Plants need nutrients, like every other living thing. Plants absorb nutrients through their root systems and through their foliage. That's why it is important to determine that your plants have soil that is well-balanced in nutrients to promote strong, healthy plants. Soil amendments and additives and regular applications of fertilizer can provide the necessary nutrients. Plants that need an extra boost may benefit from liquid fertilizers, applied as foliar sprays, as well.
Nitrogen, phosphorus and potassium are the three main nutrients plants need to grow. Nitrogen promotes growth of foliage, and increases productivity of both seeds and fruit. Phosphorus accelerates growth, producing abundant blooms and sturdy root systems. Potassium helps in building protein, plays a part in the photosynthesis process and helps fight disease, producing strong, healthy plants. In addition, there are secondary nutrients: iron; zinc; calcium; copper; manganese; sulfur; boron; and molybdenum, and micro-nutrients obtained from the soil.
Nutrients from Soil
Plants derive the three main nutrients that they need from soil, as well as secondary and micro-nutrients. Plants get the three main nutrients from decaying plant matter. Turning equal amounts of compost and peat moss assures that there are plenty of nutrients in the soil and that they will be available for plant use. Compost provides nutrient-rich humus that acts as a natural soil enhancer, loosening clay soils so that plant roots can grow freely and provides nutrients that plants need. Peat moss stores soil nutrients and releases them slowly, as plants need them.
As plants use the nutrients, the soil must be replenished, or eventually, there will be no nutrients available and plants will not thrive. Soil should also be amended to assure availability of nutrients to plants. Top dressings of compost serve as a general fertilizer application, or if soil is found to be deficient in individual nutrients, they can be replenished with top dressings of materials containing those nutrients. Bat guano, blood meal, fish meal and soybean meal are all high in nitrogen. Bone meal, colloidal phosphorus and rock phosphate are all good sources of phosphorus. Potassium is found in ground kelp, granite dust, and greensand. Regular applications of liquid fertilizers, such as compost tea, fish emulsion, kelp, or worm compost tea throughout the season, are another good way to replenish nutrients in the soil.
The amount of each nutrient required varies with different types of plants, and deficiencies in certain nutrients can stunt growth, produce weak plants or even cause them to die. Lack of nitrogen is indicated by general plant weakness and mature leaves that turn yellow, purple or red. Lack of phosphorus is seen in purplish and bronze leaves. Potassium deficiency shows in yellow leaves with translucent spots and browning edges. There are visible clues to deficiencies of secondary nutrients, as well.
Nutrients Through Foliage
Plants are also able to absorb nutrients directly through their foliage. Organic foliar plant foods are absorbed through the plant foliage and are then immediately available for use. Compost tea, worm compost tea or fish emulsion are all high in nitrogen and other nutrients that plants need. Kelp (seaweed) contain macro- and micro-organisms and trace elements that help plants grow.