Bring on the big iron! We love it. Bring out that gigantic D-9 Cat hooked to a six-foot long slip plow — an 18-inch wide, swept forward two-ton dagger that we bury in the heart of the earth.

The tracks of the Cat bite into the surface soil, dragging the massive shank through the future root zone of soon to be planted trees. A-three foot eddy of broken soil and clods sometimes as big as boulders churns up around the shank as the soil heaves and fractures. You can feel the vibration 20 feet away. Ah, this is manly work — raw power loosening the tight soil to make it easier for young tree roots to establish and grow.

However, is it absolutely necessary? Common wisdom for more than 60 years has been that a “good” orchard development should have some kind of ripping/deep tillage to a depth of three to six feet prior to planting. For many soils with hardpans and other layered structures this “just makes good sense” when considering the mechanics of root development and water penetration. Some older research under flood irrigation by Gene Begg in the Sacramento Valley found that tree size and yield in walnuts under flood irrigation increased in proportion to the amount of mixing in the top four feet of root zone — with the least benefit given by plain ripping and deep moldboard plowing giving the greatest benefit.

Fast-forward to 1997 and a deep ripping trial in Arbuckle for a new planting of almonds using micro-sprinkler irrigation established by UCCE farm advisor John Edstrom. Even though this soil has a bad hardpan and should benefit from slip plowing, as of the 2004 harvest there has been no real yield difference between treatments. The major difference between this and earlier trials is the micro-irrigation. The high frequency, uniform application of water on a more precise irrigation schedule appears to compensate for the soil moisture advantage that deep tillage provided under flood irrigation.

‘Graduated’ profile

Typical soils being developed for pistachios in the San Joaquin Valley often have a “graduated” profile that will shift from a clay loam to a fine sandy loam (or vice versa) over the top four feet of the root zone. It is not uncommon to find loose “caliche” layers of high lime (and sometimes gypsum) content or thin layers of silt in this zone on the west side of the valley. Some plow or “disk pans” are occasionally found, but rarely do we encounter the kind of hardpan found in the Arbuckle area. So are we really gaining any benefit from the $150 to $400 per acre that is often spent for deep tillage prior to planting pistachios to be grown on drip irrigation?

A deep tillage trial was established in western Kern County in December 2005 just inside the north edge of the ancient Buena Vista Lake bed where black Buttonwillow Clay is the dominant surface soil type. This ground has been farmed in cotton and wheat rotations for 30 years. Soil salinity throughout the project area runs 1.5 to 5 dS/m and averages about 3.1 dS/m in the test plot area. Irrigation is supplied by a mix of canal and marginally saline well water.

The soil profile of the test plot is typical of the whole project area: Buttonwillow clay (0-2 feet) overlaying a Garces/Lethent clay loam (2-4 to 6 feet) with a weak caliche layer about 6 to 10 inches thick around the 3 foot depth. A Kimberlina coarse to fine sandy loam underlays the entire area at a depth of 5 to 7 feet.

Five tillage treatments ranged from disking only, 3-foot cotton chisels, single-pass slip plow to 50 inches, triple-pass slip plow and full backhoe to a 7-foot depth were installed in replicated blocks. A two-foot band of sulfur was applied over the tree row at the rate of 15 tons per acre and incorporated to a depth of about two feet. All trees were planted in a standard 12-inch wide by 2-foot deep hole dug by a 3-point auger.

UCB rootstock was planted March 2006 and flood irrigated this first year as the grower was unable to capitalize the drip system until December 2006.

Irrigations

A total of one pre-irrigation and three in-season irrigations were sufficient to supply trees with adequate moisture in this heavy black clay. As of Nov. 9, 2006, root stock circumference averaged 4.76 cm for deep tillage treatments and 4.28 cm for no tillage and chisels. Tree height (whether Kerman scion or UCB shoot) averaged 94.9 and 85.5 cm for the same treatments. These differences are not obvious to the eye as you walk through the trial area and are very small in terms of overall tree size. However, they represent a statistically significant deep tillage increase in first year growth of 18 percent increased trunk caliper and 11 percent increase in tree height compared to no deep tillage.

There were no nutrient differences in leaf tissues. Trees will be drip irrigated starting this season.

Many more years of observation will be required to determine if this first year difference translates into long-term yield gains. A $300 per acre investment over a 30-year orchard life at 6 percent interest is eventually worth $1,723 per acre. That's $300,000 cash for a 1,000 acre development with an eventual investment cost of $1.7 million.

Of course, at $2 per pound for split nuts this is only 862 pounds per acre pistachios, basically 50 pounds per acre for 20 years to pay for the tillage.

We all pay for insurance, and preplant deep tillage may fall into that category. No one thinks they'll die tomorrow, but once you're “planted” it's too late to buy life insurance!