than 1.5 tons (3000 pounds) per acre and gypsum to 0.75 tons (1500 pounds) per acre.
The choice of which liming material to buy is very important and involves consideration of several criteria. No liming products are immediately soluble in water. Rather the products slowly react with the acidity of the soil to dissolve over time. The speed of this reaction increases with finer particle size. This is why lime products sold in NC must meet certain standards for particle size (reported as the percentage passing through certain mesh sizes). Products also contain different chemical compositions and amounts of calcium. Their acid-neutralizing ability can be compared using their estimated calcium carbonate equivalent. Products with values closer to 100 approximate pure calcium carbonate more closely. Liming materials can range from about 20% to 40% actual calcium. Recommendations are based on the assumption that a 20% material is used. If a more concentrated material is used, be sure to cut the application rate.
After lime is applied and has had time to dissolve (usually a year), soil tests can be used to monitor changes to soil acidity. If the pH is still low, additional lime would be recommended and could be applied. If the pH rises above optimum levels for the tree species being grown, soil amendments to reduce pH may be needed. For large reductions in pH, sulfur applications are recommended. If the needed pH reduction is small, the acidifying effects of annual nitrogen fertilizers may be enough. Soil test results can be used to make those determinations.
For more information on soil acidity and liming, go to the following resource: Soil Facts: Soil Acidity and Liming: Basic Information for Farmers and Gardeners by Carl Crozier, NC State University, and David Hardy, NCDA&CS Agronomic Division
The value presented is the suggested amount of nitrogen to be applied to the crop annually per acre. Values are based on research, not the soil sample tested. Nitrogen levels in soil will change throughout the year and do not provide a reliable indicator of need.
Nitrogen is very mobile in the soil where it undergoes a number of microbial transformations. It can leach rapidly. In periods of excess precipitation, nitrogen deficiency can develop in the course of a month. Ammonium forms of nitrogen are less likely to leach and can stay in the soil longer than nitrate forms particularly in colder months.
Nitrogen is necessary for tree growth. The greatest demands for nitrogen are in spring and early summer. Trees with nitrogen deficiency produce short needles and exhibit poor yellow-green color. New growth can be weak and spindly over the entire tree. Nitrogen needs to be applied 3 to 6 weeks before bud break to insure adequate levels are available.
Nitrogen recommendations for Christmas trees are provided in Table 2. Recommended rates increase with the size of the tree. Young Christmas trees that have been in the field one or two years need less nitrogen annually. Research has demonstrated that no additional growth occurs with higher nitrogen rates and there is greater risk of salt injury to young unestablished root systems. Remember, the nitrogen recommendation on the soil report will always be for 100 to 120 pounds to the acre since tree size is not provided. Nitrogen applied above the rates recommended in Table 2 increases the risk of ground water contamination and/or soluble salt injury (particularly when applied near the tree by hand).
|Tree Age (years)||Species||Nitrogen (ounces per tree)|
|3||Fir, spruce, hemlock||1/2 – 2/3||1/3 – 1/2|
|3||White & Virginia pine, cedar||1/2||1/2|
|4||Fir, spruce, hemlock||2/3||1/3|
|4||White & Virginia pine, cedar||1/2||1/2|
It is best if the annual nitrogen requirement is provided in two applications. In Christmas tree research studies, the best growth occurred when nitrogen was split between spring and fall applications. Split applications reduce the salts in solution at any given time. Spring fertilization can be made from up to six weeks before bud break using nitrate forms of nitrogen and even earlier with ammonium sources. Fall fertilization should occur after August 15 or when growth has fully matured and cooler temperatures are likely.
Nitrogen can be applied using several types of material. The decision of which material to use should also consider recommended amounts of phosphorus and potassium on the soil report. If both phosphorus and potassium are needed a number of different blended fertilizers can be used. Select the product that best fits the ratio between nitrogen, phosphorus, and potassium recommendations. If only nitrogen and phosphorus are needed, diammonium phosphate is the optimal choice. If nitrogen alone is needed, there are several choices. Urea is one of the most concentrated materials allowing more trees to be treated per unit purchased. It also has the lowest salt index and risk of salt injury among nitrogen sources. Ammonium sulfate is the most acidic material and can be used to make small reductions in soil pH while satisfying the nitrogen requirement. Calcium Nitrate contains a more soluble form of calcium that can be beneficial when Christmas trees are deficient in calcium and lime would be too slow-acting.
The suggested annual rate of phosphate P2O5 provided on the soil report is calculated from the P-I value, soil class, and proven requirements of the Christmas tree species being grown. Different equations have been developed for most crop and soil conditions. The rates of phosphate (P2O5) shown on a Christmas tree soil report are given in pounds per acre.
Availability of phosphorus is severely limited in many NC soils. Some native soils have little or no available phosphorus when first soil tested. When phosphorus fertilizer is applied to these acidic soils, much of it is converted to insoluble forms that plants are unable to take up. Surface applied phosphorus is often trapped in the top inch or two of the soil profile where it may not be accessible to roots.
To overcome the phosphorus-fixing capacity of soils, several strategies must be employed. The least expensive way to increase phosphorus availability is to lime acidic soils to raise soil pH. While it would be better to lime well before applying phosphorus, simultaneous applications are better than no lime at all. Any means of incorporating phosphorus will also increase availability in the root zone. Where limited to surface applications, front-loading larger applications of phosphorus early in the rotation is one way to increase nutrient availability sooner.
For a perennial crop like Christmas trees, incorporation of fertilizers can only happen during site preparation prior to planting. Tillage practices such as plowing, disking, or harrowing can mix surface applied phosphorus to some extent. A fertilizer hopper can be modified to direct phosphorus fertilizer into trenches cut during sub-soiling or field cross-checking prior to planting. These methods have the advantage of deeper placement and enough concentration of material for some of it to remain in available forms longer.
The most common phosphorus material applied in western NC is Diammonium Phosphate (18-46-0). It is slightly more water soluble than other forms and more likely to move down into the root zone. It has a 1:5 ratio of nitrogen to phosphorus providing opportunities for large applications of phosphorus. Monoammonium Phosphate (11-48-0) functions in a similar fashion to 18-46-0 but has an even lower ratio of nitrogen to phosphorus. In new field applications, 11-48-0 could maximize the phosphorus with a minimal addition of nitrogen. Unfortunately, availability of 11-48-0 is limited. Triple Super Phosphate (0-46-0) is a relatively insoluble form, but is the least salty material and works well disked-in or used in deep phosphorus applications. Growers have also used fine-ground rock phosphate (0-35-0) which can work well on acidic mountain soils. Whatever materials are used, apply enough material to overcome the fixing capacity of the soil so that trees can actually use it.
Phosphorus applications should be based on soil and tissue reports as with other nutrients. However, applications should aim not only at correcting existing deficiency but at building soil levels for future growth. Particularly in new field situations, Phosphorus applications should probably exceed soil test recommendations. Even where levels are adequate, maintenance applications are recommended to provide soluble P on an annual basis. On the other hand, extremely high levels (P-I over 100) can impact availability of several other nutrients and should be avoided. Excess phosphorus can be a water quality problem if soil moves off site. A two-depth soil sample can indicate the depth to which phosphorus is available and the need for more surface-applied fertilizer.
This value is the suggested rate of potash (K2O) in pounds per acre to be applied annually. Similar to phosphorus, the K20 rate is calculated from the K-I value, soil class, and proven requirements of the Christmas tree species being grown. Quite differently from phosphorus, potassium (K) does move down through the soil profile without being incorporated.
Lower rates of potassium are recommended during establishment of new field transplants to avoid root injury. However after the first two years in the field, vigorously growing trees have larger, more robust root systems that can withstand higher rates of fertilizer. Established trees require more potassium on an annual basis.
When the soil test recommends applying more than 100 lbs of K2O per acre, apply half in the spring and half in the fall. Applying more than 50 lbs K2O /acre at one time increases the risk of soluble salt injury to roots, especially if weather turns dry after application.
The decision of which potassium fertilizer to use should consider the risk of salt injury in addition to cost of material. Potassium Sulfate (0-0-50) is the least salty source of K and should be used during the growing season from April to October. Murate of Potash (0-0-60) is a less costly choice but contains twice the amount of salt. Murate of Potash should only be used during the dormant season when soils are cool and less likely to incur droughty conditions. Most blends contain 0-0-60 because of cost. Blends therefore represent a greater risk of salt injury since they contain the combined salt contributions of nitrogen, phosphorus, and potassium materials.
A blank or zero indicates no special need for magnesium. In this case, a calcitic lime source may be used to raise soil pH. A $ will appear in this column when magnesium is low. If magnesium is low and soil pH needs to be raised, dolomitic lime should be used to both raise the pH and provide magesium. If magnesium is needed ($), but the pH is already high enough, other non-liming sources of magnesium should be used rather than dolomitic lime. This is also true if a magnesium deficiency is induced by high levels of calcium. In most fields, magnesium requirements are satisfied in the course of liming.
Non-liming sources of magnesium that do not significantly alter soil pH include ground applied magnesium sulfate, sulfate of potash magnesia, magnesium oxisulfate, and foliar applications of magnesium sulfate or epsom salts. Blending small amounts of magnesium fertilizers with N-P-K fertilizers may ensure a more uniform application.
For most soils an application of 25 pounds per acre of actual magnesium is recommended as a corrective action. However, the soil test and information from the lime or fertilizer label can be used to calculate the actual magnesium contribution provided by a lime application and how much additional magnesium might be needed from other sources. For more information regarding magnesium fertilizers see Christmas tree note 5.
S, Mn, Zn, Cu, and B
Sulfur, a secondary nutrient, and manganese, zinc, and copper micronutrient recommendations are simply listed as sufficient or not according to the availability index on the report. A zero appears in the appropriate column if the soil level is considered to be adequate and a $ appears if an application is recommended. The $ appears if the micronutrient availability index for the crop is 25 or below. This symbol also refers to the $ Note, which provides further information on micronutrient rates of application. If soil pH is high enough to affect Mn availability, a pH$ or $pH notation will appear in the Mn column. Potential zinc toxicity is indicated by a Z under the appropriate column of the report. Possible copper toxicity is tagged by a C under the copper column. Soil is not tested for boron due to its variability in the soil. Plant tissue analysis provides additional insights regarding micronutrient levels in Christmas trees and should be considered after trees become established.
Note: Accompanying each soil test report is NCDA Note 5, "Christmas Trees" which provides additional information on lime and fertility requirements for Christmas trees.
Soil Sampling as Part of a Comprehensive Soil Fertility Program
Soil samples are the most important source of information available to support soil fertility and Christmas tree nutrition. However, they are not a stand-alone tool. They should be used as part of an overall soil fertility management plan. Other sources of information that can be vital to making the best decisions include: farm and field cropping history, influences of recent weather, the use of plant analysis test results to show how Christmas trees are responding to soil treatments, the knowledge and experience of the Christmas tree grower or manager, and expertise of NC Extension Service county agents or NCDA & CS soil agronomists. In addition, the decision on what to apply may involve soil test results from neighboring blocks or similar ages of tree, or even the remaining life of the farm lease. For instance, a grower might opt for faster-acting gypsum instead of lime if the percent calcium is low and soil pH is marginally adequate and there is only the current season to harvest remaining trees on leased land.
Further, a single soil test only reflects one point in time and the mixed soil in one sample box. The real benefit of soil and tissue sampling to a Christmas tree farmer comes with time and experience. Several samples collected over several years show trends in soil fertility. A manager can see the impact of treatments over time and get a sense of how difficult it is to correct a problem like high pH. If one sample is skewed by what went into the box, the context of other samples allows the grower to discount unreasonable results.
Soil testing and fertilization can be a little like sailing a boat. The captain knows which direction is the boat is going, but to get there he or she must tack across the wind. The boat seldom points exactly toward the objective. With soil and tissue sampling, the goal is to approach optimums for plant growth, add nutrients when results are low, and correct with balancing treatments when the mark is overshot. The real return in taking soil and tissue samples is in long-term productivity. The truth is growers who take soil samples and pay attention to the results grow better trees.
This discussion introduces 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 N.C. Cooperative Extension center, or the NCDA&CS Agronomic Division at 919-733-2656.
Publication date: April 23, 2014
Revised: Sept. 6, 2019
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