Posts Tagged: sodium
Salt, Sodium and Potassium and Drought
Potassium deficiency in avocado and citrus leaves often looks like salt stress and more specifically sodium toxicity. Plants will often look wilted with curled leaves, yellow areas between leaf veins and dead areas along the margins of the leaves. Salt stress refers to the excessive amount of soluble salts in the root zone which induce osmotic stress (appearance of lack of water) and ion toxicity (growing problems and often symptoms) in the growing plant. Among toxic ions, sodium (Na+) has the most adverse effects on plant growth by its detrimental influence on plant metabolism in inhibiting enzyme activities. An optimal potassium (K+) : Na+ ratio is vital to activate enzymatic reactions in the cytoplasm necessary for maintenance of plant growth and yield development These enzymes control such functions as the stomata which regulate water and photosynthesis control in the plant. Although most soils have adequate amounts of K+, uptake is exacerbated under sodic or saline-sodic soil conditions as a consequence of K+-Na+ antagonism. Here K+ uptake by plants is severely affected by the presence of Na+ in the soil. Due to its similar chemical properties, Na+ competes with K+ in plant uptake It would seem a reasonable assumption therefore that an increase in the concentration of K+ in salt-affected soils may support enhanced K+ uptake. And that has been noted in many plant species including citrus and avocado.
But aside from the role of potassium in drought tolerance there are many functions of potassium in plants:
• Increases root growth and improves drought resistance
• Activates many enzyme systems
• Maintains turgor; reduces water loss and wilting
• Aids in photosynthesis and food
• Reduces respiration, preventing energy losses
• Enhances translocation of sugars and starch
• Produces grain rich in starch
• Increases protein content of plants
• Builds cellulose
• Helps retard crop diseases
In the case of avocado and citrus there is about twice the amount of potassium as nitrogen harvested in the crop, yet many growers do not consider potassium in their normal practices, much less when drought has increased salt stress on the trees. The end of August through September is when leaf analysis is best used to adjust a fertilizer program.
Sodium toxicity and Potassium deficiency in avocado
avocado sodium toxicity
potassium deficiency avocado
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
Sodium chloride damage on raspberry
A few pictures here for the library on sodium chloride damage on raspberry. Leave tissue concentration of sodium is over 250 ppm and chloride over 20,000 ppm (that's 2% of total dry tissue!). Interestingly accumulation of the micronutrients zinc and copper in these plants is also notably higher than what would be normal for raspberry.
Sodium chloride damage in raspberry. Note heavily burned leaf margins, to the extent that very little green remains.
Entire cane affected by sodium chloride. Appearance of some green leaves on some laterals tells us that this plant can yet recover.
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
Sodium Adsorption Ratio (SAR), Briefly
I look at a lot of soil reports these days, and the sodium adsorption ratio (SAR) is one number that is very informative to me.
Simply put, SAR of a soil extract takes into account how much the adverse effect of sodium (Na+) is moderated by the other cations calcium (Ca2+) and magnesium (Mg2+). As you all know, calcium and magnesium can replace sodium on soil particles, subsequently permitting this toxin to be washed away from the plant roots. So the more free calcium and magnesium we have around in the soil, the better odds we have of mitigating sodium.
SAR levels below 6 are OK, levels which run above 10 mean trouble.
Although most soil reports will give you a calculated value for SAR, you can calculate SAR yourself. It's simple: SAR = Na+/((1/2(Ca2+ + Mg2+))1/2 ), where Na+, Ca2+ and Mg2+ are all measured in meq/L (milliequivalents per liter).