Dear Indranil,

If you want to measure Electrical Conductivity as a salt index the procedure explained in below:

Barradas et al (2014). Effect of Fertigation on Soil Salinization and Aggregate Stability

Hi

1. These fertilizers is mineral or organics?

2.If mineral fertilizers it is necessary conductivity meter and a set of ion-selective electrodes.

3.There are methods of photometric and spectral analysis.

4. For organic substances the most suitable general methods of chemistry - dry combustion and analysis of combustion products (gases) using absorbers (weight methods) or spectrum analysis.

Best regards.

I would like to analyze salt index with conductivity meter. Please let me know the detailed procedure as salt index relates to osmotic pressure exerted by the fertilizer compared to NaNO3. I want what proportion of of soil:fertiliser ratio I should maintain. If possible please provide me on reference paper "The Salt Index—A Measure of the Effect of Fertilizers on the Concentration of the Soil Solution,”  published in March 1943 issue of Soil Science" authored by L. F. Rader, Jr., L. M. White and C. W. Whittake

Dear Dr Das , salt index is  a  kind of numerical comparison of fertilizer compounds using sodium nitrate salt index of 100 as the standard. Sodium nitrate was choosen as the standard because it was 100 percent soluble , and it was commonly used as  nitrogen fertilizer when the salt index concept was first proposed in 1943. Before  1942, osmotic pressure  and the osmotic index number were the terms generally used to describe  what today we refer to as Salt Index.   Salt index of fertilizer is often used as a measure of the salt concentration that a fertilizer induces in the soil solution . Some of the publications , you will readily find published in 1930s and 1940s, L. F.  Rader and his co-authors were the first to coin the word SALT INDEX.   Often , estimation of a saturated extract EC , termed Salt Index using the formula: EC( Salt index)=EC(2:1)x8.0. Find below one PDF to throw more details about the calculation of salt index . Hope , it will be useful to you. 

Thank you Dr. Srivastava for your prompt response. I am unable to get the pdF paper of Radar. So I am unable to find the procedure they followed. However the pdf file you attached is already with me but if i would like to determine in the laboratory what whould be the procedure.............. that i wan't to know. That means whta would be the soil to fertiliser ratio etc.

Dear Dr. Das. According Urbano (1990) a practical way  to estimate osmotic pressure (OP) is   OP = 0,36 x EC. You  can also  determine if using an osmometer.

Best Regards

José Delatorre

• I can measure it but I would like to know the standard procedure to determine it. What would be the soil:fertiliser ratio etc

Thank you Fred for proving me necessary information about salt index. Is there any standard procedure to the factor for conversion Ec to Op

Dear Dr. Das,

several contributors have already described the various methods used to describe the 'salt index', but allow me to put some of these in perspective.

As correctly said the term ‘salt index’ was first introduced by Rader et al. in 1943 (attached). They state: “The present paper gives the results of a study of the osmotic pressure produced in the soil solution by numerous fertilizer materials and shows how these data, by means of a new quantity called the "salt index," may be used to predict the relative effect of any mixed fertilizer the formula of which is known.”

It was originally proposed with reference to the negative effects of high fertiliser doses on crop germination and seedling growth, as may be seen from the first sentence of their introduction “Germination of seeds may be prevented, or established crops may be injured by the presence of too much soluble salt in the soil.”

However, this salt index refers to the osmotic potential induced by a fertiliser in ‘the’ Norfolk sand soil. Strictly speaking, when testing really new fertilisers there is no way to measure the salt index as originally proposed by Rader et al., unless you get hold of the Norfolk sand.

As this became apparent soon afterwards, various authors proposed different ways to characterise the osmotic potential induced by fertilisers and/or proposed conversion factors, as indicated above already (e.g. Jackson, 1958). In my view most of these approaches are weak compromises, as critically discussed in the other paper I uploaded (Currently Used Salt Index Tables are Misleading, by John G. Clapp).

In any case I suggest to first clarify what the purpose of measuring such ‘salt index’ might be. Is it to do with the colloidal properties of clay particles, inhibition of seed germination, seedling growth, leave burn when top-dressing? Also, you may want to compare and compare your findings (observed effects of fertiliser application on some relevant processes) with other findings. By the way, in this regard the osmotic potential as such is not (!) an important aspect in its own right. I agree that the osmotic effects of fertilisers are important when placing them close to the seeds, but in most applications – worldwide and certainly in India – this is a minor consideration.

Hence, I suggest to carefully check the relevant literature and choose the most appropriate parameter/method based on these. Following a certain procedure by measuring the osmotic potential of an extract is usually not an issue.

Thank you Dr. Gerendas  for providing me the literature and also guiding me about the work.

Hi,

Salt index (SI) of a fertilizer is a measure of the salt concentration that fertilizer induces in the soil solution. SI produce crop injury on a particular soil, but it does allow comparisons of fluid formulations r. As we all know, placement of some formulations in or near the seed may decrease seed germination or result in seedling injury.

The SI of a material is expressed as the ratio of the osmotic pressure of the salt solution produced by a specific fertilizer to the osmotic pressure of the same weight of NaNO3, which is based on a relative value of 100. Sodium nitrate was chosen as the standard because it was 100 percent water soluble and it was a commonly used nitrogen fertilizer when the SI concept was first proposed in 1943. High analysis fertilizers usually have a lower SI because fewer ions of salts are placed in the soil solution per unit of plant nutrient when they dissolve.

SI

measures the electrical conductivity of a 1% fertilizer solution. Solutions with a high SI have a higher conductivity, an indication of the amount of salt in the

solution.

I tried to go through all the literature. I found the detailed method of Radar et al. 1943 from Dr. Gerendas. Unfortunately I can't follow entirely the Method of radar et al. as I have no provision to measure freezing point depression. So I would like to follow the same method of Radar just modifying it as ............ determination of Ec of fert. mixed soil/Ec of NaNO3 mixed soil. Further I would like to have OP by using EC x 0.3. Side by side I would also determine the EC of Fert solution/EC of NaNO3. and converting It to OP though the ratio remains same. Please suggest me what to do.

Happy to see interesting information on salt index,Dr.Gerendas.Except for sodium nitrate,do you think that the salt index has any relevance in the present day fertilizer use practices?If it is relevant, which type of fertilizer has potential to create osmotic potential  in soil sufficient to harm the seed germination and  seedling growth?

Dr. Rao, you are correct but  one should have an idea about salt index of different fertilisers as their cumulative effect may create problem someday. Besides many new complex/compound fertilisers were used now-a-days and very little information about salt index known to these fertilisers.

 Both N containing fertilizers  and K containing fertilizers  have relatively high salt index .Also the low analysis fertilizers will have high salt index.So a combination of several sources may add to salt index. So it is useful to measure salt index of complex fertilizers /fertilizer compounds.

Yes Dr. Rao, for that reason I want to know salt index of these fertilisers.

Dear Dr Das,

Allow me to add some further comments to what has been mentioned by you and others:

1.       Different salts have different specific conductivities (see below), but the osmotic pressure depends on the number of dissolved ions/molecules. That’s why in soil and fertiliser analysis we typically operate with approximations when converting EC values into osmotic pressure. For a given fertiliser of known composition this conversion can be fairly accurate, and in soil analysis it is ‘assumed’ that the salt profile is ‘typical’.

Specific conductance V (mS/cm per meg/L) (based on Harned and Owen (1964); see http://pubs.usgs.gov/wsp/2311/report.pdf)

Cation   V             Anion    V

 Ca2+      59.5        SO42-    80.0

H+          349.6     OH-        197.7

K+           73.5        Cl-           76.3

Na+        50.1        HCO3-   44.5

NH4+     73.4        NO3-     71.4

Mg2       53.0

2.       In your recent response to Dr Rao, you indicated that ‘one should have an idea about salt index of different fertilisers as their cumulative effect may create problem someday’. It is certainly correct that the prolonged excessive use of fertilisers may contribute to soil salinity, but the salt index – irrespective of how it is measured – is useless in this regard. The reason is that the SI concept treats fertiliser salts as a chemical, irrespective of the significance of its components – the nutrients – for plant growth. Consider KNO3 as an example: It is a very strong osmolyte, but contributes the two nutrients taken up (=removed) by crops in highest amounts. Hence, it has no residual, long-term effect on soil salinity. Applying these two nutrients as KCl and NaNO3 (Chile saltpetre – merely of academic interest these days… ) incurs a huge salt burden. Nonetheless, even KNO3 could easily inhibit seed germination and seedling growth. This brings us back to my original question: What is the purpose of measuring the SI?

3.       Another shortcoming of the SI that is somehow related to (2.) is the unit used. It does not take into account that trace nutrient fertilisers are typically applied as a few kg per ha, while NPK application is several 100 kg per ha. Fertiliser borates can easily induce crop damage, but certainly not because of osmotic effects. The limitations regarding urea as mentioned by Fred Vocasek are also important, even more so as in many countries NPKs are urea-based and not NPK ‘complex’ fertilisers.

Thank you Dr. Gerendas for enriching me by your valuable observations. I agree with your observations. Since NPK fertilisers are used in heavy quantities compared to trace element fertiliser, it is therefore necessary to determine the salt effect of these fertilisers in soil. I know method is not important but in the literature I found diffence in salt index values. That is hy I specified those two methods. besides I would try to compare the salt index derived from component of different mixed/complex fertilisers as i found one method where they added the salt index of each component though I am unknown about the composition about the fertiliser mixtures.