Plants make their own sugars from air and water, using the energy of sunlight. Plant “food” is sunlight. The fertilizers we apply to the soil are minerals that form some of the necessary raw materials for plant functions. Since the soil is composed of minerals, we are actually supplementing what is already there. Except in hydroponic gardening and, to some extent, container gardening, plants can grow and survive without added fertilizers. After all, wild plants have been doing it for eons.
But if we want our plants to look their best and reach their full potential of yield and growth, we must add fertilizer. We can do this in a multitude of ways. Fertilizers come as natural organic products, as soluble salts, as liquids, as pellets, spikes, and briquettes. To know which fertilizers to choose, we must understand our plants, our soil, and our own preferred way of working.
Nutrients in the Soil
Of the more than 100 chemical elements that have been isolated, 16 are known to be essential to plant growth. All 16 are essential, but they are needed in varying amounts. Three elements come to the plant from air or water (carbon, hydrogen, and oxygen). The rest—the mineral nutrients—are absorbed from soil by plant roots. They are divided into three groups:
- Three primary nutrients (nitrogen, phosphorus, and potassium)
- Three secondary nutrients (calcium, magnesium, and sulfur)
- Seven micronutrients or trace elements (zinc, iron, manganese, copper, boron, molybdenum, and chlorine)
Soil chemistry is complex and dynamic. Dozens of elements are present in soil in thousands of combinations. As the soil changes temperature, as water moves through it, as plants and animals live in it, these elements recombine endlessly, sometimes into forms that can be absorbed by plants, and sometimes into unavailable forms.
With only a few exceptions, mineral nutrients are absorbed by plants as dissolved salts. If an element is in a form that is insoluble, it is not available to the plants.