Fertilizing New Trees
The best time to fertilize in the Bandera County area is mid-March. Our usual last frost is the first week of April so fertilization should be timed to provide new growth soon thereafter. Do not fertilize after early September because the tender new growth you stimulate might be damaged by early cold spells.
What Fertilizer to Use
Remember that a 15-5-10 fertilizer means 15% nitrogen (N), 5% phosphorous (P), and 10% potassium (K). Hill Country soils are generally deficient in nitrogen, have ample phosphorous, are high in potassium. A nitrogen fertilizer is all that is really required in most cases. The least expensive one is ammonium sulfate (21-0-0). It also contains sulfur which is helpful in correcting for the high alkalinity of our soils. It dissolves quickly and goes right to work, but can burn plants if care is not taken. Because it is used up quickly by the plants, it should be applied several times during the year, i.e., every 45 days. Remember that N is what is required to stimulate vegetative growth.
Balanced fertilizers such as 15-15-15 or 10-10-10 can cause a particular kind of chlorosis by adding too much P when it is already adequate in our soils. Fertilizers with a ratio of 3-1-2 are more desirable because that is more like the ratio actually used by plants.
My favorite fertilizer is Osmocote in a 19-7-10 formulation. It is a pelletized slow release fertilizer that releases its nutrients over several months. It will not burn plants and only requires one fertilization per year. It is expensive, but very effective and mistake proof. If it is not available in small quantities locally, call Love Creek Nursery, 830 589-2588.
Natural, or organic fertilizers are very slow release, and have lower N amounts. They can be applied pretty much anytime and will release N to the plant over several years. The good thing about them is that they are complete fertilizers, supplying not only N, P, and K but many micro nutrients to the plant as well. We strongly recommended them as an addition to the commercial fertilizers.
How to Fertilize a Tree
One must get the fertilizer to the feeder roots, which are located under the outer edges of the branches. If fertilizer is applied too near the trunk (inside the feeders) little growth will occur. If the tree gets the nutrients correctly, you will see immediate, strong growth at all growth points. If it does not appear, go back and try again until you do see the good growth. For a newly planted tree you know exactly where the feeder roots are—at the edge of the old root ball—and one needs to apply the fertilizer a few inches outside of them to coax them to grow outward.
It is much better to apply the fertilizer in holes punched 6 inches or more into the soil to prevent the nutrients from being washed away by rain as well as preventing grass and weeds from stealing them. Make at least six holes around small trees, and many more around larger trees. An in-and-out pattern with a hole every few feet is needed around larger trees.
How Much Fertilizer to Apply
A good rule of thumb is that a tree needs about .2 pound of pure nitrogen per inch of trunk thickness per year. To convert this to the fertilizer of your choice, i.e., 21-0-0, 21% N is the same as .21 (to convert % to decimal fractions, move the decimal two places to the left). Now divide the amount of pure nitrogen by the percent nitrogen: .2/.21 = 1. Thus one pound of 21-0-0 equals .2 pound pure N. For most fertilizers, 2 cups weigh about one pound, so to make it simple, use 2 cups 21-0-0 per year per inch of diameter. With this fertilizer (since it becomes activated all at once and may burn the plant) it should be applied not all at once but in monthly of bi-monthly applications.
For the 19% Osmocote mentioned above (since it will not burn and is slow release) it can be applied all at once. About two cups per inch of diameter needs to be applied.
A 10% N fertilizer would require twice as much (4 cups per inch) to supply the same amount of pure N.
With a proper fertilization program any kind of tree will grow rapidly. The hardwood trees that Love Creek propagates and sells should grow at least three feet a year once the roots are established and they receive proper fertilization.