Fertilizer affects plant growth by supplementing plant nutrients, which allows plants to grow faster and thicker. However, this applies only if the soil is deficient in nutrients. Fertilizer does not compensate for other growth-inhibiting factors such as lack of water, poor soil preparation and weeds.
Fertilizer comes in two categories: manufactured and natural. Manufactured fertilizer releases its nutrients over a few days to a few weeks, with some types specially formulated to release more slowly over months. Natural fertilizer can take several months or years to release its nutrients. However, both types can contain high amounts of salt that prevent plants from growing.
Plant nutrients are divided into primary and secondary nutrients. Primary nutrients consist of nitrogen, phosphorous and potassium, which often appear in fertilizers. Plants use primary nutrients in large quantities as they grow. Secondary nutrients such as calcium, magnesium and sulfur are also used heavily by plants, but they are usually already present in the soil.
The most common cause of stunted plant growth is a lack of sufficient nitrogen in the soil. Different plants require different amounts, but an excess amount of nitrogen can also produce undesirable results. For example, too much nitrogen can cause tomatoes to produce vines without bearing much fruit.