Especially when there are no winter rains to leach accumulated salts from the root zone of trees, there is major concern about increasing the levels of salts going into the root zone. Chlorides, boron, sodium and total salts all should be minimized as much as possible in order to optimize tree production and health. Evaluating the fertilizer and irrigation management programs is important and in doing so, finding out how much is being put into the orchard.
A wonderful way to evaluate what is being applied through the irrigation system is to go online to AvocadoSource (avocadosource.com) and go to the ‘Tools' section and click on the ‘Irrigation Water Mineral Content Calculator'. Once there click on ‘Retrieve District Water Analysis Data' and there are several water qualities that can be downloaded onto the calculator.
I chose one of the Metropolitan Water District sources – Castaic Lake – which is representative of water delivered to the south from northern California. It shows a chloride level of 81 mg/L (81 ppm) which translates to 220 pounds of chloride for every acre-foot of water. Which means about 440 pounds of chloride per acre (about 2 ac-ft/ac) to grow avocado and citrus in Fillmore. And the same water coming out of Lake Skinner further south but nearly the same quality as Castaic, would be 880 pounds of chloride per acre in Fallbrook (4 ac-ft/ac).
So the question comes up about the use of potassium fertilizers. Citrus and avocado haul off about twice the potassium in their fruit as nitrogen. A typical harvest for either crop is about 50 pounds of K per acre – more fruit, more K. So to apply potassium, a grower can use several different materials – KMag, potassium thiosulfate, potassium sulfate, potassium nitrate, potassium chloride. A 100 pounds of either potassium sulfate or chloride put on about the same amount of potassium, 50 pounds. With the potassium chloride or course, there is 50 pounds of chloride.
The cheapest source of K is potassium chloride, but growers are concerned about the added chloride. The material is highly soluble and is easily injectable. It also is rapidly moved through the soil, so when it is injected through the irrigation in small amounts, the chloride tends not to accumulate in the root zone. So looking at the total amount of chloride that is applied in our normal irrigation waters, the chloride in the fertilizer doesn't represent a large proportion of the total chloride the tree sees. It could be considered in a fertilizer program, or at least a supplement to other sources of potassium.
Potassium is relatively immobile in soil, more so with more clay. Chloride on the other hand is quite mobile. It goes wherever the water goes. Applying it any time of the year basically results in its staying there until it is taken up or the soil is washed away. So applying potassium chloride in a wetter time of year, could be a cheap way to get potassium on with the least effect of chloride. Or potassium chloride could be applied in rotation with more expensive forms of potassium, such as potassium thiosulfate (KTS).
By the way, that Castaic water would contain 87 ppm sulfate and 74 ppm sodium which would mean over 200 pounds per ac-ft in the water and 110 ppm bicarbonates. The pH would be around 7.8. And this is good water by southern California standards. Many of the well water in southern California have much lower qualities than these waters from norther California and we get good yields from them. We have learned to use some pretty awful waters to grow crops here.
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 has 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.
This promises to be another low rainfall year and the customary leaching we rely upon in winter rainfall is not going to be as effective as in customary years. Irrigation is a necessary evil. Every time we apply irrigation water we apply salts, and unless some technique is used to minimize salt accumulation, damage will result. This damage can be more than just leaf drop, but also the stress that induces conditions for root rot.
Irrigation water has been applied the last four years and many trees looked stressed. Even well irrigated orchards have leaf burn due to the gradual accumulation of salts from irrigation. It is probably necessary to irrigate in many winters. With 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 first rains. Growers with water quality exceeding one, hundred, or thousand should be especially alert to the need to manage water in low rainfall years.
Assessing water quality for Southern California agriculture typically revolves around the total salinity of the water, its total dissolved solids (TDS), and the toxic ions boron, sodium and chloride. Salts are necessary to plants, because it is in the form of diluted salts that all nutrients are taken up by plants- the macro and micronutrients plants extract from the soil. High salinity leads to water imbalance problems much as if the plant were not getting adequate water. A toxicity problem is different from a salinity problem, in that toxicity is a result of damage within the plant rather than a water shortage. Toxicity results when the plant takes up the toxic ions and accumulates the ions in the leaf. The leaf damage that occurs from both toxicity and salinity are similar in that it causes tissue death known commonly as "tip burn." The damage that occurs depends on the concentration of the ions in the soil water around the roots, the crop sensitivity and crop water use, and the length of time the crop experiences the ions. In many cases, yield reduction occurs. Because crops can not excrete salts the way humans do, salts gradually accumulate in a plant. As a result plants need a higher water quality than humans do.
Much study in many countries has gone into evaluating water for crop use. Some of these studies have been on the effects of salts on soil characteristics. Generally, as sodium concentration increases, a soil will lose its aggregation, eventually leading to poor water infiltration. Many more salinity and toxicity studies have been done on plants themselves. Not all crops are equally tolerant of salinity and toxicities, and in general most plants respond to salinity and toxicities in a similar fashion. If a plant is intolerant of salinity, it will be intolerant of chloride, sodium and boron. Most annual crops are less sensitive to salts than tree crops and woody perennials, although symptoms can appear on any crop if concentrations are high enough. The reason for greater sensitivity for perennial crops is that the tree is sitting in the ground absorbing salts for a longer period than the lettuce plant that is harvested 3 months after planting. Furthermore, deciduous trees like walnut shed their leaves each winter, so they can handle salinity better than evergreens like citrus and avocado.
To manage salinity and toxicities, water management is the key. Depending on water quality, an excess of water will be applied to the soil to leach the previously applied salts away from the root zone. The poorer the water quality, the more excess water is applied.
Selecting a less sensitive crop is also an alternative when dealing with poor water quality. Some barley varieties can handle salinity similar to ocean water. Barley nets a grower $400 an acre, avocados $9,000 and $25,000 if the market is right for strawberries. Avocados are salt sensitive, so are strawberries and lemons and cherimoyas and star fruit and blueberries and raspberries and mandarins and nursery crops. We grow these because with our climate, very few other places can grow them and they return enough money for a grower to stay in business in an area where land, water and labor are expensive. We really don't have much in "alternative crops" to grow here.
Along with drought there are also concerns about water quality which has all kinds of weird units that area actually convertible. Here's a little guide for the principle water quality components and their conversions.
Common ions in water: calcium (Ca2+), magnesium (Mg2+), sodium (Na1+)
sulfate (SO42-), chloride (Cl-), carbonate (CO32-), bicarbonate (HCO3-), boron (H3BO3)
Measured as parts per million (ppm) or milligrams per liter (mg/l), which are interchangeable , or milliequivalents per liter (meq/l). A milliequivalent is the ppm of that ion divided by its atomic weight per charge.
Example: Ca2+ with atomic weight of 40 and a solution concentration of possibly 200 ppm. Ca2+ has two charges per atom, so it has a weight of 20 per charge. 200 ppm divided by 20 = 10 meq of calcium for a liter of water.
Total Dissolved Solids (TDS): measure of total salts in solution in ppm or mg/L
Electrical Conductivity (EC): similar to TDS but analyzed differently.
Units: deciSiemens/meter(dS/m)=millimhos/centimeter (mmhos/cm)=
1000 micromhos/cm (umhos/cm).
ConversionTDSEC: 640 ppm=1 dS/m= 1 mmhos/cm=1000 umhos/cm
Hardness: measure of calcium and magnesium in water expressed as ppm CaCO3
pH: measure of how acid or base the solution
Alkalinity: measure of the amount of carbonate and bicarbonate controlling the pH, expressed as ppm CaCO3.
Sodium Adsorption Ratio (SAR): describes the relative sodium hazard of water
SAR= (Na)/((Ca+Mg)/2)1/2, all units in meq/l
1.5 feet of water with EC of 1.6 adds 10,000 # of salt per acre
and that same water with 20 mg/l of nutrient will supply 80# of that nutrient/acre
Sea water has ~ 50 dS/m, 20,000 ppm Cl, 10,000 ppm
Irrigation water WATCH OUT- 1,000 ppm TDS, 100 ppm Na/Cl, 1 ppm B