Nitrogen is the most critical nutrient, and is also the most rapidly depleted. Fertilizers  should be applied to the root system as a granular via broadcast or as a soluble liquid injection within the rhizophere. Our soils, here in the Hill Country are high alkaline and usually contain phosphorus and potassium, this combination acts as an iron and mineral blocker, which can cause iron chlorosis or other mineral deficiencies. Adding disper sulfur (in pellet form to facilitate easy application with a fertilizer spreader)  will help lower the soil pH to a level rendering the insoluble nutrients to a soluble state. For trees it is best to apply a nitrogen fertilizer like Ammonium sulfate  21-0-0 with 24% sulfur. Ionate by Hi-Yield is an excellent iron supplement with a 16% iron and a 10% nitrogen and with  some trace elements. These can be acquired at your local farm feed store.

All fertilizers contain salts which can burn delicate absorbing roots. Soluble fertilizers release these salts in a relatively short period of time, increasing the risk of damage. It is best to apply a slow release type fertilizer. Water soluble fertilizers, which are suspended in water for application offer the most protection from burning. They release nutrients over time, rather than all at once and they don't readily leach away.

Trees and shrubs should be fertilized over the entire root system, which usually corresponds to the branch canopy and beyond, approximately one and a half times the drip edge.

Newly planted trees and shrubs can be fertilized by working tree fertilizer into the soil around the plant. For the first six months a miracle gro type fertilizer is best, as it will not burn the roots.

Large trees should be fertilized by using a water based suspension which is injected into the ground under hydraulic pressure for more effective dispersion of nutrients throughout the root zone. Surface broadcast is also very effective and should  be followed by thorough watering for both methods.

Watering after Fertilizing

Watering after fertilization is important in order to complete the task you started out to do.

Water properly with a low profile sprinkler system and don't be skimpy on the water or over do it either. Shallow watering will force the roots to grow too close to the surface and in the heat of our summers, the roots will dry out and die. This can cause extensive root damage and sacrifice the overall tree health. Trees don't like wet feet, but keep in mind to quench their thirst. Set out a can or bottle and when you have at least one inch or water showing, then move on to the next area of application. If you don't have an automatic watering system and you can only facilitate a few trees at a time, that is fine also, just fertilize a few trees and water them as you can afford your time until the entire task is done. Trees are very patient and they will reward you for your kindness. Of course, if you know that mother nature will help you out and provide the rain, then you can apply the fertilizer within her schedule and split the work task.
 

Trees under Stress

 

The trees should have enough carbohydrate reserve to weather these conditions. I would recommend a slow release, preferably organic, fertilizer. Trees under stressed conditions require a lot of  energy to metabolize nitrogen, whether NH4 or NO3 and then apply a light sustainable amount, rather than a heavy application.  A good application of Protium Hydroxide will provide the absolute best results. This product is not available from any of your local garden supply stores, but can be found in your local grocery store, and best of all, it is found at the end of your garden hose (it is water) and apply it liberally, abundantly and, properly, but not excessively as trees don't like wet feet.