Proper soil evaluation and preparation is essential to the ultimate success and profitability of the olive orchard. The best time to prepare the soil is before the rain starts in the Fall of the year before Spring planting. New growing sites should be evaluated for rooting depth. This is commonly done with a backhoe that can rapidly dig test holes for observation. Holes should be dug at the drip line of any existing trees on the property. The cut surface should be observed for the presence of roots that will reveal whether conditions are acceptable for growth. Careful attention to these soil evaluation steps is imperative to ensure the best orchard location, considering the narrow profit margins of the industry, and the expense of thorough preparation (leveling, deep ripping, fumigating).
For more in-depth information, we recommend the books from Louise Ferguson and G. Steven Sibbett, and from Paul Vossen, listed in our Sources below.
A benefit of leveling is that maximum production occurs when all the trees are uniformly supplied with water. How much to level the site depends on the terrain, the type of soil, and the cost of moving earth versus the cost of an irrigation system designed for rolling or sloping land. The use of pressure regulating drip emitters will frequently eliminate the need for any leveling. That being said, there are significant economic advantages to harvesting a level orchard.
Once the land has been leveled, the next step is deep tillage. This process breaks compacted, cemented, or textural layers that impede water movement within the soil profile. Olive trees are killed by poor drainage where saturated soil conditions exist in the root zone. Soils with 4 feet (1.2m) of uninterrupted profile do not usually require deep tillage.
‘Ripping”, pulling a 3 to 5-foot shank through the soil profile by a track-layer tractor, breaks up cemented hard-pan layers.
Slip plows are used to invert the soil profile where the blade is placed. This practice is more costly, since it usually requires more power than ripping, but it is more effective if the layers are compacted or textural and not cemented.
A backhoe can be used to excavate individual tree sites but the local “flower pot” effect may eventually impede water drainage. In the long term, it is probably better to excavate the entire site.
Some soils can be improved by installing sub-surface drain lines, and improving surface drainage by planting tress on raised berms.
Various amendments such as well rotted animal manure, Azomite, or lime, can be added to the soil before ripping if necessary. In many instances, it is not economically viable.
A weed management program should start before trees are planted because the more difficult-to-control weeds are easier to manage then. It is especially important to control established stands of perennial weeds before trees are planted so that potential injury to young trees from herbicides can be avoided. Perennial weeds that can be especially troublesome in California are field bindweed, johnsongrass, dallisgrass, bermudagrass, and nutsedge.
Weed seedlings and established annual weeds can be controlled either with pre-emergence or post-emergence herbicides before planting. Use a pre-emergence herbicide before planting an orchard only in conjunction with a rotation crop. Make sure the residual period of the herbicide is not long enough to preclude planting the trees. Post-emergence herbicides generally have a short soil residual and are safer to use before planting trees. To avoid possible exposure of newly planted trees to herbicides in the backfill soil, many growers prefer to use pre-emergence herbicides only after trees have been planted.
An especially effective method of weed control before planting is to cultivate, then irrigate to germinate new weeds, and shallowly cultivate again to destroy weed seedlings. Frequent cultivation lowers weed seed populations in the soil, thus reducing weed growth. At least two cycles of cultivation, irrigation, followed by a shallow cultivation are needed for a marked reduction in weed seedlings. Unfortunately, this method is not effective on established perennial weeds.
A method of control for perennial grasses such as bermudagrass and johnsongrass is to cultivate the soil when it is very dry. Cultivation cuts the rhizomes into small pieces so they can dry. Rework the soil frequently using spring-tooth harrows to pull new rhizomes to the surface so that they will dehydrate. If the soil is irrigated or rain occurs before total control of the perennial plant is achieved, the rhizome pieces will begin to grow and the effectiveness of this practice will be reduced. By the same token, working the soil when wet can increase the population of perennial weeds, because each piece of cut rhizome can root and develop into a new plant.
Field bindweed growth can be reduced for 1 to 2 years by deep plowing or with a reclamation blade (a large V-shaped blade) to cut the roots 16 to 18” (41 to 46cm) deep in dry soil. Populations of nutsedge can be reduced by deep plowing with large moldboard plows to bury the nutlets to a depth of at least 12” (30cm).
Soil solarization is a non-pesticidal method of controlling soilborne pests by placing clear plastic sheets on moist soil during periods of high ambient temperature. The plastic sheets allow the sun's radiant energy to be trapped in the soil, heating the upper levels. Solarization during the hot summer months can increase soil temperature to levels that kill many disease-causing organisms (pathogens), nematodes, and weed seeds. It leaves no toxic residues and can be easily used on a small or large scale. Soil solarization also improves soil structure and increases the availability of nitrogen (N) and other essential plant nutrients. (For additional information see UC ANR Publication 21377, Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds)
In non-organic orchards, fumigation destroys parasitic nematodes and eliminates weeds. It is an expensive practice that must be done correctly to be effective. (See our Organic farming page for suggestions on best organic practices.)
Louise Ferguson and G. Steven Sibbett: Olive Production Manual, University of California.
Paul M. Vossen: Organic Olive Production Manual, University of California.