Pistachios can handle salty irrigation water to a point

Pistachios are the most drought-and salt-tolerant of the nut crops, but there are limits, and researchers are discovering where to draw the line.

Recent research shows that there can be salty hot spots within an otherwise healthy orchard, and that young pistachios in these hot spots may unexpectedly suffer extreme frost damage.

"Over the last three years, young second- to seventh-leaf pistachios have died or had severely restricted spring shoot growth in marginal to severely saline hot spots in some Westside orchards in Kern and Fresno counties," said Blake Sanden, University of California Cooperative Extension farm advisor.

It is even possible to put a number on the level of salt in the soil it takes to subject young pistachio trees to the danger of frost damage.

"We may have identified a threshold. It appears that average root zone salinity greater than 6 EC (electrical conductivity), soluble salt greater than 920 parts per million and salt-to-calcium ratios greater than 15 can increase young pistachio frost susceptibility," Sanden said.

Sanden presented his attempts to unravel the mystery of selective frost damage as pistachio growers attended a series of research meetings held throughout the state.

"The symptoms of this decline—dead bark, brown phloem tissue near the bud union on dead trees—appear the same as frost damage and have generally been located in a lower part of the field. However, some adjacent trees at virtually the same elevation appear to have no symptoms at all," Sanden said.

The affected trees had high sodium levels in the bark near the bud union, but not in the leaves. The high salt levels in the bark can leave the trees unable to harden off in the winter, making them more vulnerable to frost damage than neighboring trees.

Salty irrigation water can also have a long-range effect on young pistachios, with trees looking just fine for years, then suddenly growing more slowly than pistachios irrigated with cleaner water.

Sanden has taken a close look at the cumulative impact since 2004 of trying to grow cotton and pistachios together with salty well water.

The salty water produced a cotton crop, while saving quite a bit on irrigation costs. And for years there was no great difference in the growth of the young pistachios receiving the saline well water.

But over the years the well water, with an EC of 5.1, dumped an additional 100,000 pounds of salt per acre on the soil. The salt built up, and by the end of the 2011 season there was a significant difference in the size of the trees, depending on the salt in the irrigation water.

"Cotton income plus saved water costs are about $6,200 an acre since the inception of the project. However, cotton plantings may have contributed to slower tree development," Sanden said.

One way to improve water penetration and drainage is to rip the soil, and it is common to spend as much as $400 per acre on pre-plant tillage to improve drainage before planting pistachios. But this expense may not pay off in the long term, he said.

Trees planted in ground that had been worked with a backhoe to a depth of seven feet grew faster the first year than nearby trees planted in ground worked only with an auger. But by the seventh leaf, this advantage from extensive pre-plant tillage had disappeared.

"The deep tillage treatments gave an 18 percent increase in tree circumference at the end of the first year over the auger only. After six years, however, there is no significant difference in the rootstock circumference of any treatments," Sanden said.

Next year will be the first commercial harvest, which should tell if the investment in pre-plant tillage paid off in more pistachios.

(Bob Johnson is a reporter in Magalia. He may be contacted at bjohn11135@aol.com.)