Posts Tagged: salts

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

Soil Moisture Sensors

There's been a lot of salt damage to fruit trees after two years of drought using poor quality water and not doing adequate leaching.  The situation could be improved by knowing when to irrigate.  Here's some guidelines on selecting soil moisture measuring devices.

Soil moisture sensors fall into two broad categories, volumetric and tensiometric methods. One tells you how much water is in the soil and the other tells how difficult it is to remove water.  Volumetric methods require a calibration of the sensor to the soil, whereas tensiometric is good to go when installed.  The case in both methods is the grower learns to keep soil moisture within a given range of values and in theory, the plant is kept in a better condition with improved health and yields. 

The most common volumetric methods rely on measuring the dielectric constant of the soil which determines the velocity of an electromagnetic wave or pulse.  These sensors have become widely used because they have a good response time, do not require maintenance and can provide continuous readings, allowing for automation.  There are several different methodologies used Time Domain Reflectometry,  Frequency Domain Reflectometry (Capacitance), Amplitude Domain Reflectoremtry (Impedance), Phase Transmission, and Time Domain Transmission.  There is quite a range in prices for these different devices, but they generally do not require maintenance once installed and not require close soil contact.

The tensiometric methods include Tensiometers, Gypsum Blocks, Granular Matrix Sensors, Heat Dissipation and Soil Psychrometer.  These techniques rely the sensor to come into equilibration with the soil moisture and generally are unaffected by soil salinity. Gypsum blocks and Granular Matrix are not very responsive in sandy soils and require good soil contact.

Volumetric Sensors

	TDR	FDR	ADR	PT	TDT
Cost (including logger/reader)	$400- 20,000	$100-3,500	$500-700	$200-400	$400-1,300
Field Maintenance	No	No	No	No	No
Affected by salts	High levels	Minimal	No	>3dS/m	High levels
Installation method	Buri9ed	Buried	Buried	Buried	Buried
Soil type not recommended	Organic, salt, high cay	None	None	None	Organic, salt high clay

 

Tensiometric Sensors

	Tensiometer	GB	GMS	HD	SP
Cost (including logger/reader)	$50-75	$400-700	$200-500	$300=-500	$500-1000
Field Maintenance	Yes	No	No	No	No
Affected by Salts	No	>6 dS/m	>6dS/m	No	maybe
Installation method	Buried	Buried	Buried	Buried	Buried
Soil Type not recommended	Sandy	Sandy, high  clay	Sandy, high clay	Sandy	Sandy, high clay

 

tensiometer2

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