Tree 101: Does Your Tree Really Need Fertilizer?
by Ann Barklow, Consulting Arborist

Fertilizers are sometimes used to excess so it’s important to determine what your tree needs before deciding whether or not to fertilize.

The two most direct ways to find out if your tree needs fertilizing is to observe its growth rate and examine the leaf color.

Look closely at your trees smallest branches - do you see new growth? Young shade trees between 1-5 yrs. old with twig growth of 9-12” per year, as well as mature trees with twig growth of 6-8” per year will probably not be helped by additional nutrients. However, if your tree is lacking new branches, growing slower than the above mentioned rates, or showing considerable loss of branches and leaves, especially in the very top of the tree - this could be a sign of nutrient deficiency.

Leaf color should be typical for the species - watch for unusual yellowing which may indicate a specific nutritional deficiency. In citrus, for example, yellowing is common between leaf veins, often indicating iron deficiency. Note also whether new or old foliage is affected by the problem. A tree requiring nitrogen may have healthy new foliage which it displays at the expense of yellowed, brown or withered mature foliage. Is your tree deciduous? Our native California sycamore’s leaves may look withered as it starts to lose it’s leaves in autumn - this natural process should not be mistaken for nutrient deficiency.*

For most shade trees, nitrogen (N) is the key element needed for optimum growth. Nitrogen, along with phosphorus (P) and potassium (K), two other key nutrients, may all be present in the soil but become unavailable to the plant because of special circumstances.

One such special circumstance is the pH factor of the soil. For most of Southern California the soil pH is alkaline, making it high on the pH scale (pH >7.0). High pH levels, especially exceeding above pH 7.5, can chemically “tie-up” nutrients such as iron, manganese and phosphorus, making them inaccessible to some trees. Excess zinc in our soils can also make iron unavailable.

Our alkaline soils can create the iron deficiency often seen in citrus, azalea and hydrangeas, as well as other commonly grown plants. Elements such as calcium, magnesium, sulfur and the other micronutrients that are usually sufficient in most soils can be missing as well. Acidic sandy soils (pH <7.0) that are low in organic matter can make potassium inaccessible. Most shade trees prefer a soil with a pH range between 5.2 to 6.2, well below the range in many Southern California gardens. There are some trees, including California native and mediterranean species, that do well in our alkaline soils.

A soil test is key to giving you an accurate description of your particular soils chemistry while offering suggestions for improvement. Often for low pH, an application of gypsum and sulfur is recommended, as well as mulch. We use Wallace Labs, located in El Segundo, California, for our soil testing needs.

Another common problem is improper watering. Overwatered trees can produce symptoms of nutrient deficiency as the excess water drowns out the plants ability to take up proper nutrients. Drought stricken trees may exhibit symptoms such as a loss of leaves and twig die-back and can also succumb to pests that take advantage of their weakness.

One of the simplest ways to make sure your trees are being watered correctly is to take samples of the soil around your tree and use the look and feel method to determine moisture content.

A general rule for watering trees is to water deeply and infrequently rather than frequent shallow watering. Most trees being watered on the same schedule as thirsty turfgrass will suffer. Sprinkler heads watering on or too near the tree's trunk should be redirected to the drip-line and beyond. A thick layer of mulch is always recommended to help retain moisture. If your tree is not mulched, especially in sandy soils, it's difficult to keep the soil properly watered.

If you determine that a fertilizer is necessary, all-purpose organic-based fertilizers such as cottonseed meal, for example, have a good balance of nutrients. Usually a bag of cottonseed meal consists of 6.5% nitrogen, 3.0% phosphorus and 1.5% potassium. Other all-purpose organic fertilizers with similar NPK % may also be used and may contain a mixture of cottonseed meal, fish bone meal, kelp, pulverized feathers or other organics. Some include beneficial soil microbes known as mycorrhizae. Some fertilizers are made specifically for certain trees, such as citrus or other fruit trees.

Place fertilizer around the tree starting at the drip-line area and well beyond. (see above diagram) For most trees, absorptive roots start a few feet from the trunk, then spread way past the outermost branches (known as the drip-line) by 40%-50% or more. In fact, for most trees, research shows that 2/3 of absorptive roots for the entire tree are located past the drip-line. It is not uncommon to find the absorptive roots of large urban trees occupying an entire lawn or backyard of and beyond if physical barriers are absent. Trees growing in sandy well-drained soils may have more extensive root systems than those in finer textured clays. Never fertilize trees near their trunks as the high concentration of mineral salts may damage the root collar or trunk base.

Broadcast organic dry fertilizer on the soils surface using a cyclone seeder or lawn spreader for even distribution. Fertilizer should be watered in thoroughly, with a garden hose if possible, to move the nutrients down into the root zone. A second watering the following day will further aid distribution of the material, especially in heavy turf. If your tree's root zone is in the lawn and you are already fertilizing the lawn - you are also fertilizing your tree, so don't over do it. Your chosen fertilizer should give an accurate formula on the box or bag to help you calculate how much to use depending on the size of your tree.

Applications of fertilizer should be timed so that nutrients are available when your tree needs it the most - during periods of rapid growth. For some trees, this occurs once in the springtime. Spruce and fir trees, for example, have a single flush of growth in the spring. This is also true for deciduous trees such as liquidambar, jacaranda, birch and maple. In general, to stimulate spring growth of trees, apply fertilizer in the late summer or early fall. Tree roots will continue to absorb nutrients until soil temperatures fall below 40 degrees.

Many more trees, especially some of our shade tree favorites, have two or more periods of growth, the most important in the spring and another in midsummer. Holly and yew, for example have two growth flushes. A few, such as juniper, grow continuously during the growing season. For these, fertilizers may be applied after the spring growth flush. Fertilizers should not be applied in early spring; applications are sometimes lost through leaching by spring rains.

Keeping your garden properly mulched and recycling organic matter back into your garden soil as compost is an excellent way to add nitrogen and other nutrients to your soil, decreasing the need for store-bought fertilizers and increasing the chances of success for your trees health. Healthy trees, especially mature trees, showing no symptoms of nutrient deficiency can be harmed or even killed by too much fertilizer, so use care and avoid applying fertilizer indiscriminately.

*Most native trees do not require fertilizer