|MadSci Network: Botany|
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
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