By John P. Edstrom, Orchard Farm Advisor, Univ. of California Cooperative Extension

Almonds represent the major orchard crop in Colusa County with acreage expanding in recent years to 20,000 acres. Much of this expansion are attributed to the successful adoption of low volume (drip) irrigation in the Arbuckle/Williams area. Soils in this region, Arbuckle and Arand, are coarse textured with low moisture holding capacity. Surface layers are shallow in the Hillgate soils and underlain with a dense clay layer. Under such conditions, dramatic yield increases can result from the switch from sprinkler irrigation to drip irrigation that applies water frequently in small amounts to each individual tree. Soil limitations such as low water holding capacity, slope, and limited rooting depth can largely be overcome with the use of low volume irrigation systems coupled with the frequent injection of nitrogen and potassium fertilizers through the drip systems.

In general almonds still prefer deep well drained soils represented by the Westfan and Arbuckle series. Although Grandbend, Moonbend, and Vina are well suited, these soils reside along the Sacramento River at low elevations where frosts are common during almond bloom. Primarily due to the frost-free microclimate of the Arbuckle area and nearby low foothills, almonds are concentrated on the Arbuckle, Hillgate, Westfan, and Arand soils. Higher density plantings are required on these soils to approach the yields produced on prime soils. Typically, 100 or more trees per acre are planted versus 70 to 80 trees per acre on deep alluvial soils such as Vina and Grandbend. High-density orchards are generally planted in a north/south orientation where trees within the row are spaced closer together than the distance between rows. Trees should be planted on mounds or berms to protect the tree from excessive moisture from irrigation or rainfall.

Wide spread use of the soil modification technique known as slip plowing, has significantly altered the soil profiles in thousands of acres of local almonds. This operation partially mixes soil layers of different textures to a depth of five or six feet. This results in improvements in water infiltration and drainage that in turn favor vigorous and more uniform tree establishment. Root systems are generally deeper and more expansive under the modified soil conditions. This method is practiced mainly on the Hillgate and Arbuckle soils. However, the long-term benefits to orchard productivity of slip plowing have not been documented.

Walnuts are generally more demanding with respect to soil type. Deep, well-drained, moderately textured soils are required for top production. Most Colusa County walnuts are planted adjacent to the Sacramento River on soils represented by the Grandbend, Moonbend, and Vina soils. Walnuts are also successfully grown southwest of Williams on the Westfan soils.

Excessive water can also be a problem along the Sacramento River where surface flooding is common inside the levee system and subsurface seepage from the river during periods of high river flows injures orchards outside the levees. In locations prone to flooding or seepage, walnuts should be planted on mounds or ridges to lessen root/crown damage. Walnut culture has expanded away from the traditional Sacramento River area into heavier textured soils with higher clay content such as Corbiere, which have poor drainage, were formerly farmed to row crops.

Prunes are the third major crop in the area and the local tree crop that is the least demanding in soil requirements. Prunes can be successfully grown on a wide range of soils but generally do best on deep well-drained soils. The most productive orchards are planted on Grandbend, Moonbend, and Vina soils along the Sacramento River.

River flooding and seepage is less detrimental to the water tolerant plum rootstocks used for prunes. Surface, flood irrigation is most commonly used for prunes. Moveable pipe sprinklers are suitable for flood prone orchards. Permanent sprinklers and all low volume systems should not be installed in orchards where soil erosion or deposition is likely. Minor plantings of prunes are located in the Arbuckle area where Arbuckle, Hillgate, and Arand soils are suitable for prune production given the use of low volume irrigation.