Maintenance II: Fertilizer application
Drought tolerant plants generally do not require fertilization unless nutrient deficiencies occur. In fact, most soil generally is fertile as is for trees and shrubs, primarily because mycorrhizae (special fungi that live in and on plant roots) help plants exploit nutrients in larger volumes of soil. Consequently, avoid over-watering, soil compaction, excessive fertilizing, and exposing the soil and plants to harmful chemicals and pollution since all of these may harm the mycorrhizae fungi and other useful microorganisms available in the soil.
Unless otherwise advised by a professional, fertilizers should not be applied to more mature trees or shrubs. Keep in mind that every plant part– leaf, flower, fruit, stem, or root – removed from the soil takes some nutrients with it. It therefore is advisable to allow plant residue to remain in the soil and decompose in place.
Fertilizers serve to replenish soluble salts. If at any time the soil becomes deficient in any one of a number of minerals (such as nitrates, which are a source of nitrogen; phosphate, which is a source of phosphorus; and compounds containing potassium, calcium, iron, magnesium and aluminum), fertilizers can replace them. Base the need for fertilizers on plant performance, on visual clues such as lack of vigor, sparse foliage, light green or yellow leaves, twig die-back, gradual slowing of growth, and on a comparison with adjacent plants. Before assuming nutrient deficiency, consider other external environmental effects such as irregular watering, disease, or exposure that might adversely influence plants.
When diagnosing nutrient deficiencies, keep in mind that both nitrogen and iron deficiencies can produce chlorosis – yellowing of plant tissues. Nitrogen deficiency causes the older leaves of the plant to become chlorotic first; new leaves may follow. In the case of iron deficiency, however, plant leaf veins remain green, but the rest of the leaf turns yellow. New leaves become chlorotic first, and older leaves may follow.
In the spring, when growth is lush, there is no advantage to forcing growth with fertilizers. If fertilizers are necessary due to signs of nutrient deficiency they should be applied right before new growth begins in late winter and early spring. When applying fertilizers, always do so before a scheduled irrigation, not after the soil is already wet, and always irrigate sufficiently when applying fertilizers since nutrients must be dissolved in water to enter the roots of the plant. Exercise care when watering, however, as watering too deeply can cause nutrients to move below the root zone, where they are of no use to the plants. This process, which is known as leaching, can be reduced by applying water to moisten the root zone and not beyond.
Use discretion when applying any fertilizer. Too much potassium inhibits the uptake of nitrogen and calcium. Too much nitrogen stimulates lush leaf and stem growth, reduces root development, lowers carbohydrate reserves, and increases susceptibility to environmental stresses such as disease. In general, do not use more fertilizer than is recommended. Since fertilizers dissolve in soil water, the application of too much fertilizer will result in high salt concentrations outside the plant’s roots, which in turn can cause the plant cell membrane to reverse the flow of water. This will result in what is known as physiological drought. “Fertilizer burn” or scorched foliage is the visible symptom of this form of dehydration within the plant.
| Fertilizer checklist |
|---|
| Too little fertilizer | Too much fertilizer |
|---|---|
| - Slow growth | - Wilted or malformed leaves |
| - Little resistance to disease or attack by pests | - White crust on clay pots and over the surface of the potting mixture |
| - Pale leaves, sometimes with yellow spotting | - Winter growth is lanky while summer growth may be stunted |
| - Flowers may be small, poorly colored or absent | - Leaves may have brown spots and scorched edges |
| - Weak stems | |
| - Lower leaves drop early |
Consequently, read and follow label directions for applications rates and guidelines, and do not use higher amounts of fertilizer. Also, keep in mind that accepting a lower growth rate for your plants can minimize or even supplant the use of fertilizers. In addition, once plants are established, reduce the amount of nitrogen applied as well as the application rate and frequency of application. The application rate stated on fertilizer labels is intended for optimum growth, and thus can be reduced after plant establishment.
The upcoming article of this series will discuss pruning practices.
This article is part of a series of articles prepared by the Center for the Study of the Built Environment (CSBE) on water conserving landscapes.
For additional information on water conserving gardens, visit the CSBE web site at www.csbe.org
Support for the CSBE project on water conserving landscapes is provided by WEPIA (Water Efficiency and Public Information for Action), a program being implemented in collaboration with the Ministry of Water and Irrigation and funded by the United States Agency for International Development (USAID).