Nutrient Management for Christmas Trees

As commercial Christmas tree production increases in South Carolina, so do the problems growers are experiencing. Tree quality is improving, but it needs to continue to improve to meet consumer expectations. Problems such as slow growth, poor tree color, poor tree vigor, and susceptibility to insect damage and disease demonstrate that more attention needs to be paid to site fertility. With proper planning and careful management, Christmas trees can grow successfully in South Carolina.

Understanding that growth is very slow in the first year and that it increases rapidly with age will help growers make better fertilizer management decisions. Under ideal conditions, maximum growth occurs during the year prior to harvesting. Rapid growth generally occurs the third year after planting for most species. On excellent sites with high fertility, rapid growth may begin in the second year. On fertile soils with adequate moisture, the growth rate will continue to increase after 3 years.

FERTILIZATION
Most trees are not grown under ideal conditions. On poor sites, growth rates are generally very slow, both in terms of shoot growth and root spread. Fertility problems can be overcome with soil testing and treatment before and after establishing the trees. The critical period is during early growth and through the stages of rapid growth in the first 3 years.

Effective fertilization depends on understanding how a tree grows, how fertilizers move in the soil, and when a tree can absorb nutrients. It is very important to keep in mind where the roots are so fertilizer can be distributed around the tree to coincide with root spread and control of competing plants.

Following is a list of popular species in the state and desired pH ranges for their successful management.

 Table 1.   South Carolina Christmas Tree Varieties

Establishment
It is strongly recommended that representative soil samples be obtained before planting and analyzed at Clemson’s Agricultural Service Laboratory for proper nutrient recommendations. All recommended fertilizers, lime, and gypsum (except nitrogen) should be incorporated thoroughly into the top 6 to 8 inches of soil prior to planting. This will ensure adequate availability of nutrients in the root zone, especially of non-mobile elements like phosphorus. Buildup of nutrients is not desired since the growth rate is relatively slow during the first year. Soil samples should be obtained again at the end of the first year to ascertain the nutrient levels. Fertilizers must be uniformly surface-applied since incorporation is not possible after planting.

Maintenance
It is important to maintain soil pH in the desired range through regular soil tests. Improper pH levels affect the availability of nutrients and result in reduced growth and delayed maturity. While soil pH higher or lower than the desired level is not always indicated directly by any plant symptoms, higher soil pH can result in a general yellowing of the foliage.

The amount of lime recommended is designed to raise the pH level to the desired ranges, given in Table 1. Lime may not be recommended for most South Carolina soils because generally pH tends to be just above or in the desired range for most species. Lime, if recommended, should be surface-applied around already established trees even though it is expected to alter pH only in the surface 3 to 4 inches. Each surface application of lime should not exceed 1.0 to 1.5 tons per acre.

Occasionally there may be a need to lower the soil pH. However, another soil test should be done to confirm soil pH before any corrective action is taken. In sandy textured soils with lower Cation Exchange Capacity (CEC) and where nitrogen is needed, ammonium sources—preferably ammonium sulfate—will help lower the pH to the desired range. Alternatively, for heavy textured clayey soils with higher CEC, 150-200 lb/acre of elemental sulfur can be used for effectively lowering the soil pH.

Table 2: Fertilizer Recommendations for Christmas Trees (lbs. of N - P₂O₅ - K₂O per acre)
for soil groups 1, 2, 3, or 4, desired pH 5.5-6.0

* rate of nitrogen varies depending on age of trees
x = 50 lbs. N/acre in year 1
x = 75 lbs. N/acre in year 2
x = 100 lbs. N/acre in years 3 and beyond

Common tree spacing followed in South Carolina is 6 feet by 6 feet, yielding 1210 trees per acre. When treating just a few trees, the per-acre amount should be divided by 1210 to get the fertilizer amounts for each tree.

The nitrogen rates in Table 2 should be split, with the first half applied prior to the active growth stage (early spring) and the rest in May. The nitrogen dosage should be placed around the tree in the root area  and not broadcast on the whole field.

Since most Christmas tree species grow best under acidic soil conditions, the most economical way of lowering pH is by using ammonium-based nitrogen fertilizer sources like ammonium nitrate and ammonium sulfate. Ammonium sulfate is a preferred source because of its higher acidic residual effects.

At the time of establishment, recommended phosphorus should be spread uniformly and incorporated. Soil should be tested again at the end of the first year, and if phosphorus level is Medium or lower, the recommended phosphorus should be surface-applied. Higher levels of soil phosphorus are required for improved bud setting and better tree development. Phosphorus availability may be reduced in acid soils due to increased fixation. Any lime recommended should be incorporated along with, if not prior to, phosphorus application.

Potassium should be applied at 150 lb. K₂O/acre when establishing new plantings if the soil test shows potassium levels of medium or lower. For existing plantings, any potassium application above 100 lbs./acre should be split, with half applied during early spring and the other half in the fall. Availability of potassium may be reduced if gypsum is applied during these stages. Soil potassium levels should be monitored by soil tests at regular intervals.

Soil levels of available calcium and magnesium should also be monitored through regular soil testing. Maintaining an adequate supply of calcium will prevent needle drop, and a sufficient level of magnesium is critical for maintaining the trees’ healthy color. Gypsum is a preferred source of calcium when no lime is recommended. Lime should not be applied unless recommended because it can raise the pH, resulting in poor growth. In turn, gypsum should not be used in place of lime. Table 3 gives amounts of gypsum required to raise calcium to 55 percent in the base saturation. This ensures an adequate calcium supply to the trees without disturbing the pH balance.

Table 3 Gypsum Application Rates (lb/acre)
target Ca percentage is 55.

For further assistance, contact your local county Extension agent.