Very intelligently placed question on a very intriguing issue , which needs to be addressed in providing the comprehensive schedule of fertigation , instead of regulating NPK through fertigation and practicing foliar spray for micronutrients. Lets listen from our colleagues , why there are limited efforts on fertigation of micronutrients, regardless of crops . Is it because of better efficacy of foliar spray over fertigation  or any other reasons .

Fe-EDDHA (and Fe-HBED) are the worldwide standards for Fe fertigation in Citrus and other fruit trees. Please ask for the so called ortho-ortho content (obligatory label value in Europe) of the product. Only the ortho-ortho isomer will be able to keep the Fe in solution in calcareaous/alkaline soils. For Zn and Mn, EDDHA is not suitable; here the Zn-EDTA and Mn-EDTA are used in fertigation. These two can also be used foliar, but often fertilgation is more practical if the drip lines are functioning well. You can test this with Fe-EDDHA: it gives the irrigation water a reddish color (natural color of Fe-EDDDHA). You can mix Fe-EDDHA, Mn-EDTA and Zn-EDTA if you want, if you use high quality, fully chelated products. Sulphates are much less reliable; they are cheaper per kg, but if you weight them against the effects (yield and quality increases), they turn out not to be very efficient.

Marcel,  very impressive response . But , I was looking at any efforts so far made to domesticate the conventionally used non-chelated fertilizers through fertigation .  

In case there is a need for micronutrients (based on leaf or soil analyses or leaf color or deficiency symptoms):

EDTA chelates of Mn, Zn and Cu will be needed in case of soils with a higher pH (let say above 6 or 6.5) to avoid precipitation, or in organic soils to avoid adsorption to the organic matter of the soil. Chelates of Fe will be needed always, since there are no other reliable Fe fertilizers for fertigation. Fe complexes (like amino acids) and Fe sulfates will precipitate in almost all soils. The choice of chelate depends on the soil pH: Fe-EDTA when soil is below 6, Fe-DTPA for soils up to 6.5, and Fe-EDDHA (or Fe-HBED) for soils above pH 6.5.

So, conventionally used non-chelated fertilizers could be an good option only for Mn, Zn and Cu, for most acidic soils. Do not mix sulfates with Calcium products. Also do not mix non chelated metals with Fe chelates: you risk a swap of metals with the Fe (result: Fe is lost due to precipitation).

Thanks Dr Marcel, for your valuable input regarding the micro-nutrient fertigation specially in citrus fruit crops. Fe-EDDHA (and Fe-HBED) will be tried for  Fe fertigation in Citrus fruit trees. 

Micronutrients are needed to satisfy yield and quality fruit production in citrus crops. Citrus crops are classical example to work as indicator plants for exhibiting deficiencies of micro nutrients.  Zinc, Iron, manganese and boron are the micronutrients needed in the largest quantities to ensure that growth and quality is not limited. But the missing link I think which we need to ascertain is correct balance of these trace elements which is essential for growth and development of citrus plants.

I am extremely thankful for you Sir. You have corrctly said about the amount and combination of these micronutrients for the fertigation in citrus crops. The answer of Marcel Bugter explains the use of Fe-EDDHA  and Zn as well as Mn-EDTA for micro-nutrient fertigation in citrus crops. I hope these will be soon popular for the growers in place of ZnSO4 or FeSO4 solid fertilizers. Once again thanks for you interaction on this topic. 

Rightly pointed out  Dr Malhotra , and  equally responded well by Dr Shirgure . Let me now add  my bit. No doubt , citrus is considered highly sensitive to micronutrients , regardless of any other citrus cultivars or soil type growing the crop. We know very little about the kinetic behavior of these nutrients inside the plant . How  does  the co-kinetics of  these micronutrients affect their uptake , translocation and utilization efficiency in response to rate , dose , source and time of application  across agro-pedological conditions.  These limitations further limit our understanding to upscale the present set of information into fertigation . And , this becomes still very difficult with use of water soluble or nutrient-based chelates. No doubt again about their better efficacy over conventionally used straight micronutrient fertilizers. This is the reason , we advocate the aggressive research on domestication of conventional micronutrient fertilizers under fertigation to have some value added crop responses on , both production as well quality , coupled with better economic returns. 

Here is some interesting work, Dr Shirgure..

Humic acid improved efficiency of fertigation and productivity of  tomato

ABSTRACT: Studieswere conducted during 2010–2012 at University of Horticulture & Forestry, Solan, Himachal Pradesh, India. Four fertigation levels were tested with humic acid (combined fertigation) and without humic acid (sole fertigation or fertigation alone). The experiment comprising eight treatment combinations in Randomized Block design, replicated four times. Investigations revealed that combined fertigation significantly increased plant height (5.7%), total dry matter (7.7%), leaf area index (3.2%), chlorophyll content (4.7%) and fruit yield (9.6%) over sole fertigation. Higher available N and K in was recorded in surface soils whereas, better translocation of available P was noted at 10–20 cm soil layers under combined fertigation. Higher nutrient recovery and fertilizer use efficiency was also noted in combined fertigation. Fertigation along with humic acid resulted in 20 per cent fertilizer savings over fertigation alone. It is concluded that efficiency of fertigation can further be increased by using humic substances in any agricultural production system.  PDF enclosed fro further reading...

Source :JOURNAL OF PLANT NUTRITION 2017, VOL. 40, NO. 3, 439–446, http://dx.doi.org/10.1080/01904167.2016.1245325