Even experts often have considerable difficulty identifying individual nutritional deficiencies, this with plant in hand; so identifying one from a net picture would be even more difficult. A large part of the reason is because, from plant to plant, symptoms vary and overlap. You should be thinking in terms of an o/a nutritional supplementation regimen, rather than trying to identify and correct any singular elemental deficiency. Additionally, plants use about 6X more N than P, and 1.5X more N than K; so since you're providing NPK in a 1:1:1 ratio, you should expect a deficiency of N to be a limiting factor before either P or K. Further, even if it WAS a P or K deficiency, adding elements of compounds targeted at providing only 1 element usually cause more harm than good. I can go into more detail on that topic if you wish, but we'll see what you think after I finish.
As a bonsai practitioner with more than 20 years of experience at keeping trees covering dozens of genera and hundreds of species in health excellent enough that the trees are able to withstand the added stresses of bonsai culture, I can say your tree doesn't have the look of a tree that's going to be brought back from the brink by correcting a single elemental deficiency. Your 10-10-10 supplies much more P than your plant can use in relation to N, which is the standard that professionals use to judge the needs of other nutrients, simply because plants use more N than any other nutrient. Your tree has the look of one suffering from a drought response. Very commonly, drought responses are brought on by a high level of soluble salts in the soil, under-watering, or over-watering. The easiest way to get to the root of your plant issues is to eliminate those things it can't be, than concentrate on the possibilities. W/o asking more questions, my guess is your plant is suffering from a high level of soluble salts in the soil and may be suffering from over-watering. Though I can't choose one over the other by looking, I can say these two areas are favored by odds. Can you eliminate any of these three possibilities?
Under-watering is the easiest. It can be eliminated if you water faithfully within a short time (a day or two) after your finger, stuck deep in the soil, reveals the soil dry to the touch. A sharpened dowel stuck deep into the soil is a good gauge. If it comes out cool or colored from the wet/damp soil, wait to water.
Over-watering obviously occurs when you water too frequently, but over-watering can be a condition that almost can't be helped. If you water properly, so the entire soil mass is saturated and at least 10-15% of the total volume of water applied exits the drain hole, most bagged or commercially prepared soils are left with a soggy layer of soil at the bottom of the container. This soggy soil kills roots and impedes a plant's root function by making it difficult for the plant to efficiently move water to the canopy. The accelerated transpiration in low humidity conditions (indoor/winter) exacerbates the problem. If you suspect this, let me know and I'll help you deal with the heavy soil.
Last, and what I most suspect, is an accumulation of soluble salts from fertilizers and tap water in the soil. High levels of ANYTHING soluble in the soil makes it difficult, or impossible in some cases, for the plant to pass water through cell membranes into cells. The result is a condition botanically known as plasmolysis (because cytoplasm is torn from cell walls as the cells dehydrate, resulting in death of cells and the tissues they comprise) and commonly known as fertilizer burn. Fertilizer burn can occur even w/o the addition of fertilizer after a period of improper watering, so it really isn't exclusively dependent on how much fertilizer is applied - only on the level of solubles in the soil (TDS/EC - total dissolved solids/electrical conductivity).
I wouldn't be so quick to jump to pot up, and you certainly shouldn't repot at this time (Let me know if you don't know the difference). If you are having an over-watering issue, potting up will make things worse. It will also make it more difficult to water properly if a high salt level is at the heart of the issue. Being rootbound doesn't 'suddenly' become a critical issue, so I think it would be wise to isolate the probable causes before you take off in a direction that can complicate the issues. That whole 'look before you leap' thing. ;o)
First, examine very carefully for insect infestations like scale & mites. Then, consider the 3 causes I mention that are favored by odds. Then let's talk more.
There are two ways to approach keeping a plant in good health. One is to learn to recognize problems and treat/correct as they occur, which is what we are doing now. The other, and the one that makes far more sense, is to adopt a way of dealing with your plants that helps to ensure that they remain healthy enough so they can do the fighting (fighting off insects and diseases because their vitality/energy levels/metabolic rate are much closer to the potential Mother Nature endowed them with.
I can help you with 3 of the 4 most important things, which are the soil, your nutritional supplementation program, and proper watering habits. The 4th is light, and that will be up to you. Let me know if you're interested and willing to put forth a little effort. I'm not just blowing smoke - I've helped literally thousands of people reduce or nearly eliminate their container gardening frustration and increase the satisfaction.
Al
