Soil Management in Olive Farming

The natural environment, the selection of a suitable field and locally adapted varieties, as well as the application of appropriate cultivation techniques are all factors affecting the fate and productivity of an olive grove.

Soil is usually an underlooked critical factor that can dramatically influence the final yield of the trees.

Selecting a proper field for establishing olive grove

Olive trees planted in flat locations and areas surrounded by hills are exposed to spring frosts and run the risk of severe frost damage during the winter. A field with a slightly downward slope, ending in a flat surface, where cold currents can easily escape, is usually a suitable place for installing an olive grove. Completely flat fields with no history of frosts or cold winds are also appropriate. The average olive tree needs enough sunlight to produce a good yield and hates excess soil moisture. Therefore, the olive grower must choose well-drained soil, in which rainwater cannot easily stagnate.

Soil management in newly established olive groves

Before planting our young olive trees, we should keep the field we choose fallow for 2-3 years to decrease the pest and pathogen populations.  Then, in the fall of last year, we can sow the field with vetch, an excellent plant for green manure, and incorporate it into the soil a few months later. This method will significantly reduce some harmful perennial weeds and enrich the soil with nutrients like nitrogen.

Afterward, we need to level the ground in our field to install an irrigated olive grove. This activity takes place before the deep plowing of 45-50 cm (1.6 feet), aiming to destroy the remaining perennial weeds. It also makes the soil less compact and impenetrable, contributing to the better development of the root system of young trees. However, before plowing, it is suggested to perform a basic soil analysis. The results will determine the type and amount of chemical fertilizers needed to improve the soil (seek advice from a local licensed agronomist). In many cases, farmers add 2 to 3 tons of manure per stemma (12 tonnes per acre, 30 tonnes per hectare). In general, when preparing the ground for planting olive trees, we should avoid using heavy diggers and disc harrows.

Before applying any fertilizer in our olive grove, we must check the natural properties of the soil and the levels of available nutrients. These factors affect various other variables that ultimately influence the productivity of the trees. Thus knowing the amount of certain nutrients in the ground will help us make more suitable and focused actions. For example, to deal with the lack or excessive concentration of an ingredient avoiding stress the trees. Two very interesting parameters are the soil’s pH and the soil’s calcium content since both affect the absorption of certain nutrients added to the soil by fertilizers. The optimum pH for the olive tree is about 6.5, but the average olive tree can adequately yield, even in soils with a pH of 5.5 to 8. A standard action for correcting soil pH before planting new olive trees is the incorporation of lime (CaO) into the soil (consult your local licensed agronomist).

Soil management in olive groves with trees older than 7-8 years old.

Some techniques and cultivation methods applied in many modern-intensive crops accelerate soil erosion. Soil erosion causes reduced fertility, subsidence of the phreatic zone, and negative changes in natural geomorphology. Soil is considered a non-renewable resource. It takes 500 to 1,000 years to create a 2.5 cm thick layer of soil. Nonetheless, we often take this precious natural source-pore for granted, believing that it is something stable. Olive growers can regularly take some actions to help the soil regain the desired physicochemical structure and restore its fertility, moisture retention, flood prevention, etc. Improving soil’s attributes must be continuous as we cannot achieve it with a single action. A combination of different measures is needed, with some performed annually and others every 3 to 4 years.

When the trees have reached maturation, many olive growers apply 30 kg (66.14 pounds) of well-digested manure per tree every 3 to 4 years, combined with the standard annual fertilization. They spread the manure around the tree’s trunk, maintaining a distance of 50 cm (1.6 feet) from the trunk. Then they carefully plow and incorporate it into the soil. This technic improves soil characteristics, such as cohesion.  It also improves the texture of heavy soils, maintains soil moisture, increases water capacity, activates soil microorganisms, and facilitates nutrient absorption.

Green manure

The application of green manure in olive groves has become a method increasingly popular in recent years. It begins with sowing an annual or perennial plant (vetch, alfalfa, forage bean) in the olive grove immediately after the harvest of the olives. Just before the legume reaches full bloom, we incorporate it into the soil, usually by plowing. This method improves soil fertility and soil structure. Additionally, it increases water absorption and the ability of the soil to retain moisture and contributes to weed management of harmful perennial and resistant weeds (especially if we sow vetch).

Legumes such as vetch, alfalfa, reptile clover, lupine, livestock bean and pea, etc, which enrich the soil with nitrogen, are widely used in green olive manure. Similarly, farmers may partly use cereals such as oats and barley. Given that these plants absorb significant amounts of nutrients, incorporating them into the soil provides readily available nutrients for our plants. In case the olive grower decides to apply green manure, it is necessary to use propagating material that is not Genetically Modified (GMOs)

Recycling of nutrients inside the Olive Grove – Exploitation of cut woods after pruning

After pruning, rather than removing cut branches or burning them (as is usually the case in conventional farming), organic farmers crush them with special machinery, decimate the tree’s branches, and deposit the sawdust on the soil. This has beneficial effects, as it has been calculated that for every 1,000 kg (2,205 pounds)of olive wood (with 50% moisture) incorporated into the soil, 4 kg (8.8 pounds) of nitrogen, 0.5 kg (1.1 pound)of phosphorus, 4 kg (8.8 pounds) of potassium, 5 kg (11 pounds) of calcium, and 1 kg (2.2 pounds) of magnesium are added to it (Amirante et al., 2002). This method reduces the need for additional chemical fertilizers, the majority of which are not allowed in organic farming. In this way, we have the least possible inputs and outputs and promote the recycling of the elements within the olive orchard. There are, of course, cases in which tree branches must be removed immediately from an organic orchard, for example, when a tree tissue suffers from a pest or a disease.