Signs of excessive fertilizer use are all around us -- nitrogen and phosphorus from overfertilized agricultural fields run off into lakes and streams resulting in environmental problems: overgrowth of algae, decreased oxygen levels in water, injury and death to fish and other wildlife, atmospheric warming, dead zones in our oceans and contaminated drinking supplies. Because fertilizer is in essence salt, excessive use, even on a small scale in our home gardens, has serious negative effects.
One of the first signs of excessive fertilizer use in the home garden is a white or brownish crust of fertilizer that forms on the soil surface. For container plants, this evidence may also be seen as a ring of salt deposit around a drain hole or as a buildup on the exterior of terracotta or clay pots. If more fertilizer is applied than the plants can use, the unused minerals, dissolved in water, leave this crust behind as water evaporates from the soil.
If fertilizer salts are allowed to become more concentrated in the soil, the next sign of excessive fertilizer use is injury to the plant's roots. Evidence of root injury includes direct damage to the roots themselves, which is also called root burn. Plant roots may be browned or blackened, limp with dead root tips. In this weakened state, plants become susceptible to disease such as root rot. In addition, the roots will struggle to take up water and nutrients required for healthy plant growth and development.
With time, evidence of excessive fertilizer use is seen above ground in the form of foliage problems. As a consequence of a damaged root system, water and plant nutrients cannot be delivered and distributed throughout the plant properly. Leaves will turn yellow and the bottom leaves of a plant will wilt. In addition, leaves with browned tips and margins indicate root burn caused by overfertilization. Defoliation, or leaf drop, can also occur.
Like humans who suffer from stunted growth as a result of an inadequate diet and supply of nutrients, plants will be begin to show slow or no growth as a result of overfertilization. The excessive use of fertilizer damages their vascular system, quashing the effective relay of nutrients and water. At first, plants appear dehydrated. With time, young shoots on vegetable plants like squash turn yellow with scorched tips and are poorly developed.
Overapplication of fertilizer is also evidenced by reduced yields in vegetable production. Cucurbits like the aforementioned squash, as well as cucumbers, pumpkins and watermelon will produce fewer fruit when excessive fertilizer is used. Other vegetables that suffer reduced yields from overfertilization include beans, corn, peppers and tomatoes.