Fall Fertilization After a Drought

A drought is a particular stress that can have short and long-term effects on tree health. These effects should be taken into consideration when planning a fall fertilization treatment subsequent to a severe drought.

Effects of Drought on Tree Physiology

During a drought, there is little or no available soil water to dissolve minerals for subsequent absorption by roots or mycorrhizae. Therefore, mineral absorption is minimal. As a result, growth slows or even halts. Eventually, the tree can suffer numerous deficiency symptoms even if soil analysis shows adequate soil fertility. This happens because nitrogen and minerals can only be absorbed when they are in aqueous solution. Without water, the minerals remain undissolved and therefore, unavailable.

Under prolonged conditions of dry soil, fine feeder roots will die back. This exacerbates the problem, because it reduces the ability of the tree to absorb water and minerals even during an occasional light precipitation. In the absence of feeder roots, the tree cannot effectively absorb water rapidly, should a brief but light rain occur.

How Does this Affect Fertilization Programs?

After a prolonged drought, trees can suffer from a broad range of nutrient deficiencies, so a comprehensive fertilization treatment is needed. This should include N-P-K as well as various micronutrients. However, fertilization will not provide effective treatment unless the drought is ended. No fertilization treatment will replace the absolute need for water.

What Kind of Fertilization is Recommended After a Prolonged Drought?

After a prolonged drought, it is best to provide a comprehensive mineral treatment that is low in soluble nitrogen, but rich in micronutrients. Such a treatment will not dramatically stimulate top growth at a time when there are not enough feeder roots to support the increased water and mineral needs of a larger canopy. The ideal treatment will provide mineral nutrition that will support root growth and reverse deficiency symptoms without stimulating additional foliage. A combination fertilizer containing low levels of nitrogen, or slow-release nitrogen with phosphate and potassium, along with iron, magnesium, and manganese, is a good practical treatment that can promote the desired results.

What Would LebanonTurf Recommend?

Roots PHC for Trees SRN 11-22-22 is an excellent formulation for treatment of trees after a prolonged drought. It contains relatively lower levels of Nitrogen, and it contains soluble forms of iron, manganese, molybdenum, copper, boron and zinc.

Another effective post-drought formulation is Roots PHC BioPak Plus. This provides 3-0-20 NPK, along with magnesium, sulfur, boron, iron, manganese and zinc. BioPak Plus can be used alone, or it can be added to your existing fertilization treatment to boost the micronutrient content, and to provide a bacterial component that will promote soil fertility over the long term. In addition, PHC BioPak Plus is formulated so that all the minerals remain soluble regardless of soil pH.

Both products, Roots PHC for Trees 11-22-22 and Roots PHC BioPak Plus include six species of rhizosphere bacteria to increase beneficial microbial activity in the root zone to take advantage of sustainable biological fertility provided by these microbes.

Is There a Way to Promote Growth of New Fine Feeder Roots?

The fertility treatments recommended above will help promote new growth of fine feeder roots. However, an inoculation with mycorrhizal fungi will further enhance the efficacy and reach of these roots, so inoculation with mycorrhizal fungi should be given serious consideration, especially if the tree is in serious decline. Inoculation can be done by soil injection, using Roots PHC Injectable for Trees or Roots PHC Ecto Injectable. Alternatively, it can be done by core aeration techniques, using Roots PHC Vertimulch. All these products are compatible with Roots PHC BioPak Plus.

What About Effective Water Penetration?

Very dry soils are usually hydrophobic, that is, they resist wetting and resist penetration of water. This reduces the depth and spread of fertilizers and other products applied in liquid form by injection or drench. This problem is easily addressed by adding a wetting agent to your liquid injection or spray tank. Roots PHC for Trees fertilizers already contain an advanced wetting agent. For other products, PHC offers a natural-based wetting agent called Yuccah®. Yuccah is a soluble liquid that, when added to any liquid spray or injection treatment, will significantly improve both its penetration and spread through the soil. Yuccah is recommended as an important adjuvant when applying dissolved fertilizers, especially in droughty soils.


Post-drought fertilization will only be effective provided several conditions are met:

  1. The tree must have prolonged access to adequate water (that is, the drought must be ended).
  2. The tree must re-grow new feeder roots to replace those that had been lost during the drought.
  3. Mycorrhizal fungi (native or applied) must colonize the new feeder roots.
  4. The fertilization treatment should address numerous mineral deficiencies at once.
  5. A wetting agent should be employed to reverse the tendency of dry soils to resist water penetration.