By Joan Jubela
The shelves of garden stores are well stocked with bags and bottles of fertilizers designed to improve the quality of plants and increase the quantity of fruit and flowers. Soil tests, gardening articles, nursery tags, and other sources frequently make recommendations for fertilizer use.
A string of three numbers is included in these recommendations. An example is 5-10-10, a common formula recommended by Penn State Extension for vegetable gardens. Combinations abound: 5-1-1, or 30-3-12, maybe 12-0-0, and so on. What do these numeric formulas mean? Whether cultivated in a garden or growing in a ditch, along with sunshine, air, and water, plants take-up nutrients present in the soil. Seventeen known nutrients are deemed important for normal plant growth, but there are three that are essential to plant life: nitrogen, phosphorous, and potassium.
The role each plays, both alone and in conjunction with one another, is complex and beyond the scope of this article, but in very simplistic terms nitrogen is known as integral to vegetative growth, potassium is necessary for the development of young plants and seedlings, and potassium promotes reproductive growth, as with flowers and fruit. This threesome is referred to as NPK, the symbols for those elements in the periodic table. NPK is key to understanding fertilizers too.
In physical bags of fertilizer, however, these elements exist in chemical form. Nitrogen ingredients are listed as (N), phosphate (P2O5), and potash (K2O). Fertilizers, both solid and liquid, display their numeric formula as a grade based on the percentage of the weight of each nutrient relative to the weight of the overall container. The formula represents a guarantee from the manufacturer as required by law. A 50-pound bag of fertilizer with an N-P-K of 5-10-10 means that there is 2.5 lbs. of a nitrogen compound, or 5 percent of 50 pounds; 5 lbs. of a phosphate compound, or 10 percent; and 5 lbs. of some form of potash, another 10 percent. The total is 12.5 pounds of NPK. The other 37.5 pounds, or 75 percent, are other compounds; material intended to improve soil health or quality in some way, or filler.
Follow instructions carefully. Overuse of fertilizers can have negative consequences. Solubility is an advantage that fertilizers offer over soil amendments, like manure. Fertilizers become available as soon as they dissolve in water whereas nutrients in manure breakdown slowly over time. Highly soluble in water, chemical forms of nitrogen, like nitrates, can easily leach out of soil into waterways, resulting in algae blooms and pollution. It is important to understand that fertilizers are salts and repeated applications can result in a build-up of these salts in the soil, drawing water away from roots, robbing plants of hydration and resulting in burn.
In addition, excessive uptake of nitrogen causes lush green growth that weakens plants making them not only droopy or spindly, but also susceptible to pests and disease. Too much fertilizer can alter even the flavor of vegetables. While fertilizers can be useful, there are many reasons to exercise caution.
Source : psu.edu