Fertilizers That Make Plants Greener

Nitrogen is a double-edged sword in the garden. It provides the fuel needed to turn leaves green, but too much nitrogen can burn a plant's leaves and even kill it. But you definitely want greener plants, and to get them, judicious use of nitrogen (the "N" in the NPK in all fertilizers) is key.

All commercially sold fertilizers list values of "NPK," which describes the product's percentage of the primary nutrients nitrogen, phosphorus, and potassium, in that order. For example, a ratio of 30-10-10 means that the fertilizer contains 30 percent nitrogen, 10 percent phosphorus, and 10 percent potassium.

Nitrogen is naturally in the soil but can be depleted through leaching or nutrient uptake by plants. Further, plants can't absorb it from the air even though it's abundant in the atmosphere. For this reason, all plants, especially annuals and vegetables, need regular nitrogen feeding throughout the growing season.

Phosphorus is a nutrient required for the development of a healthy reproductive system, flower and fruiting capabilities, and the strength of plant tissues. Because plants' ability to absorb phosphorus is slow (called "low mobility"), this nutrient should be built up in the soil over time. Good sources of phosphorus include bone meal, alfalfa meal, fish meal, and bat or seabird guano. You can also choose a specially formulated high-phosphorus fertilizer if soil tests indicate a deficiency in your yard.

Potassium is important to strengthen a plant's immune system and the development of roots. Usually, potassium is already present in the soil but can be augmented in small amounts by applying a balanced fertilizer. Potassium is found in kelp meal, greensand, guanos, and nitrate, phosphate, chloride, and sulfate potassium.

Nitrogen is the key nutrient in helping a plant to produce chlorophyll, which is necessary for photosynthesis. As every high school biology student knows, chlorophyll is the magic that makes leaves green. Too little nitrogen can stunt a plant, result in yellowed leaves, and reduce flowering and fruiting through a general degradation of plant health.

To know the right amount of nitrogen to apply, know your plants. Some plants are widely known as "heavy feeders," including many vegetables and most annuals. At planting time, incorporate a high-nitrogen fertilizer into the soil; then supplement with a water-soluble product throughout the growing season, depending on the plant.

Most landscape shrubs and trees don't need regular fertilizer but can sometimes benefit from an annual application of a balanced mixture. Some experts recommend applying small amounts of fertilizer, including nitrogen, throughout the season when the leafy canopy is actively growing.

Using the old adage that "if some is good, more is not better," too much nitrogen can burn the leaves so that they appear charred and shriveled (starting at the tips). It can also encourage so much leafy growth that the plant can't get around to creating flowers or fruit. In addition, nitrogen can leach into groundwater and eventually into the ocean, causing environmental damage such as algae blooms.

If a tree or plant has been fed an overabundance of nitrogen, it will develop a lot of leafy growth at the detriment of flowers and fruit. While this may be attractive on shrubs if you don't care about flowers or fruit, it's not healthy for the plant because it impedes the penetration of sunlight and creates a more humid environment within the shrub that invites fungal diseases.