Most of the nutrients needed for the growth and development of plants are absorbed from the soil by the roots. Over the seasons, these soil nutrients become depleted and must be replenished or our garden plant health will decline.
There are 17 chemical elements known to be essential for plant growth, flowering, and fruiting. The primary macronutrients, nitrogen (N), phosphorus (P), and potassium (K), are used in the largest amounts by plants and are thus prone to deficiency in soils. These nutrients are the primary ingredients in most garden fertilizers and the percentages of each are prominently displayed on the bag as the N-P-K numbers (always presented in that order).
Nitrogen is required for healthy vegetative growth (leaves and stems) and is especially important in young plants. High levels promote dark green leafy growth but not fruits and flowers. Thus a fertilizer higher in nitrogen is great for lawns and leafy vegetables but disastrous when you are trying to grow tomatoes!
Phosphorus is important in all functions of plant growth but especially for root development and growth, and in the production of flowers, fruits, and seeds. Starter fertilizers, which can be used when transplanting trees, shrubs, and perennials, are much higher in phosphorus than nitrogen and potassium. They stimulate root growth and help avoid transplant shock. “Bloom booster” fertilizers with 50%-60% phosphorus help promote flower bud formation.
Potassium is important for the overall vigor of plants. It promotes disease resistance, root formation, and cold hardiness. Plants deficient in potassium will have weak roots and stems.
The secondary macronutrients are calcium (Ca), magnesium (Mg), and sulfur (S). These nutrients are very important to plants but are used in smaller amounts than the three primary macronutrients.
Micronutrients, also known as trace elements, are not nutrients of lesser importance to plant health but those that are required in minute quantities. With the exception of iron and manganese, micronutrients are seldom deficient in our garden soil, however, some can become unavailable to plants when the soil pH is either too high (alkaline) or too low (acidic). Maintaining your soil pH between 6.0-6.5 will keep these nutrients available to the plants. Some fertilizers are fortified with micronutrients.
There are several ways to improve the fertility of your soil but the most common way is to apply fertilizers, either organic or synthetic (chemical fertilizer). There are advantages and disadvantages to using either form, however, in André’s opinion, organics are by far the way to go.
Organic fertilizers such as the Espoma “Tones” (check out Espoma on the web) release nutrients slowly over time and thus need to be applied less often than chemical fertilizers (usually once in the spring and a light application in the fall). Organic fertilizers will rarely burn the roots or foliage of your plants.
Synthetic fertilizers are more concentrated and must be applied carefully to avoid damage to growing plants. They also could cause water pollution because they quickly release nutrients which can then leach into the groundwater in a short time. In addition, because they release nutrients quickly, they must be reapplied more frequently than organic fertilizers.
More significantly, organic fertilizers add all important organic matter to the soil, thus improving soil structure. Remember, good soil structure improves drainage in clay soils and increases the water-holding capacity of sandy soils. Some organic fertilizers add mycorrhizal fungi as well as nutrients to the soil. The benefits of mycorrhizal fungi in garden soil are becoming increasingly recognized worldwide.
Mycorrhizal fungi form a symbiotic relationship with the plant roots to improve the uptake of water and nutrients. Tremendous numbers of mycorrhizal threads attach to the roots of plants and then fan out into the surrounding soil. These "extensions" of the root systems exponentially increase the area that the plant can exploit for the raw materials needed for growth. The mycorrhizal filaments capture and assimilate nutrients, minerals, and water which can then be transferred to the plant. This relationship has been shown to greatly increase plant tolerance to drought stress.
Mycorrhizals are known to secrete chemicals which dissolve hard to capture nutrients like phosphorus and iron, making them more available to plants. They even produce antibiotics which attack and kill many disease organisms that enter the "root" zone! The benefits of inoculating your garden soil with mycorrhizal fungi are astounding!