Although the major nutrients are needed in the greates quantities, a deficiency of any one nutrient can prevent growth or make plants more susceptible towards disease.
You may have noticed, when looking at fertilizer products recently, that Phosphorus is slowing disappearing from the store shelves. Why is this? The uptake of phosphorus by plants is highly dependent upon the pH of the soil. Soil pH should be neutral for phosphorus to be taken up efficiently by plant roots. Over the years fertilizer products have included varying amounts of phosphorus (i.e., "Bloom Booster" fertilizers with very high percentages of phosphorus). It has been determined that we have been adding too much phosphorus to our soils. What we should have been doing is adjusting our soil pH so that the plants can take up the phosphorus which occurs naturally. Phosphorus is expensive to produce, and in excess, is harmful to our environment (Canada has already banned it, the U.S. may soon follow).