"Obesity" - Lol - good one. ;o)
Is there a visual feedback that I can use to tell whether I have fertilized too much or too little?
Your question needs to be answered in two parts. First, there will be a very visual attention getter if you fertilize so that the level of TDS (total dissolved solids) is so high that it prevents water uptake. This is called plasmolysis, and we commonly refer to it as fertilizer burn. Necrotic leaf tips/margins or dry, dead spots in the leaves are the first signs, with total plant collapse occurring in more extreme cases. Think of fertilizer like salt, and if you add too much salt, instead of plant cells pulling nutrient-laden water into the cell, the highly ionized water that occurs in soils after heavy fertilizer can actually reverse the process and PULL water OUT of cells, and we know that's not good. (Think of how salt pulls water from ham or bacon - same thing.)
The reason I said it needs to be answered in two parts is because we can still over-fertilize while still maintaining the level of TDS within bounds. Lets use the example of the high P formulations known as 'bloom boosters' or some similar name. If you use the popular 10-52-10 fertilizer that is touted as a bloom booster, and you are providing N at the levels the plant needs to grow normally, you're adding about 32 times the amount of P the plant could EVER hope to use.
These high levels of P do two things. They raise the level of TDS unnecessarily, because the plant will never use all the P, and the ultra high levels of P inhibits iron and manganese uptake by roots, and deficiencies of these two metal micronutrients causes interveinal yellowing. You see the interveinal yellowing and say, "Wow! My plants must need more fertilizer." So you sock them with more bloom booster, which of course, exacerbates the problem.
Two other common 'over-fertilizing' problems are adding iron supplements that don't have Mn included, and using Epsom salts when there is a low soil Ca content. Iron and Mn have an antagonistic relationship & should be present in soils at approx a 2:1 Fe:Mn ratio. Ca and Mg also share an antagonistic relationship, and should be in soils at approximately 3-4:1 Ca:Mg. Simply adding Mg via Epsom salts can cause an antagonistic Ca deficiency and cause weak, distorted growth, even if there is ample amounts of Ca in the soil.
. . . fertilized too little?
You'll need to look for deficiency symptoms. The list is too long for me to go into, but you can become a very good detective if you know your business. ;o) I should also tell you that you can fertilize at exactly the correct rate & ratio of nutrients, and STILL have culturally induced deficiencies. Examples besides the antagonisms I already mentioned are things like a chill induced P deficiency. Plants have difficulty taking up P when soils are cold. Another very common cultural problem is caused by saturated soils that drain poorly, low oxygen levels cause Fe and Mn to become electron acceptors. This leads to their precipitation (iron and manganese) and, even though these elements might be adequately present in soils, the plants inability to assimilate them can become obvious in deficiency symptoms. Soggy soils also make Ca difficult or impossible for uptake, as does very humid conditions or prolonged periods of cloudy weather.
Finally, fertilizing at extremely high levels of any one element, even if the o/a level of fertilizer (I'm actually talking about the level of TDS) in the soil is favorable, you can expect deficiency symptoms of the element that was supplied at above luxury levels when the level of the element drops to normal. Example: Over-fertilize with N, and when the N levels return to a more favorable level, the plant can exhibit N deficiency symptoms, even though the N level in soils SHOULD be perfectly acceptable.
Or alternately, can I determine the maximum amount that will NOT cause root burn and just apply the maximum amount?
Yes and no. If you are growing in a fast container soil that drains freely, and you are watering properly, it will be very difficult to over-fertilize, unless you exceed the recommended solution strength. With these fast soils and proper watering, the soil is flushed every time you water, so salts do not appreciably accumulate. With a slow container soil, you need to be very careful. If you are watering in sips, virtually ALL the unused salts from fertilizer and those that naturally occur in irrigation water will quickly accumulate in soils. When someone complains of wilted plants while the soil is damp, look first to a salt accumulation in soils (possibly a temperature issue, too).
The maximum you can apply will always depend on the TDS in the soil before you fertilize + what you're adding, so you can actually cause severe fertilizer burn with a 1/4-1/2 strength solution if the soil is already high in soluble salts.
I assumed you were asking this last question about containers, because of the extremely high # of variables that would affect the answer if you were asking the question about in situ soils.
That cover it?
Al
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Belinda’s Dream
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