[compost_tea] Re: Fert, salt indexes

From: <soilfoodweb_at_aol.com>
Date: Tue, 12 Oct 2004 16:06:57 -0400
I was asked the following -

It would be handy to have the method of calculation so that if we get any further salt indexes , we can work them out ourselves .

Method of calculating salt index - This is a measure of osmosity, or the ability to hold water against the application of pressure.  Plants apply pressure to pull liquids into their roots.  So if salts hold water more strongly than the roots can apply pressure to remove it, the plant will die.  Same for microorganisms. 

Sodium chloride is set as the standard, of 1.00, or in the scale of the data that was sent you, the relative unit for sodium chloride would be 100. 

The easiest way to interpret that is that 100 pounds more or less of salt per acre has no discernable effect on the biology in the soil.   When the salt index is 100, and you apply less than 100lb/ac, we do not measure an effect.  Probably there is one, but we can't detect it with current methods.  Let me point out that plate count methods won't  show these negative effects, because plate count methods assess spores and other dormant stages.  So, ACTIVE organisms are being killed, and you'll miss it altogether if you rely on plate count methods. 

If more than 100 pounds per ac of NaCl is applied, a detextable number of organisms will be killed.  The higher the amount of salt applied, the more organisms will be killed.   

The cause of the effect is holding water so organisms have no water available to use.  Available water decreases below the threshold value needed by the biology to survive.  The higher the salt index, the more water that salt will hold against the pull of pressure, and so the less usable water for the organisms, so more species die.  

So how to use the below salt index.  If salt has an index value of 100, meaning 100 lb/ac can be used without significant harm, then you can put together a ratio to calculate the others. 

So, super phosphate has a value of 8, while sodium chloride has a value of 100.  Thus you could put on 100 divided by 8 times more super P than salt beofre you would see a killing effect from the super P.

NaCl index/Material of interest salt index =

Multiply the 100 lb/ac amount by that ratio.

100/8 times 100 = amount of material of intereat that can be put on before damage to the microbes occurs.  

1250 lb/ac of super P before microbes will be harmed. 

Now, let me point out that this does not jive with what I've seen in the field.  I think they gave you a value for rock phosphate, not super P.  Or the value for super P was determined using coarsely ground super P, where most of the mineral did not react in their test.   If they grind it finer, the salt index would go way up. 

I also have a hard time believing the surpentine super value, whatever that is.  A value of 0 means the material does not dis-associate in water, so it will never dissolve.  Why would you put something into your soil that is not capable of going into solution?   Your plant will never be able to take it up. 

Let's try another one

Pot chloride   
100/114 times 100 = 87 lb/ac of potassium chloride could be used, not 100 lb/ac.  Or, if 100 lb/ac of KOH was used, organisms would be killed.  The negative impact would be discernable. 

SALT INDEXES

Surpentine super        0
Super phosphate         8
Pot. chloride           114
Pot. sulphate           46
CAN                     82
Nitrophoska 12/10/10    56
Te Nitrophoska blue     60
Urea                    75
Patent Kali              28 

The table of salt index values was supplied by Graeme Reid, an SFI advisor in New Zealand, who obtained the data from Ravensdown, a government soil chemistry group in New Zealand.

Hope this was clear.  Any questions, please ask. 

--
Elaine Ingham
President, Soil Foodweb Inc.
SFI Corvallis, OR
SFI Port Jefferson, NY
SFI Lismore, NSW, Australia
SFI Cambridge, New Zealand
SFI Culiacan, Mexico
SFI Jerome, Idaho
SFI South Africa
http://www.soilfoodweb.com


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Received on Wed Oct 13 2004 - 01:04:14 EDT

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