One, hundred, thousand

This little mnemonic, or memory aid, in the title is helpful in remembering the critical levels of toxic constituents in irrigation water. The “one” stands for 1 part per million (ppm) of boron (B), the e” hundred” flags 100 ppm of sodium (Na) and (Cl) and the “thousand” represents the level of total soluble solids (TDS or slats) in water. Levels exceeding the critical values for any of these constituents can present problems for tree growers. The problems typically show themselves as tip-burn and defoliation. The B, Na and Cl are toxic elements at relatively low concentrations, but symptoms appear similar to the damage caused by high salinity.

Water that exceeds the critical levels mentioned in the mnemonic have a greater tendency to cause damage if sufficient leaching is not applied. It doesn’t mean the water is impossible to use, only that greater attention needs to be made to ensure that these salts are adequately leached. High levels of these salts accumulate in the soil with each irrigation, and the salts are absorbed by the tree and end up in the leaves where they do their damage.

So why talk irrigation water in the winter? Because this promises to be a low rainfall year and the customary leaching we rely upon in winter rainfall is not going to be as effective as in previous years. Irrigation is a necessary evil. Every time we apply irrigation water we apply salts, and unless some technique is used to minimize slat accumulation, damage will result. This damage can be more than just leaf drop, but also the stress that induces condit5ions for root rot.

We have the two irrigation years of 1996-97 and 1997-98 as reference. The period from the last rainfall of January 1, 1997 to October 1997 was the longest period without rainfall in the 20th century. Irrigation water was applied throughout the winter, spring, summer and early fall and many trees looked stressed. Even well irrigated orchards in the fall of ’97 had leaf burn due to the gradual accumulation of salts from irrigation. From the fall of 1997 to the spring of ’98 we had protracted rains that not only flushed salts, boron, sodium and chloride from the soil, but also delayed irrigations so that the full irrigation season did not really begin until July of that summer. That meant less water was put on with its concomitant salts. More trees in the Ventura and Santa Barbara areas looked good that fall, even though persea mite was still having an effect.

It will probably be necessary to irrigate in many winters. To add to the lack of rain problem, it may be necessary to irrigate even if there is rain. The wetted pattern that is created by a drip or microsprinkler emitter also creates a ring of salt in the outer band of the wetted patter. If there is less than an inch of rainfall to push this salt down, this salt tends to diffuse towards the tree where it can accumulate back in the root system. Orchards with even good water quality would find it advisable to run the irrigation system with the fir5s rains. Those with poor water quality definitely should the microsprinkler system an equivalent of on-half applied water (13,500 gallons per acre) during or soon after the first events of less than on-half inch rainfall. Growers with water quality exceeding one, hundred, or thousand should be especially alert to the need to manage water in low rainfall winters.