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WATER QUALITY CONCERNS
WATER QUALITY CONCERNS-
As irrigation supplies from the Rio Grande diminish and as growers turn to wells to keep their orchards going, the quality of well water becomes a major concern. Often, however, we are more concerned with the quality of the water than we are with the effect of such water on the soil and, by extension, the effect of the soil on the tree.
Citrus is relatively intolerant of saline conditions. Rootstock does make a difference, i.e., Cleo is more salt tolerant than sour, which is more tolerant than trifoliate and many of its hybrids.
In that regard, it is interesting to note that the usual salinity content of our river water is 800 ppm or more, which level is classified as high salinity (750 to 2250 ppm). Florida researchers concluded that no yield reduction occurs from salinity levels of about 1100-1150 ppm, while a 10 percent reduction occurs at about 1500-1550 ppm. Similar results have been reported in California.
Research in Texas has shown that citrus can survive and grow with irrigation water having salinity levels that are classified as very high, i.e., greater than 2250 ppm, provided that the sodium content is not over 60 percent of the total salts, that the sodium absorption ratio is below 18, that boron is less than 1.0 ppm and that proper irrigation amounts and frequency are carried out on soils that are deep and have good internal drainage.
The more important issue is that of soil salinity. Trouble starts when soil salinity in the root zone exceeds about 2600 ppm. Yield of California navels reportedly declines about 36 percent at that level. Obviously, a yield decline of such magnitude would be preceded by serious damage to the tree.
The key to using saltier water is to use more of it and at greater frequency than usual. Each irrigation must provide more water than is needed to replenish the soil profile, the excess to leach soluble salts from the root zone and maintain a salinity level in the root zone of nearly that of the water being used. In other words, you cannot just replace the soil moisture that was used since the previous irrigation, as that leads to an accumulation of salts.
As the roots extract the available soil moisture from the soil, soil salinity increases. For example, a soil salinity level of 2205 ppm represents three tons of salt per acre foot of water in the root zone. When half that water has been depleted (by roots and evaporation), the three tons of salt are still there, but the concentration has doubled to 4410 ppm. Consequently, irrigation must be applied more frequently to reduce the concentration effect.
Ironically, sandy soils can show salt damage sooner than loams despite the fact that sandy soils are highly permeable and leach readily. The reason is that sandy soils have very low soil moisture capacity, so the concentration of soil salinity increases more rapidly.
SYMPTOMS OF SALINITY-
The general symptoms of salinity damage in citrus include dull color, slight bronzing, necrosis and defoliation. Over time, trees suffer twig dieback, are thinly foliated with smaller leaves and smaller fruit. If the conditions persist over an extended time, growth is greatly reduced and the trees cease to produce adequate crops.
The accumulation of excess chlorides in the foliage will cause the leaves to burn and fall within a few months, while moderately high levels result in bronzing of the leaves.
The symptoms of boron toxicity start as yellow dots on the leaves, then orange-mottling and tipburn followed by defoliation after several months. Where both chlorides and boron accumulate at damaging levels, the leaf symptoms are not so easy to distinguish.
The trees will grow and survive in spite of leaf burn and the loss of several crops of leaves. Neither is necessarily permanent-on well-drained soils. With the return of good water, the trees will recover to normal appearance, growth and productivity in short order. Even bronzed leaves will regreen with good water and cooler temperature.
The trees will dry up and die without adequate irrigation or rainfall-that is a certainty. The potential damage to the orchard (and soil) from using well water having a salt content two to three times that of normal river water is less certain, being dependent on the soil, the water, their management, the frequency of leaching rains and the time to return to normal reservoir levels.
While perusing the literature of the 50's to research the previous two articles, I ran across one about fruit growth in times of water stress. Since few orchards are receiving as much water as usual, this research is rather timely. The article is by G.R. Schulz, H. Peterson and S. Deckard in the 1958 Journal of the RGV Hort. Society (12:22-26).
They measured soil moisture at 6, 12 and 18 inches and tracked the increase in fruit size from June to November. The trees were six-year-old Ruby Red grapefruit.
Generally, the fruit increased rapidly in size immediately following irrigation-for about two weeks-then no further size increase occurred until the next irrigation. Although the fruit stopped enlarging, the trees reportedly looked healthy with no evidence of wilting.
In looking at their data, fruit enlargement stopped when soil moisture at the 6-inch depth was depleted about two weeks after a June 22 irrigation, although soil moisture at the 12-inch level was at two-thirds depletion and that at the 18-inch level was at one-third depletion.
Following a July 10 irrigation, enlargement slowed when the 6-inch moisture level was depleted and stopped when the 12-inch level was depleted and the 18-inch level was at two-thirds depletion. Enlargement started again with an August 22 irrigation but slowed again when moisture at the 6-inch level neared depletion. Enlargement stopped altogether when the 6-inch level was totally depleted and the 12-inch level was at two-thirds depletion (the 18-inch level was still at one-third depletion).
Irrigation was again applied September 20-but the fruit continued to increase in size despite the fact that soil moisture at the 6-inch level was depleted about October 8. However, moisture at the 12-inch level was less than one-third depleted. Irrigation was applied on October 10, after which soil moisture remained near field capacity through November-and fruit size continued to increase steadily during the period.
While the authors concluded that enlargement ceased when soil moisture was depleted at the 6-inch depth, their graphic data suggest that enlargement slowed at that point and ceased only when soil moisture at the 12-inch depth was two-thirds depleted.
This supports the classic work by Bob Koo at Lake Alfred in which he showed that irrigation in the last half of the year should be applied when available soil moisture in the upper 12 inches is two-thirds depleted.
Be that as it may, the long and short of it is that grapefruit does not increase in size when available soil moisture in the upper 6 to 12 inches of soil is depleted or nearly so-and it does not start again until water is applied. The unanswered question is whether or not such lulls in growth reduce the ultimate potential fruit size.
Because at least seven of the 24 districts are out of agricultural water at the present time, the question arose as to how many acres of citrus are situated in those seven districts. According to my best calculations, there are approximately 150,000 acres of irrigated land in those districts, which numbers were more or less verified by the districts.
Of the seven districts, two report that they have no citrus-the other five provided estimates that totaled a little over 8,500 acres of citrus, or about 25 percent of the total industry. I'm not really too confident about this number of acres of citrus inasmuch as the estimates for the depleted districts in Cameron County totaled more than the total citrus acreage in Cameron-leaving none for Bayview, Harlingen and Brownsville. Consequently, I had to make some downward adjustments to the estimates.
This effort may have been an exercise in futility, but if the estimate is even close, the industry might have a better idea of the impact of the drouth on the 1998-99 crop. I intend to try to refine the estimates during the next couple of weeks-especially since it doesn't look like it will ever rain again.
According to what I've been told, seven irrigation districts have run out of water since early June. Unless well-water is available, that means that an awful lot of citrus is in desperate need of a good, soaking rain. There's no question that fruit size in dry orchards has not increased since a couple of weeks after their last irrigation was applied.
About the only good news for those orchards is that pest problems have been pretty light to almost non-existent-see Dr. Anicso's Pest Update following. Should the weather change for the better, you should expect most pest populations to increase rather quickly-both in the dry orchards and in those still having irrigations.
CITRUS PEST UPDATE-
Rust mite populations pretty much have remained at non-detectable levels this summer due to the very dry conditions. The armored scale complex, especially Florida red scale, has been increasing to treatable levels this summer even though chemical control has proven to be difficult in the past. In general, this pest is kept under control by beneficial parasitoids but the widespread presence of Florida red scale indicates that these beneficials may not be present. One of these parasitoids, Aphytis holoxanthus, which was introduced in 1959, has kept the Florida red scale as a minor pest but recent outbreaks appear not to have been pesticide-induced, indicating a need for further investigation and study. Chemical control using Lorsban or Supracide has proven ineffective in the past for Florida red scale. Citrus leafminer populations have been increasing in some areas (Cameron County) on the new growth but still remain at low levels in these orchards. Citrus blackfly populations continue to increase but are still at low levels. False spider mite damage on grapefruit has been reported and this is not unusual during very hot and dry summers like the one we are experiencing. False spider mites can cause serious damage on grapefruit and should be quickly treated; little to no damage generally occurs on oranges. Texas citrus mites, mealybugs, and whitefly are all rapidly increasing but these populations are quite varied and some groves may require action for these pests.
JULIAN W. SAULS, Ph.D.
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