Preparation for planting a new walnut orchard should start with a soil evaluation. Walnuts have traditionally been planted on class one soils — the deepest, most uniform well-drained soils.
Recent research and grower experience has shown that with the right preparation and planting system walnuts can be successfully grown on less-than-ideal soils. Soil evaluation will help determine the steps to take prior to planting to ensure successful results.
Soil survey maps are a good place to start. Soil surveys are available at local Natural Resources Conservation Service office, Cooperative Extension offices, and can be found online at http://websoilsurvey.nrcs.usda.gov/app.
Soil surveys provide information on the type of soils present and the distribution and acreage. It describes each soil type and provides information about drainage, flooding, exchangeable sodium content, and other details important to successful orchard establishment. The soil survey cannot provide every detail that may be necessary.
Using a backhoe to further explore the soil can provide valuable information necessary for orchard development. Digging backhoe pits 5 feet to 6 feet deep in strategic locations where soil differences are expected will allow first-hand examination of the soil.
Look for stratified soil, compacted zones, hard pans, and clay pans. Your local farm advisor may be able to provide assistance in the evaluation of the backhoe pits.
Abrupt changes in soil texture can result in a perched water table which is unhealthy for walnut roots. If soil modification is necessary, it will be much easier to accomplish before planting. Modification should be done in the late summer or fall when the soil is dry to ensure the most disruption possible while allowing winter rains to settle the soil before planting. Touch-up leveling or smoothing can be done in the spring before planting.
Leveling will be necessary if the orchard will be flood irrigated. With low volume (drip or microsprinkler) or solid set irrigation systems leveling to grade is not necessary. Leveling to smooth out low spots or improve surface drainage should be done to keep the future orchard healthy.
Deep uniform soils may only require shallow ripping (1.5 feet to 3 feet) to loosen the soil. Stratified soils or soils with hardpans or claypans will require deep ripping or slip plowing (3 feet to 6 feet) to disrupt layers. Ripping is less effective for clay pan soil because of the elastic nature of the clay which will flow around the ripper and reseal in a short period of time.
A slip plow, a ripper shank with an iron plate coming from the point of the ripper at a 45 degree angle to the surface of the soil, can lift soil at the bottom of the shank to the soil surface and permanently disrupt a clay layer. Ripping and slip plowing are typically done in two directions, with the second pass diagonal to the first.
In a walnut trial at the Nickel’s Research facility in Arbuckle, a backhoed trench 10 years after slip plowing revealed a disrupted claypan and deeper rooting in the slip plowed area compared to the non slip plowed area. Surprisingly no increase in tree size or yield was noted. This may be due to the low volume irrigation (drip) system and frequent fertilization.
Soil physical characteristics can to some extent be overcome by the use of low volume irrigation, especially under close tree spacing. Soil can be modified to a depth of 6 feet but large equipment is necessary and it is expensive ($300-$500 per acre). Backhoeing tree sites to mix the soil may be practical on sandy soils where it can be done quickly. The costs may be prohibitive on heavier soils.
Planting trees on berms is recommended especially on heavier soils. Ridge berms in the fall after soil preparation to allow for settling over the winter.