Although your tree does not look fabulous, it also does not look all that bad either. It is definitely showing a fertilizer deficiency, especially nitrogen. When a tree it greatly deficient in nitrogen the leaves will be totally yellow with no variation of color, or yellow/orange veins with some green out on the far sides. When the deficiency is less severe, as what your tree is showing, Less severe shows up on older leaves with newer leaves still retaining some green. Your tree is currently showing a medium nitrogen severity. At temperatures above 95-F growth stops in citrus trees. This is the time of year nearing the time to stop fertilizing. I would suggest you first foliar spray the foliage with a nitrogen solution such as low biuret urea, or a fertilizer with a higher nitrogen content. You might also apply a light fertilizer to the roots. Do what you want, but for this tree I would not apply a organic fertilizer due to the trees immediate needs.