Hey Rkymtnman Boron is a good one , a very good one but Boron goes way behyond ppm it effects the whole nutrient solution in one way or another, I'll try not to write a book but hopefully I'll give you some useful info.
so what is Boron ,well its is an essential micronutrient necessary for optimal plant growth and development.pretty much all of a plant’s boron is found in its cell walls. its most important role is aiding in cell wall formation functioning and strength.Boron has also an important role in meristematic tissue, especially in root points, pollen growth and shoot growth points,the number of bud sites, flower growth etc. .Potassium and phosphorus nutrition is enhanced by boron.It is available as boric acid as well
well if your familiar with photosynthesis i'm sure you know that The four major nutrients that enable photosynthesis are calcium, phosphorus, magnesium, and boron. so one should be very mindful of enhancing availability of these elements. B and Ca have a very close realtionship that most don't understand Ca and B support each other in the plant.low deficient levels of B will reduce the uptake of Ca, If Ca is deficient, B uptake is also reduced . You can have high Ca, but with low B, that Ca is not absorbed. ok back to the relationship between Phosphorus and Magnesium, and between Calcium and Boron. Calcium is necessary for the transport of all other nutrients in the plant. Calcium should be balanced with potassium, magnesium and sodium Too much potassium inhibits the uptake of both Calcium and Magnesium. A plant needs Boron in order to take in Calcium.high nitrogen levels can reduce the availability of boron,potassium and copper. so i could still go more indepth but hopefully you can now all understand why a balances solution is important.
we do know B absorption is very slow and is relatively immobile in the plant, transpiration plays a significant role in translocation.here is where people running a recycling system run in to trouble very very easily and don't have a clue why. your plants uptake nutrients at different rates , there is basically 3 different classes N,P,K,Mn are absorbed the fastest , Mg,S,Fe,Zn,Cu,Mo,and CI have an intermediate uptake and Ca and B being the slowest so the nutrients that returns to the reservoir is altered from the inital fed , meaning the fastest nutrients get depleted while the less needed nutrients accumulate in the solution, the process keeps repeating and within a day or days the ratios of nutrients to each other are fucked,thats why I laugh when people say they don't change the solution for weeks. also remember Ec readings do not distinguish between the different types of nutrients ions and that given N.P.K,Mn are taken up at higher rates then other elements N,P,K,Mn may be deficient even though the EC is ideal.
so as far as ppm there are 3 different guidelines for Boron from the US EPA sensitive crops citrus and trees 0.3 to 1.25 semi tolerant crops cereals and grains 0.67 - 2.5 tolerant crops includes most vegetables 1.0 - 4.0 . Me I run around 1ppm to 1.3 through veg. pre-flower I bump it up as the plant is doing a lot of things and building its self, stretching/growing , transitioning to flower etc. and then another bump for the heaviest 2 weeks of flower, I go as high as 4-4.5 and then right back down to 1 ppm. but remember I don't just change the Boron everything in the solution changes weekly to keep all the elements in balance
. I just smoked a big fat one with a buddy that used it on his soil grow and honestly it tasted very nice, burned well, and he said the plants were happy and large, he never ph’d water from a well and had them under two 600’s. Back when I started the larger field grows in the mid 80’s we used pounds of the stuff and never had any nutrient issues. I may try a run next and see what happens in my trays