All plants need nutrients to grow and, given good soil, sunlight and water, most are able to make their own food through photosynthesis. Fertilizers provide extra helpings of substances that encourage plants to excel in qualities gardeners desire.
Ancient humans used decomposing matter to encourage their crops, but the first manufactured fertilizer was an early 19th century "super-phosphate," made of bones and sulfuric acid. Chemical fertilizers developed by the Tennessee Valley Authority revolutionized fertilizer production.
Modern fertilizer is manufactured or processed in liquid or dry forms and applied with water or broadcast from mechanical "spreaders." Slow-release forms minimize fertilizer "burn," or overfeeding.
Fertilizers may be chemical, using ammonia nitrates and phosphates, granular urea or other chemicals and minerals. Organic fertilizers use decaying plants, fish, bone or other animal waste.
Commercial and homemade fertilizers contain nitrogen, phosphorus and potassium (N-P-K) mixed in proportion to function. Limestone and nitrates are often added as soil conditioners.
Nutrients benefit different aspects of growth. Nitrogen and potassium encourage basic plant growth. Potassium promotes root growth, and phosphorous aids growth and fruiting (blooms). N-P-K numbers on packaging guide gardeners to the proper use of different blends.
Time fertilizers right to avoid overfeeding or "burning" plants. Overfeeding can contaminate groundwater and cause harmful plant growth in waterways.