Sitting silent and immobile, it is sometimes difficult to imagine plants eating, but they do. Although they do not stalk or hunt prey or gather food from their environment, the root systems of plants allow them to uptake nutrients from the soil and the leaves convert sunlight into sugar that they use as energy. Plant foods restore the soil with nutrients plants need to grow, flower and produce fruit.
Although plants produce their energy through photosynthesis, they take up water and essential minerals through their roots. The nutrients found in soil are finite, so plants can deplete them and, eventually, starve. Poor soil nutrition causes yellow leaves, poor growth and dehydration. When added to the soil, plant foods restore the nutrients that the plants have depleted, allowing the plants to continue to thrive.
Nitrogen, phosphorus and potassium--abbreviated NPK and also known as primary macronutrients--are the three most essential ingredients required for plant health. When you buy a bag of fertilizer or a container of plant food, the nutrient analysis usually lists three numbers. This is the NPK ratio, and it explains the proportion of ingredients in the plant food or fertilizer. All fertilizers--even organic substances--have an NPK ratio. For example, fish meal has an NPK ratio of 5-3-3: For every five parts of nitrogen it provides, it also provides three parts phosphorus and three parts potassium. When selecting a plant food, the NPK ratio is often instrumental in making your selection.
The secondary macronutrients include calcium, magnesium and sulfur. As the North Carolina State Department of Agriculture explains, because these nutrients are found naturally in the soil and plants do not use them to the extent that they use NPK, it is rare to have to apply them as part of a fertilization program. However, for soils that lack one or more secondary macronutrients, applying additives such as fish emulsion, gypsum and limestone restores them.
Also called minor elements or trace elements, micronutrients play a variety of roles in plant health, including metabolic regulation, photosynthesis, enzyme production and growth. Plants require seven micronutrients, which--while also naturally available in the soil--can be boosted, if needed, using organic or chemical fertilizers containing them.
Choosing a Plant Food
As the New Mexico State University Extension notes, selecting a fertilizer is not always an easy choice. Smart selection begins with soil testing to determine what your soil needs, as providing too much of certain nutrients can be damaging to both your plants and the local ecosystem. After determining nutritional needs, the extension office recommends considering solubility, effect on soil pH and salt concentration and suggests providing nitrogen through nitrates rather than ammonium, which can become toxic in high quantities. Agricultural extension offices can usually test your soil and make recommendations of the best fertilization program for your soil's needs.