The Ph(acidity or alkalinity) of your nutrient solution determines how the plant absorbs it.

Look at the chart. At about 6 all of the major nutrients are being absorbed in decent amounts, but at a ph of 4 many of the nutrients cant be absorbed.

You can fertalize the crap out of a plant and it wont do any good unless the PH is dialed in. The nutrients are still present, just not available by the plant because the PH is off.