Hi Diane
@ShakespearesGarden
I wish it was as simple as that but the truth is:
N = everywhere in the plant
P = everywhere in the plant
K = everywhere in the plant
And all the other essential elements are active everywhere in the plant. That's why they are essential - all 20 of them.
Larry
@Seedfork is right - too much of anything in the soil is just that - too much! The growth of the plant will be negatively affected in some way(s). Too little of any essential element - the plant will be underdeveloped in some way (s).
In plant nutrition there is a principle called "the limiting element". Adding additional quantities of an element that is insufficient will increase growth until the point is reached at which the available quantity of some other element is insufficient.
In many soils, the most common limiting element is nitrogen (N). It is held only in the organic matter. OM is often low especially in soils that have been cultivated for a long time without a fallow period (planted to a years-long deep rooted stand of perennial plants). That is why a fertilizer with only nitrogen will improve the growth of most plants in these soils.
Beyond that general fact, it's very hard to make any general statements about how plants will respond to adding any given fertilizer.
Gardeners should always be leery of adding phosphorus (P) without a soil test. It is generally the most likely to be excessive especially if the gardener has added "balanced" fertilizers like 8-8-8, 10-10-10, 12-12-12 and 5-10-5. Contrary to growing roots, excess P will stunt the root growth!
Adding lime without a soil test is also risky. The calcium (Ca) can interact adversely with some other essential elements.
Some fertilizers are less likely to cause long-term problems because they don't linger long in the soil, such as most forms of potassium (K).
There just aren't any generalizations that I would make about how much of any fertilizer to use. Getting a soil test is essential. Adding a tissue analysis for the plant(s) you are most concerned about will give a fuller picture of the situation in your own garden.
Pat
