Following are some basic concepts about fertilizing olive trees. For more in-depth information, we recommend the books and articles by Paul Vossen, as well as the book by G. Steven Sibbett and Louise Ferguson, all listed in our Sources below.
How do olive trees respond to over-fertilization?When it comes to fertilizing olive trees, more is not always better. In fact, more fertilizing often creates negative results. “Olive trees are not big feeders”, says Paul Vossen. “They are semi-wild, hardy, tough plants that will tolerate poor growing conditions, especially low fertility, better than most plants. They also tolerate a very wide range of soil pH.” In fact, olive trees tend to grow too vigorously in very fertile soils. They grow too tall and produce little fruit. Often, when pruned to limit their growth, they respond by sending long and vigorous non-fruiting shoots. The oil quality may be poorer in very fertile soils.
Paul Vossen also observes that a common mistake of new growers is to over-fertilize when their trees are not growing adequately, rather than make sure that irrigation and weed control are adequate. Olive trees respond much better to correct irrigation (enough but not too much) and weed control than to fertilization. There are many myths and exaggerated claims (sometimes encouraged by fertilizer manufacturers) that fertilizing will magically cure or prevent all sorts of problems and result in better quality olives and olive oil. It is often the opposite, however. In order to know when fertilizer is needed, observation and analysis are vital.
DIAGNOSING NUTRIENT AND SOIL PROBLEMS
Three general categories of observations can be used: visual symptoms, plant or tissue analysis, and soil and water analysis.
Diagnosis from visual symptoms alone can be difficult. A tree may have already suffered in growth or in yield by the time visual symptoms appear. Problems other than nutrition may cause similar problems. Borderline deficiencies or deficiencies involving more than one element may be difficult to diagnose. It is best used in conjunction with plant or soil analysis.
Plant or Tissue Analysis
Olive leaves are sampled and chemically analyzed for mineral deficiencies and toxicities. A leaf’s mineral composition depends on its maturity, current climactic conditions, availability of mineral elements in the soil, cultural practices, and other factors. The mineral nutrient level in the leaf integrates all these factors and thus reflects the nutrient status of the tree.
Although there are exceptions, the general rule is that nutrient deficiencies are best measured from late June through early August. The concentration of mineral nutrients in leaves changes as the leaves first emerge and then expand to full size. For many elements, the smallest change in concentration occurs during that period. Olive leaf samples should be taken then because critical levels have been established for that time period.
Leaf samples should be taken from the middle of non-bearing, current season shoots. A sample of 80 to 100 is sufficient. Ideally, a sample should be taken from similar trees. Samples from different varieties, or from parts of the orchard with different soils, microclimates, or irrigation systems should be sampled separately. Samples should consist of a few leaves of as many similar trees as possible, selected at random throughout the orchard. Avoid any leaves that are abnormal in appearance or from abnormal trees unless this is the specific problem to be solved. In this case, the abnormal leaves or trees should become a separate sample.
Olive trees, like most tree crops, have extensive root systems that occupy a larger volume of soil than those of most annual crops. Soil can vary widely within such an area. Thus it may be difficult to take a soil sample that accurately represents the root area and nutrient levels that the root extracts. In general, soil analysis is not accurate enough to diagnose fertility in olive trees.
It is a good idea, however, to do soil and water analyses before planting a new orchard as discussed in our Soil and Water Analysis page. It is also useful in mature orchards if a soil problem is suspected. It can help diagnose toxicities from excessive concentration of sodium, chlorine, and boron, as well as other problems, which helps in planning corrective procedures.
In general, soil samples can be taken at any time, as soil characteristics and nutrient levels are relatively stable. There are a few exceptions. Nitrogen levels can be depleted after a long wet season. In many soils, nitrates, chlorides, and boron can be leached by winter rains and irrigation. The sampling procedures depend on the problem and area involved. Is one tree affected or many? Is there a visible pattern? Ideally, 3 to 10 spots in a site should be sampled. Because soils differ in composition at different depths, the top 6 to 12” (15 to 30cm) should be a separate sample, as well as each subsequent foot downward. Samples taken from different distances from the trunk may be combined, but different soil depths should be separate. Samples should represent the effective rooting zone. A soil auger may be used to obtain samples. Generally, about a quart (1L) of soil per sample is adequate. The testing lab will often provide an interpretation of the results as well as suggestions for corrective action.
CORRECTING NUTRIENT PROBLEMS
A lack of nitrogen (N) is the only common deficiency in olives. Potassium (K) and boron (B) deficiencies are quite uncommon. Deficiencies of other nutrients are rare.
Nitrogen levels directly affect fruit set, yield, and shoot growth. It is important to remember, however, that both nitrogen deficiencies and excesses can hurt yields. The goal is to maintain leaf nitrogen levels of 1.5 to 2.0%. Deficiency level is less than 1.4%. In deficient trees, the leaves are small and yellowish. Shoot growth is less than 8”. These symptoms often occur in soils that are cold and wet during the winter when nitrogen is not as readily available, but disappear in the early summer. Nitrogen can be applied either with organic or with conventional fertilizers.
Organic applications can be done either with certified organic materials such as feather meal, blood meal, or compost, or with a leguminous cover crop. Rates of applications should be approximately 40 to 50 pounds of actual nitrogen per acre, per year, in a mature orchard, taking into account the rate of nitrogen in the fertilizer. Organic fertilizers such as compost can take quite a while to decompose and release the nitrogen in a form that the plants can absorb. They decompose over a period of about 15 years with the most intensive release during the first year or two. Annual applications over many years can build up high nitrogen levels.
Conventional nitrogen fertilizers such as urea, ammonium nitrate, ammonium sulfate, potassium nitrate, or calcium nitrate also work. The important thing about nitrogen is to make periodic applications in accordance with leaf analysis and adequate shoot growth of between 8 and 20” (203 to 508mm). It is also important to note that nitrogen can carry over in the soil for several years, especially in heavy clay. In sandy soils, it tends to leach and can cause ground water and run-off pollution, so make sure not to apply too much. It may not need to be applied each year.
For dry farmed trees, it should be put on just before a rain in mid to late winter. Under irrigated conditions, it can be applied periodically throughout the growing season and watered in. For drip irrigation, it can be placed right under or through emitters.
Note that olive trees cannot tell if the nitrogen comes from an organic source or from a conventional fertilizer. From a chemical point of view, it is all the same to the tree. There are different reasons to choose one or the other, however. Organic materials have the characteristic (which can be desirable or not) of being released slowly. They are less likely to leach into ground or surface water. Conventional sources are generally cheaper and easy to handle. But the differences between the two are getting more subtle with the availability of slow release conventional fertilizers and high analysis organic fertilizers which are easy to apply. Costs do vary considerably, however.
Low levels of potassium in olive leaves and deficiency symptoms have been observed only in rare cases. Note that soil levels of potassium do not correlate well with leaf nutrient levels or deficiency symptoms. Potassium levels below the critical leaf level can occur long before any symptom appears. Adequate concentration is over 0.8%. There is a deficiency at less than 0.4%. Deficient leaves are light green, with tip burn, and there may be dead areas in the trees. Similar symptoms sometimes appear because of poor soil drainage.
Most potassium fertilizers are mined form natural sources and classified as organic. The rate of application to correct a deficiency is generally 10 to 20 lbs of potassium per tree at the drip line. It can be dug in to be more effective or applied right under the drip emitters or through the drip system in soluble forms. Composts and other organic fertilizers all contain some potassium, so regular application would most likely never allow a deficiency to arise.
Boron deficiencies are rare as well. In California, they have been observed in Butte County. Olive trees are much less sensitive to high boron levels than most other crops. Adequate boron concentration is 19-150ppm. The tree is deficient below 14ppm and there is an excess concentration above 185ppm. In deficient trees, the fruit is misshapen (“monkey face”). The growth is short and branched. There is limb dieback, rough bark, and small leaves with top dieback.
Deficiencies can be corrected by applying a pound of borax to each tree, which will last many years. It is important not to apply too much because of the risk of toxicity.
Phosphorus deficiencies have been observed in Europe. Adequate concentration is 0.1-0.3%.
Also see this UC Davis Overview of the new California Fertilization Guidelines funded by a grant from the California Department of Food and Agricultural Fertilizer Research and Education Program (CDFA-FREP).
A Note about Foliar Sprays
Beware of foliar sprays. All the necessary nutrients for olive trees are available via ground application, and the effect is much more lasting. Research trials have proven that all nutrients can be taken up by olive roots. Foliar sprays of nitrogen, boron, and potassium will be rapidly taken up by the tree. Nutrient levels will increase dramatically in the leaves, but only for a short while, usually just a few weeks. There is no measurable effect on shot berries, fruit set, shoot growth, number of flowers, number of perfect flowers, fruit yield, or fruit size. All of this has been measured in University of California trials. Foliar sprays also have had no effect on alternate bearing in various experiments around the world. Foliar applied compost tea will not control or prevent peacock spot on olives, nor has it been shown to have any other positive effects. The best plan is to have adequate nutrition for the tree through ground application. Foliar sprays can sometimes be useful if a quick and short-lasting effect is needed.
CORRECTING SOIL PROBLEMS
Various soil problems identified through soil analyses can be corrected or at least diminished. Following are the most common soil problems and corrective measures with amendments.
Correcting Soil pH
As noted above, olive trees are quite tolerant when it comes to soil pH. When soils are overly acidic, lime is commonly used to correct the pH. The amount required varies with soil texture. The approximate amount of finely ground limestone needed to raise the pH of a 7-inch (18.cm) layer of soil by one pH unit from an initial pH of 4.5 or 5.5 ranges from about ½ ton per acre for sandy soil to about 2 tons for a clay loam. Usually, only the surface becomes acidic enough to require liming.
Compensating for Low Cation Exchange Capacity
The cation exchange capacity (CEC) indicates the ability of the soil to hold many mineral nutrients against leaching. These nutrients are absorbed by tree roots from the soil solution. Soils with a higher CEC are typically more fertile, as they have a greater capacity for mineral nutrients. The CEC depends directly on a soil’s clay and humus content. At one end of the spectrum, a sandy or loamy sand soil has a CEC of 2 to 7 milliequivalents (meq)/100g. Clay or peat soils have a CEC greater than 40 meq/100g. As a rule of thumb, fertilizers should be applied more frequently and in smaller amounts to soils with lower CEC.
Correcting Sodic (Alkali) Soils
Soils that contain excessive amounts of exchangeable sodium in proportion to calcium and magnesium are termed sodic or alkali soils. They are characterized by a dispersion of soil particles that reduces the soil permeability to water and air. By definition, a sodic soil has an exchangeable sodium percentage (ESP) of greater than 15. This means that 15% of the soil exchange capacity is associated with sodium, and the rest with calcium, magnesium, and other cations. Olive trees are affected when ESP levels reach 20 to 40. Soils with this problem are generally planted with annual crops.
This can be corrected by the application of gypsum. How much can be determined by a lab analysis. After the gypsum is applied, the displaced sodium must be leached below the root zone. Organic materials such as manure, cover crop, or crop residues may help improve the soil structure for leaching. In established orchards, heavy irrigation during the dormant period minimizes the damage to tree roots from lack of aeration.
There is a close association between the composition and concentration of soil salts and salts in irrigation water. When used for irrigation, water with high sodium relative to calcium and magnesium is likely to result in a sodic soil. This is something to watch for.
Correcting Water Penetration Problems
Irrigating with water that is very pure (low salt) may slow infiltration into sandy loam or fine-textured soils. It can drop to less than 0.1” (2.5mm) per hour, making it difficult to satisfy the orchard’s water requirements. Gypsum application can help correct this water penetration problem. The infiltration rate can be increased by as much as fivefold by applying 1 to 2 tons of gypsum per acre in late spring or early summer, just before peak evapotranspiration. Gypsum is generally beneficial for three to five irrigations. Infiltration can also be improved by mixing gypsum directly into the irrigation water with specially designed equipment. Manure applied to the wetted areas can add soluble salts to the irrigation water.
Paul Vossen: What is a “Chemical” Fertilizer
G. Steven Sibbett and Louise Ferguson: Olive Production Manual, University of California.
Paul M. Vossen: Organic Olive Production Manual, University of California.