CRFG "Fruit Gardener" August 1993
How to Diagnose Plant Health Deficiencies
Without Calling The Doctor
You are alarmed to see that the leaves of your lemon tree have paled to a sickly yellowish color while remaining green along the midribs. Your newly sprouted peach leaves have mysteriously grown out purple. Something is clearly wrong. You see no bugs, and you suspect that some nutrient must be missing from your soil. But which one?
Answers to these and other questions were addressed by Dr. Ben Faber, Ventura/Santa Barbara County Soils and Water Advisor, during the Seventh Annual Los Angeles Home Garden Conference at Pierce College in Woodland Hills, Calif. His focus was to suggest ways for home gardeners to diagnose nutritional deficiencies in their own gardening projects without resorting to expensive tissue testing.
Dr. Faber started by reminding his audience that the predominant part of any plant is water—about 80% water to be precise. Most of the problems in agriculture that he sees are generic problems related to water, such as excess salinity in the water, which then gets deposited into the soil. Other generic problems have to do with fertilizers or soil fertility. Although deficiencies in these areas can be diagnosed by tissue testing at a lab, to do so is expensive; practically speaking, most deficiencies can be ascertained simply by eyeballing the plant since the lack of certain nutrients often shows itself through specific symptoms.
Soil and Its Functions
Dr. Faber listed the following as the four most important functions of soil: to provide a continual and adequate supply of nutrients, aerate roots so as to provide oxygen, offer mechanical support, and serve as a water reservoir. Adding or removing certain substances can modify these functions.
Plants manufacture needed sugars by means of photosynthesis. Other than moving or transplanting a plant from one level of light intensity to another, the grower really has little control over this process. However, the chemicals a tree or vegetable takes up by its roots from the soil can be controlled.
Modem horticulture recognizes certain chemicals as essential to healthy plant growth. The ones needed in greatest quantities by the plant—nitrogen, phosphorous, potassium—are called macronutrients. Certain other chemicals—also needed by plants but in much smaller amounts—are called micronutrients. Their necessity is more subtle but just as essential. A shortage of these will create noticeable symptoms and even seriously impair the plant’s ability to grow and produce fruit.
Fertilizer is defined by law as having a certain percentage of macronutrients. For instance, a 10-10-10 bag contains 10% nitrogen, l0% phosphonis and 10% potassium. Although compost is often 1-3% nitrogen by dry weight, the nutrients vary depending on the source and time of year. Therefore by law, compost cannot be called a fertilizer even though it does contain certain chemicals important to plant growth.
The advantage in using rotted organic matter is that soil microorganisms, in breaking down this material, slowly release nutrients to plants. Although the overuse of chemical fertilizer can seriously damage or even kill plants and contaminate the environment, Dr. Faber warned that an excess of compost can also cause nutrient overload. He advised adding to your soil only well-composted material—compost being defined as the product of anything that has reached a state of low activity in decomposition. Although, despite popular belief, composted eucalyptus mulch is not toxic to citrus and avocados, Dr. Faber said only compost, not "green" cuttings or clippings, should be used. Any fresh material added to soil will rot, and decomposition releases heat. This heat will kill your plants as well as rob them of nitrogen. Freshly chipped materials can be used successfully as a mulch or surface application. In a choice between compost or peat moss (partly decomposed moss which tends to be acid, if Canadian), compost is better because it’s cheaper.
Look at the Leaves
Fruit growers who wish to determine soil fertility might do well to grow some sweet corn, a really good indicator plant. Sweet corn will demonstrate very clear deficiency symptoms. If the sweet corn planted shows older, yellowing leaves with a "V"-shaped patch of green, your soil definitely needs something. In this case it is nitrogen.
Look at the leaves of your plants they can reveal much to the experienced eye. With most plants, nitrogen deficiency shows up as a general yellowing of the older leaves. Yellowing is not always the case, however, since in some peach varieties a nitrogen-deficient plant turns purple.
In California, phosphorous deficiency is rarely seen on trees, and only occasionally on vegetables. In tomatoes, a lack of phosphorous shows up as purple on older leaves and a stunted plant. To counter this, apply superphosphate, chicken manure or fish emulsion.
Potassiumdeficiency in prunes and potatoes is likely to show up as leaf scorching (brown margins, brittle and dried-up looking). If growing corn (and using it as an indicator), look for crinkled edges along the margins of lower leaves. Potassium deficiency is very common in sandy, shallow soils.
Now for the Micronutrients
What about a deficiency of micronutrients? If it is the younger, smaller leaves on your orange or apple tree that are showing problems, you are possibly missing some micronutrient. California has mostly young soils, reasonably rich in micronutrients. Old soils tend to be deficient in these micronutrients because rain, time and weathering have washed them out. Still, deficiencies can occur.
A zinc deficiency on corn shows up as young leaves that are green on the margin and mid-ribs, yellow in between. High pH soils tend to be zinc-deficient. Another symptom indicating deficiency of this micronutrient is abnormally small leaves at the ends of branches. With apple trees, leaves will be plentiful at the bottom of a branch, but very sparse at the terminals. In peach trees, a rat-tail appearance at terminal points, where there maybe some flowering but no leaves, indicates a lack of zinc.
Younger leaves that have turned yellow indicate a lack of iron. In most plants, inadequate iron shows up as interveinal chlorosis—that is, yellowing between the veins, green right around them—but only on leaves at the growing tips. Iron deficiency can be corrected using compost, fish emulsion or iron chelate. Blood meal has iron but it is an expensive way to get iron to a plant. A very cheap solution is to soak a bunch of rusty nails in a bucket of water. Iron from the nails leaches into the water. Pour this water around the iron-deficient plant.
Manganesedeficiency is commonly found on subtropicals in our area. A plant needing this micronutrient will show a yellowing between the veins, but with wavier margins than the very sharp interveinal chlorosis caused by a lack of iron. Such deficiency is common in orange and loquat trees.
Nickelhas just been discovered as a micronutrient. Because nickel is found in the air in sufficient quantity to fill the needs of plants, and because plants are able to ingest it in this form, a need for nickel was never suspected, and its discovery points up how much is yet to be learned regarding the micronutrient needs of plants.
For a rule-of-thumb, remember that if your problem involves older, larger leaves, suspect a need for nitrogen, phosphorous or potassium—the macronutrients. If the younger leaves show problems, look further for a possible lack of some micronutrient.
Dr. Faber explained that leaf and growth abnormalities, i.e., symptoms, are the result, challenging the gardener or grower to find the cause. And this is not always easy. For instance, Phytophthera root rot causes the plant to wilt just as does drought. A lack of water, or saline water, can cause salt buildup which is also toxic to plants. Anything that affects the roots would show up as symptoms of poor irrigation. But is it really poor irrigation or something else? When confronted by a plant with drooping, water-starved appearance, start by checking the ground. If the soil in which the plant grows is moist to a reasonable depth, you’ve got some other problem, perhaps a disease involving the roots, or salt buildup in the soil. At least by checking moisture, you have eliminated a possible cause.
On the other hand, you may indeed be facing a toxicity problem—a non-pest condition affecting vegetable, ornamental and fruiting plants. Toxicity can be defined as an overdose in the soil of a particular salt or salts in general. Many plant problems are due to poor irrigation practices such as shallow watering. Toxicity is the result.
Sodium toxicityshows up as marginal necrosis, patches of dead tissue at the edges of leaves, as does boron toxicity. Chloride toxicity starts as necrosis at the tip of the leaf Add nothing but water to plants suffering such symptoms. A good leaching should flush out the salts beyond the root zone.
Most water, unless distilled, has salts. The problem is that salt pulls water towards it and away from roots, thereby making whatever water is in the soil unavailable to plants. A salt-stressed tree appears to be poorly irrigated. When a chemist or botanist speaks of, say, 800 parts per million total dissolved solids, he or she is talking about salts. Under sparse watering conditions, the plant pulls out the water and the salts carried by that water start to accumulate. When that happens, you see tip or edge bum.
The reason toxicity shows up at the edges of leaves is because the edges dry out first and the cells are actually being killed.
A salt problem will be found in older leaves first. Annual plants are more salt tolerant than perennials such as fruit trees. The reason for this is that annuals spend a shorter time in the ground. To make certain you are watering deeply enough to prevent salt buildup, Dr. Faber suggests shoving a piece of rebar into the ground. It will stop when it hits a dry patch. Soil should be wet to about three feet down or to the rooting depth of the plant.
Other problems can be caused by a high or low pH. Dr. Faber urges gardeners to test the pH of their soil and recommends the economical test kits found at garden centers. A 6 to 7 pH indicates a good soil. Greater than 7 or 8 and your soil is too basic. Your plants will suffer zinc, iron, copper or manganese deficiencies. Correct this by adding elemental sulphur. Dr. Faber cautions that although sulphur is a nutrient, it has a salt effect. He recommends 10 tons of elemental sulphur per acre or a 25-pound bag per 1000 square feet. This must be incorporated into the soil, not left on top.
Most California soils tend to be basic. However, some local patches have acid soil of less than pH 6. Add dolomite, calcium carbonate or lime to correct excess acidity in your soil.
A Parting Word
A final piece of advice from Dr. Faber addressed a common (and ill-advised) horticultural practice. It concerns tree planting. Unless you have a sandy soil, do not use planting mix when putting in a tree. You’ll create a hole that has a texture different from its surrounding soil and water might stay in that hole, or roots will ring it the way they do in a pot that is too small for the plant it holds. Plant directly into natural soil instead. For containers, he advised that something coarse, such as sand, should be added to prevent compacting.
Alice Ramirez is a contributing editor of the Fruit Gardener