Dear Manoj Kumar,

The deficiencies of micronutrients are of critical importance for sustaining high productivity of rice in India. Zinc is one of the essential plant micronutrients and its importance for crop productivity is similar to that of major nutrients . The average response in dry matter yield (%) at optimum level of applied Zn in soils below critical limit ranged from 51.2 to 89.6 per cent with mean value of 68.5 per cent. With increasing the level of Zn content in soil above the critical limit, the percentage responses also decreased which varied from (-) 5.2 to 70.3 per cent with a mean value of 19.9 per cent. On the other hand, the application of Zn increased the average shoot yield from 8.4 to 11.76 g pot-1 up to 5.0 mg Zn kg-1 soil below the critical value .The positive effect of Zn might be due to its beneficial effect on metabolism (Rashid and Fox 1992). Whereas, the critical value, the application of Zn fertilizer increased the average shoot yield marginally from 11.38 to 13.04 g pot-1 up to 2.5 mg Zn kg-1 soil. However, a substantial amount of average dry matter yield decreased with increasing levels of applied zinc fertilizer in the soils containing Zn above critical limit up to 5.0 mg Zn kg-1 soil. The decrease in dry matter yield at higher Zn levels may be ascribed to Zn toxicity in plant tissue . Based on critical value of available Zn, soils were grouped into deficient and adequate classes . Considering critical value of Zn in soils (0.75 mg kg-1), 13 soils were rated to be adequate and 53.8% soils belonging to this category responding to Zn application. Whereas, 100% soils below the critical value showed the positive response to Zn application and number of soils in this category were nine (9). However, closer examination indicated that the magnitude of mean percentage response due to 2.5 mg kg-1 level of Zn application over 0 mg kg-1 was found to be maximum. This indicated that Zn can be applied for rice @2.5 mg kg-1 in the soils of alluvial zone of West Bengal, where the Zn content was below 0.75 mg kg-1. reported that the application of zinc as Zn-EDTA @ 1 kg ha-1 to low land rice soil of West Bengal resulted the 37.8% and 20.9 % greatest increase in grain and straw yield of rice respectively, over the control.

Regards,

Prem Baboo

Dear Manoj , are you talking about the soils of different mineralogy growing paddy .... or on same soil type , paddy responds so differentially  at different critical levels of avialable Zn...?

Dear colleagues

In my opinion, different soils are containing different amounts of Zn if we detected this element by one method.

Regards

I encountered a production system where DTPA Zn was 2 ppm and that it still responded. I am trying to find a reason behind this.

It is generally assumed that critical limit for paddy is 0.75 ppm.

 Mr.Manoj,Good question but difficult to answer. I appreciate the pertinent and detailed answers of Dr. Baboo and Dr.Raj. Most of the critical limits for zinc or other nutrients were generated under pot culture conditions and used for interpreting crop responses  to zinc application under both pot culture and field conditions.Crop nutrient requirement is met from  both soil supply and external source(fertilizer and manure).Crops and crop varieties differ in their ability/efficiency to utilize soil and fertilizer nutrients.Inefficient crop varieties(which cannot utilize soil nutrients efficiently), need invariably fertilizer nutrient  application and their requirement will change with yield level of crop. So,crop varieties which are not efficient to utilize soil available nutrient  may respond to fertilizer nutrient application, even though the soil shows adequate available nutrient content ,under high yield environment.Two things are very important in micronutrient response studies-validation of critical limits under field conditions and determination of upper critical limit for crop response beyond which the toxicity will set in or manifest in micronutrients.

The key problem is that we cannot say with any certainty what value of, e.g. DTPA Zn, is critical without doing the experiment with the soil in question.  That is, it is a big leap of faith to regard a chemical extraction to be a measure of bioavailability.

Paul , you are right , I do not think , there is any universal kind of extractant , which would be effectively extracting the plant available  Zn  on soils of different mineralogy and other associated physical properties. Probably , this si where , we need robust calibration strategy to propose different critical limits of DTPA-Zn in response to crops through field experiment   . Very often , a critical limit suggested through a pot experiment , fail to testify  the same results under field conditions. 

Dear Dr. Anup, thanks for such valuable inputs in this discussion.

Yes. FYI, I am referring to an RBD trial on added Zn in paddy crop.

The grain yield response up to 50% has been observed. Now, after having discussed this much, I feel that is reality.

I thank every scientist in the discussion who made precious comments. 

Being in industry, I have various queries in mind. I look forward to having your guidance in future.

Keep contributing!!

Thank you so very much Dr Manoj Kumar . You are most  welcome at any time .  I have just one point to flag off further. In a critical limit concept  , single value very often  overlooks the fertilizer response of crop . Therefore , it is more advisable to have a optimum limit having a range of soil test values  at which there is a possibility of securing maximum crop response.  We need to work such optimum ranges with respect  to a given crop and under varied soil mineralogy  to say the least. But , all these must accrue through filed experiments to have wider application of such results. 

Critical limit of any nutrient in particular soil depends upon soil type, climate and adopted cultural practices for many years. At present Indian soils are deficient in 8 nutrients due to faulty agricultural practices. Adoption of organic farming practices viz; crop rotation, inter cropping, cover cropping, green manuring, mix cropping etc is only alternative to sustain the soil health.   

Dilute hydrochloric acid has been found to be a very good extractant of zinc from rice-growing soils. Critical limit of 0.05 (M) HCl extractable Zinc in rice soils has been established as 1.0 mg/kg. To know relative superiority of this extractant over other commonly used extractants, I suggest you to read the following paper: Ponnamperuma et al. (1981) Plant and Soil 61(3): 297-310. 

Dr.Milham,the whole concept and use of soil testing depends on the  big faith that the soil test  indicates bio-availability of nutrients.We have faith in nutrient extraction chemistry,correlation with crop response and calibration and interpretation(for generating recommendations).We continue to use the soil tests for P(Bray and Olsen methods) developed 60-70 years back.As you are aware, we use both soil and plant diagnostic tests and also see the correspondence between the two .Discrepancies are noted sometimes,possibly because of more requirement of a particular nutrient by crop or the chemistry of soil and extraction procedure mismatch.The DTPA extraction method is widely used in India and many other countries for Zn and Fe,Cu,Mn.When I examined the critical limits for Zn in over 10 states in India varying in soil characteristics and in a large number of crops(around 40) the critical limits are varying from 0.35 to 0.99 ppm .But the critical limits are predominantly in the range of 0..48 to 0.75 ppm and a critical limit of 0.6 ppm is considered a safe critical limit for a range of crops.I believe that at least 60-70 percent of crops in diverse soils  having

Thanks, Dr. Rao for sharing your experience. I wish to draw your attention to a recent publication of Pradhan et al (2015) Evaluation of .. different extractants for zinc and copper in soils ... Plant Soil Environ. 61(5):227-233. Based on their comprehensive evaluation of different extractants including DTPA: they have concluded that Mehlich 3 and dilute HCl method of Ponnamperuma et al. (1981) could be effectively used for estimating Zn and Cu availability in soils of India. This work was carried out at BCKV, West Bengal, India by Prof. Biswapati Mandal's research group in collaboration with five other State Agricultural Universities of India. Much lower cost of dilute HCl compared to that of DTPA as extractants for Zn also warrants consideration. I seek your opinion on this issue.