Re: How do the amounts of fertilizer affect plant growth?

Insufficient fertilizer can slow plant growth or cause deficiency symptoms such 
as leaf chlorosis (yellowing), leaf death and stunting. Lack of fertilizer 
means the plant has inadequate levels of mineral nutrients it needs to make 
essential compounds or for general metabolism, such as nitrogen for proteins 
and chlorophyll, magnesium for chlorophyll, and several micronutrients as 
enzyme activators. A severely nitrogen deficient plant loses its normal green 
color because it cannot make enough green chlorophyll. However, most plants in 
nature do not look chlorotic but would respond with more rapid growth and get 
greener if given nitrogen fertilizer. Thus, in nature, nitrogen typically 
limits plant growth; plants merely slow their growth to fit the available 
supply of nitrogen.

The mobility of a mineral nutrient in the plant determines where the deficiency 
symptoms occur. Mobile mineral nutrients such as nitrogen, potassium and 
phosphorus, have deficiency symptoms first on old leaves because existing 
mineral nitrients are transported to growing points when a deficiency occurs. 
Root and shoot tips of calcium deficient plants die because calcium is needed 
for cell walls, and calcium is not mobile in the plant. There may be excess 
calcium in old leaves but the plant cannot transport it to deficient growing 
points, it must get calcium from the soil for the growing points.

Having excess fertilizer often causes a salinity problem because the excess 
salts make water less available to the plant due to an osmotic effect. This is 
usually the case for mineral nutrients such as potassium, calcium, magnesium, 
and nitrate-nitrogen, which do not produce a specific toxicity. Excesses of 
these mineral nutrients may induce a deficiency of other mineral nutrients. For 
example, magnesium excess can cause a calcium deficiency.

Excesses of other mineral nutrients, such as boron, manganese, copper, zinc, 
and ammonium-nitrogen, cause specific toxicity symptoms. One of the mineral 
nutrients that has the smallest range between sufficient and toxic is boron. 
Excess boron causes leaf tip death because it is carried in the transpiration 
stream to the leaf edges where it accumulates.

References


What is Plant Nutrition?


Plants and Boron


Color Pictures of Mineral Deficiencies in Potato


Fertilizer Deficiency and Toxicity Symptoms