Plants use light and water to make their own food, but they also need the basic building blocks of plant nutrition. These major or macronutrients, supplemented by minerals and trace elements found in most soil, control the way a plant grows. Plant foods meet the nutrient needs of plants that require more than a soil has to offer.
Soil is dissolved rock mixed with organic matter. Its character depends on minerals present in local rocks and the plants and animals that live, die and decay in the soil. Flower gardens, lawns and trees disrupt the natural evolution of the soil; nutrients needed to sustain soil fertility disappear when plant matter is harvested or mowed. Non-native plants compound the problem because they have different requirements than native, adapted plants, which have adjusted to soil in a certain area. Plant foods restore soil balance and provide nutrients for non-native or densely planted or flowering and fruiting plants.
Growing plants crave nitrogen; when plants begin to lay the groundwork for reproduction, their nutrition needs change. Each stage of a plant's life requires a different balance of macronutrients for the plant to grow and reproduce successfully. A soil test at a local university agricultural extension can answer questions about soil and identify needed macro- and micronutrients.
Plant macronutrients in the soil include nitrogen, phosphorus, potassium, sulfur, calcium and magnesium. The first three must be replenished as they are consumed; they are the building blocks that produce flowers, fruits and seed. Plant foods and fertilizer are described using the percentages of these "consumable" macronutrients present in the mix; their scientific abbreviations are "N,""P" and "K." To find the weight of N, P and K in a bag of fertilizer, multiply the percentage of each by the weight of the bag.
Lawn foods contain more nitrogen because turf must grow continually. Phosphates encourage strong root growth, which is important in spring and fall. Ornamental and vegetable plant foods contain higher percentages of phosphorus and potassium for the needs of flowering plants. Certain groups, like roses, bulbs and vegetables, share macronutrient needs, and plant foods may be labeled for them. Sulfur and aluminum sulfate may be added to foods for acid-loving plants. Specialized lawn foods add minerals or trace elements where the local soil has known deficiencies.
Organic fertilizers use natural substances such as compost, fish and plants, like kelp, to provide nutrients. Most organic plant foods release their nitrogen slowly. Organic foods are often nitrogen-rich and may be heavy in salts; they require knowledgeable use, as does any plant food.
More is not always better with plant foods. Heavy fertilization creates phosphorus runoff and nitrogen overdose, also known as fertilizer burn. Frequent feedings with dilute solutions rather than monthly full-strength feedings make healthier houseplants, and slow-release nitrogen fertilizers make more vigorous lawns. The best way to choose plant food or fertilizer is to read the label carefully to find plants and application methods for which it is manufactured.