Hey Cody

.. Magnesium is in fact highly mobile (low retention) in colloidal systems including soils - displacements occur most frequently in strongly acidic, or sandy media, where the magnesium can be readily leached into various mineral 'sinks' (sites of accumulation) by cation-exchange. When an Mg deficiency is accompanied with excess potassium availability, the situation is magnified. Despite it's profoundly important role in enzyme synthesis and related processes, magnesium forms the central cation in the chlorophyll molecule and plays an important role in fluorescence - the driving force behind the photosynthetic process.
Therefore, without sufficient amounts of magnesium - especially when the plant is cultivated under optimized photosynthetic conditions such as those provided by LEDs, the plants will begin to metabolize the chlorophyll in their existing leaf mass leading to senescence and abscission. This is often (but not always) indicated by the common, observable symptoms of magnesium deficiency, such as chlorosis, or yellowing of tissues connected to vascular structures which remain green in appearance - this tends to give the leaf a marbled appearance.
Due to the highly mobile nature of this element, the plant will adapt to a magnesium deficit by turning to it's internal source (or 'sink') in older leaves and translocate it to younger tissues - distributing the resource to meet it's nutritional demands. Therefore, the first sign of magnesium deficiency is usually the chlorosis of old leaves which tends to continue as the situation worsens.