My interpretation of the FAO report is that hey are strictly referring to its status as a negatively changed ion in fluids.  The question of "essential elements" usually involves whether another element could be substituted.  I think any non-toxic counterion such as sulphate or bicarbonate could replace it as simply a counter-ion to  maintain ionic strength. It does happen to be the usual counterion to sodium, which is essential.

 I agree with the response from Roberts. Let me put some experimental proofs for chlorine to be classified as micronutrient .

The chlorine requirement is not small as compared to other micronutrients, for in the leaves of tomato plants suffering from chlorine deficiency disease, the chlorine concentration is in the order of 7 micromoles per gm dry weight (250 ppm). Data bearing on this point will be presented in another paper. By comparison, the presently recognized micronutrient which is required in the least amount is molybdenum and

plants showing an equivalent degree of visual stress by reason of molybdenum deficiency would have in the order of 1.4 x 103 micromoles per gram (0.1 ppm) or less of molybdenum. Thus, on a mole basis, the minimal quantity of chlorine required in plant tissue is several thousand times greater than for molybdenum. Plant  requirements for Fe, B, 'Mn, Zn, and Cu are intermediate, and reasonably within the

order given.In recognizing chlorine as a plant nutrient, it is with the realization from this experiment that Br can substitute at least partially in plant functions normally

dependent on Cl, in a way reminiscent of the sparing effect of Na for K or Sr for Ca. Whether or not Br can substitute completely for Cl must remain a subject for future investigation.

Earlier , controlled culture solution experiments of Eaton , Raleigh  showed highly significant increases in yields of tomatoes and cotton, and of beets, respectively, when supplied with additional chlorine. It also supports earlier work of Lipman as early as n 1937 ) who, after directing his attention specifically to chlorine as a growth factor for buckwheat concluded that "if chlorine is not essential, it is certainly highly beneficial." Based on exerpts from publication of Broyer et al .,

Plantphysiol. PDf is enclosed for further reading .

we consider a Cl concentration of 100ppm in younger fully developed leaves of dicots and monocots as the lowest concentration required for maximum yield:

Schnug E and Haneklaus S (2008) Evaluation of the relative significance of sulfur and other essential mineral elements in oilseed rape, cereals and sugar beet crops. In. Jez J (ed.) Sulfur: A missing link between soils, crops and nutrition.  Chapter 17. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, 677 South Segoe Road, Madison, WI53711, USA.

Dear colleagues,the FAO report mentioned above refers to animals,fish and shrimps not plants.Though the essential nutrients are required for both plant and animal systems,there are some differences in requirements of different kind of elements and their magnitude of requirement.There is no dispute for chlorine as an essential micronutrient for plant life.In animal system as per FAO report P K S Ca Mg are macronutrients similar to plants but  in in addition Na and Cl are also macronutrients(not in plants though Na can substitute for K in some crops like cotton,  sugar beet  etc.).The report also lists several other micronutrients/trace elements/beneficial elements for animals which are not required in plants in  like iodine,fluorine,tin,nickle etc.

Dear Portia,

certainly an interesting question, but I think you and others - except the most recent answer by Annangi Rao - have been misled. From the list of essential elements including F, V, Cr, I, Sn it’s obvious that the authors referred to animal systems. The chapter was published by the ‘Fisheries and Aquaculture Department’ concerning ‘The nutrition and feeding of farmed fish and shrimp; a training manual’.

Well, I’m not an expert on fish nutrition, but the elements listed above are not even discussed as being essential for plants. I would always classify Cl as an essential micronutrient (critical concentration of 100 ppm as Ewald Schnug pointed out), in agreement with just any textbook on plant nutrition. I would also always follow the concept that the definition of micro vs macro is solely based on the requirement of the element under consideration to fulfil its essential function. When such element is present in concentration well above this critical concentration – as is often the case with chlorine – this element still doesn’t qualify as a macro nutrient element. As far as plants are concerned chloride is most effectively replaced by sulphate and nitrate.

I refrain from elaborating on the three criteria established by Arnon in 1950; that’s found in textbooks.

(mostly written BEFORE Annangi Rao provided his input)

Dr. Gerendas,I appreciate your answer and wish to highlight two points from your reply.When an element is not in critical concentration (adequate or high) in soil-water-plant -animal system,it does not matter whether one call it macro,micro or non essential element.Second point,as far as plants are concerned chloride can be effectively replaced by sulphate and nitrate,is not so clear.Please elaborate this point.

Chloride is only required by plants in small concentrations so according to the usual definition that makes it a micronutrient.  The fact that plants typically contain macro concentration is irrelevant

Dr.Paul,I agree with your first sentence. Dr. Gerendas has also concluded similarly.Probably as it is not present in low or critical concentrations in soil-plant  system it is not receiving the attention of researchers like other micronutrients. Functionally  in plants can sulphate and nitrate replace chloride?

Dear Dr Rao,

I still don't understand whated you wanted to say in your first statement, posted in the first message after I provided my input.

Regarding my statement "As far as plants are concerned chloride is most effectively replaced by sulphate and nitrate" this refers exclusively to the statement "I think any non-toxic counterion such as sulphate or bicarbonate could replace it [chloride] as simply a counter-ion to maintain ionic strength" made by  Robert G. Qualls referring to animal systems.

Chloride is mostly found in macronutrient concentrations (Paul Milham, above) and as such makes a substantial contribution to the osmotic potential of plant cells. In this regard, it could effectively be replaced by nitrate and sulphate, particularly in the vacuoles. Phosphate ion concentrations are usually too low to be effective.

Dr Gerendas , you still consider chloride as micronutrient or macronutrient ?. And essential or non-essential ?

I consider it a micronutrient (have never seen evidence to the contrary), and it's essential.

Dr.Gerendas,interesting discussion on chlorine.The essentially of chlorine as micronutrient in plants was established in 1954 by Broyer et al. The critical limit of chlorine ranged from as low as 2-20 ppm,followed by 100(Schnug) and then 70-700 ppm( HLS Tandon compilation).In plant material the concentration ranges from 0.2-2.0%(200-20000 ppm).That is why I said it is immaterial whether you call it macro or micronutrient in plant.Any way it is macronutrient in animals.However we all agree that it is micronutrient in plants as per its metabolic requirement. Dr.Gerendas you have mentioned that sulphate and nitrate can substitute for chlorine in osmoregulation ( and probably tissue hydration) function in plant or animal cells.What are the other specific functions of chlorine in plant metabolism like enzyme activation etc. which needs certain concentration of chlorine in plant system?