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Soil Modifications- A Must for Almond Orchards

Mario Viveros

UCCE Farm Advisor, Kern County

Deciduous Tree Fruits and Nuts


July 23, 1999

Soil Modifications: A Must for Almond Orchards

Almonds live in an environment which includes both the air and the soil. The tree above the ground is easy to see and study. We investigate the nutrition of a tree by leaf analysis and we modify the aerial environment by pruning and spraying. The soil environment is also important but it is a lot more difficult to see its effects. If we could see root systems as easily as we can see branches and leaves, we would give more thought to soil selection and soil preparation before an orchard is planted. The best time to do soil preparation is in mid-summer and early fall.
Almonds are most productive in loam-textured, deep and uniform soils. However, soils with such characteristics are very few in numbers. Most of our soils including Delano, Milham, Kimberlina and Wasco soil series require some kind of physical modifications.
Soil compaction, clay and plow pan, sand silt or clay layers are among the physical soil limitations that affects tree development and subsequent tree productivity. These limitations must be diagnosed, evaluated and eliminated before the trees are planted in an orchard. A series of backhoe pits, five to eight feet deep, will clearly show the numbers and types of soil layers, the depth of the layers and the variability of the subsoil throughout the orchard site. This information can help determine the most economical method of soil modification, how to properly set up and use deep tillage equipment and when to conduct deep tillage.
Deep tillage is a general term used to describe the breaking up, loosening, or mixing of restricted subsoil layers which are below the depth of normal cultivation. The purpose of deep tillage depends on the nature of the restricting layer. Some of the major purposes are the following:
1) To break up man-made compact layers in the top two feet of soil. This will improve water penetration, soil aeration and improved root growth.
2) To break through natural subsoil claypans, hardpans, and dense layers. This will improve drainage and water penetration, better salinity control and deeper root growth.
3) To mix stratified soils and eliminate abrupt boundaries between unlike textures. This will improve internal drainage and improve deeper root growth.
The equipment used for deep tillage are rippers or subsoilers, slip plows, disc plows, moldboard plows, backhoes and trenchers.
Rippers or subsoilers vary in sizes and styles. Their typical depths of tillage operation range from two to seven feet. They break up hard layers by cracking and shattering. There is a little mixing or dislocation of layers. However, ripping is most effective in dry, brittle soils and hardpan but least effective in moist loams and clays. Very compacted sandy layers will shatter even when moderately moist.
The slip plow is a combination of a vertical ripping shank with a wide (12 to 15 inches) inclined beam extending to the rear from the ripping point at an angle up to the soil surface.
There is considerable mixing and lifting action in the soil due to the wide inclined beam of the slip plow. Subsoil chunks torn loose by the point, slide up the beam toward the surface and are permanently dislocated from their original position. Other loose soil falls down the channel to create some permanent mixing and an improved zone for water and root penetration. Slip plowing is usually done to a depth of three to six feet.
Disc plows commonly have three to five discs mounted on individual hubs that do not limit the depth of penetration. Therefore, disc plows can penetrate the soil up to about two-thirds of their disc blade diameter. For example, disc plows with 44-inch diameter discs can plow 30 inches deep.
Disc plows are most useful for relieving soil compaction in the surface two feet and in mixing shallow lying layers. They also do a good job of turning-under crop residue and mixing manure and other organic materials with the compacted soil layers.
Moldboard plows were capable of plowing four to six feet deep in the 1950s. At the present time, however, there are very few left capable of plowing three to four feet deep. The action of moldboard plows in loosening and mixing is similar to that of disc plows.
The backhoe has been used successfully to modify orchard soil problems in the upper San Joaquin Valley. However, they have not had the same success in the southern end of the San Joaquin Valley.
Trenchers are machines used for digging trenches for water and drain lines. They have been used in Kern County, however, the use and success has been limited to individuals that own this type of equipment.
The goal of modifying soils with restricting subsoil layer is to deepen the zone favorable for plant roots. This involves breaking up, loosening and mixing subsoil layers that restrict root growth by their hardness, their clay content, or their layers of sand and silt which interrupt the normal flow of water and air through the soil.