Nitrogen levels are naturally low in almost all California soils and phosphorus levels may be low in reddish soils that have hardpan or claypan layers in the subsoil, according to the 2015 edition of the University of California Master Gardener handbook.
In the central San Joaquin Valley, much of clay soil that’s common to our region will be low in both nitrogen which promotes green growth and in phosphorus which encourages flower production. Low annual rainfall amounts (averaging 11.6 inches per year in the Fresno/Clovis area) causes high pH levels or high alkalinity in local soils. Iron deficiencies are common to plants, especially shade-loving plants, grown in alkaline soils. High pH levels also prevent roots’ uptake of soil phosphorus.
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Valley gardeners will have the best chances for successful gardening if they can recognize the effects and signs of naturally occurring nutrient deficiencies and then apply fertilizers and supplemental nutrients to help correct those deficiencies.
Nitrogen (N) percentage in fertilizer is identified by the first number of three on the label. Given that nitrogen levels are low in California soils, you might think that the higher the percentage, the better. This is true for crop-bearing trees including citrus and regularly mown lawn grasses which should be fed with fertilizers such as ammonium sulfate with 21 percent nitrogen or a good quality lawn food (which can be used for both citrus and lawns) with a similar percentage of nitrogen as well as additional micronutrients. For most other plants and trees, regular, consistent feeding with a lower-nitrogen slower-release fertilizer will be best.
Plants which develop chlorosis or yellow leaves with green veins (very common in gardenias in the Valley) can be treated with chelated iron products in combination with soil sulfur which will help lower soil pH levels.
Phosphorus (P) percentage is the second number given on most fertilizer labels. Phosphorus deficiencies can cause stunting and purplish leaves. Additional phosphorus can be added to the soil with ammonium phosphate (20 percent P) or bone meal (15 percent P). Apply supplemental phosphorus close to plants’ roots since phosphorus does not move well through soil.
Potassium (K) percentage is the third number on a fertilizer label. Potassium encourages good root development and health. Since sufficient potassium is found in most California soils, supplemental potassium is seldom required.
The primary N-P-K numbers may be followed by a list of macro and micro nutrients contained in the fertilizer. These lists are getting longer as manufacturers now include beneficial microorganisms, humic acid and beneficial fungi to the ingredients list. Tomato fertilizers should contain extra calcium (Ca), at least 3 percent to help prevent blossom end rot. Shade plant formulations should contain additional sulfur (again at least 3 percent) to lower Valley soil pH levels.
So how many fertilizers and supplemental nutrients are on the shelves in my garage? There are just four: bone meal, soil sulfur, a tomato and vegetable food (also used for roses and other flowering plants) and an azalea, camellia and gardenia food applied to all flowering shade plants as well as potted Japanese maples. We have no lawn, so no need for a high-nitrogen lawn food. The tomato and vegetable food has a 4-5-3 formulation with 3 percent calcium and 1 percent sulfur. The shade plant formulation is 5-5-3 with 3 percent calcium but a low 1 percent sulfur.
Next week: a growing-season fertilization schedule for most of the plants and trees in our gardens