Plants require 17 elements for normal growth. Carbon, hydrogen, and oxygen are found in air and water. Nitrogen, potassium, magnesium, calcium, phosphorous, and sulfur are absorbed from the soil. The latter six elements are used in relatively large amounts by the plant and are called macronutrients. There are eight other elements that are used in much smaller amounts; these are called micronutrients or trace elements. The micronutrients include iron, zinc, molybdenum, manganese, boron, copper, cobalt, and chlorine. The nutrients that are most likely to limit plant growth are nitrogen, phosphorus, and potassium.
Nitrogen is a primary component of proteins and is a part of every living cell. This nutrient is usually more responsible for increasing plant growth than any other nutrient. Shortages can cause slow growth, reduced leaf size, yellowing, short branches, premature fall color and leaf drop, and increases the likelihood of some diseases. An over abundance can cause excessive shoot and foliage growth, reduced root growth, low plant food reserves, and increased susceptibility to environmental stresses and some plant diseases.
Nitrogen is a mobile nutrient it is in constant motion. Nitrogen applied to the soil can be used by plants, washed off the soil surface, lost to the air as a gas, or leached through the soil). Nitrogen from granular fertilizer can enter streams from surface runoff. Nitrogen loss is higher when a heavy rain immediately follows a surface application of fertilizer, especially on sloped areas. Incorporating fertilizer into the soil or lightly watering (1/4 to 1/2 inch) after making a surface application will reduce the amount of nitrogen loss.
Phosphorus (P) plays a role in photosynthesis, respiration, energy storage and transfer, cell division, and cell enlargement. It promotes early root formation and growth, and the production of flowers, fruits, and seeds. Many of our urban soils are low in phosphorus. Cultivated farm land often has a high phosphorus level from years of fertilization. In these cases, the addition of more phosphorus is not going to increase yields and can potentially harm the environment.
When applied as fertilizer, phosphorus is quickly bound by soil particles. Phosphorus is extremely immobile in soils (except sand); it moves about 1 inch from its original placement. Unless phosphorus is incorporated into the soil, watered in, or applied as a band, plants may not be able to use it.
Potassium in involved in many plant growth processes; it is vital to photosynthesis and helps regulate water in plants. Potassium fertilization helps plants overcome drought stress, increases disease resistance, and improves winter hardiness. Potassium can be leached through the soil by water, but not as quickly as nitrogen.
Nutrient uptake by plants
Nutrients in the soil can be in a solid form (granular fertilizer, organic matter), attached to the soil particles or dissolved in soil water. For an element to be absorbed by plants it must be in a form the plant can use, and present (dissolved) in soil water. Water and oxygen are required for nutrient movement into plant roots. Without adequate oxygen in the soil, there is limited nutrient absorption.
Anything that lowers or prevents the production of sugars in the leaves can lower nutrient absorption. If the plant is under stress due to low light or extremes in temperature, nutrient deficiency problems may develop even though adequate nutrients are available in the soil solution. Diseased or damaged roots, improper soil pH, water logged sites, and plantings that are too deep can result in inefficient nutrient absorption. Adding fertilizer under these conditions will not enhance plant growth, and may actually cause plant damage.
The stage of growth or how actively the plant is growing may also affect the amount of nutrients absorbed. Many plants go into a rest period, or dormancy, during part of the year. During this dormancy few nutrients are absorbed.