In soil, as long as your pH is somewhere between 5.5-7.5, odds are you will not have lockout. A lot goes on in soil/humus to keep nutrients available for plants.
Other than that, iron and magnesium 'lock out' at more or less opposite ends of the spectrum: Iron becomes less available at the extremely high pH whereas magnesium would become less available at the lower pH end. Mg also becomes less (but more so) availble at the extreme alkaline end.
You should learn how to interpret plant problems. When we have a problem, like a deficiency, the first thing you want to do is ask yourself what the problem is and where it occurs. Nutrient deficiencies more or less all present with different symptoms in different locations. For example: chlorosis (yellowing), necrosis (dead tissue spots), leaf veins still green (intervenial chlorosis) or yellowing between the margins? If necrosis what does it look like and where do the spots occur?
Location is the other key to this, because plant nutrients are more or less mobile within the plant. Mobile nutrients like N, P, K, Mg will go from the old growth to support the new growth if there is a deficiency, so the symptoms start on/nearer the bottom of the plant and move up. For nutrients that are not so mobile, like Fe, Sulfur and most other trace minerals the symptoms tend to exhibit in the newest growth.
