Posts Tagged: sodium

One, one hundred, one 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 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.

irrigATING CITRUS

Salt and Gypsum

With the drought our perpetual salt problems are exacerbated due to less water and often more saline water.  The question keeps coming up if gypsum (calcium sulfate) can help correct the problem.  And the answer is maybe, but along the coast, probably not.  The problem there is confusion about what is a saline soil and what is a sodic soil.  A saline soil is one that is dominated by salts, but has a pH below 8.5 and can have a white crust that will actually taste salty.  A sodic soil is one dominated by sodium, has a pH above 8.5 and can be saline, as well.  Often though, there is a brownish cast to the surface salt crust.  This is caused by dispersion (dissolved) of soil organic matter caused by the high pH.  It's like cooking with vinegar when you make ceviche out of fish.  Saline soils often have a high calcium content and may have sodium, but at a very low ratio compared to calcium.  Most of the sodic soils in California are found in the Central and Imperial Valleys.  Along the coast, the soils, if they have a problem, are largely saline.

The way gypsum works, is that the added calcium displaces soil sodium, pushing it lower in the soil column.  The process also requires a lot of water to move the sodium through the soil column.

So the answer is, along the coast, gypsum is unlikely to improve soil conditions.  However, there are other instances where it might help. In the San Luis Obispo area there are lots of serpentine derived soils that have a high magnesium content relative to calcium.  And they commonly aren't saline, just an imbalance between the two cations. This can lead to infiltration problems and calcium deficiency in plants.  Apples are especially sensitive to this high Mg:Ca ratio and develop a condition called “bitter pit”, a surface, brown pitting in the skin. There are other crops, like celery that are especially sensitive, but even avocado can be mildly affected. In the case of magnesium imbalance, gypsum can help.

sodic-crust stutsman-co

Water Quality and Southern California Agriculture

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.

avocado fruit

Water Quality Terminology

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.

 

Water Terminology

 

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

avocado water

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