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There are several nutrients needed for proper plant growth. The first three are non-mineral nutrients: hydrogen, oxygen and carbon. Water, atmospheric oxygen and carbon dioxide supply these requirements for plant growth.
During photosynthesis, light energy is collected by chlorophyll in leaves, and part is used to split water molecules into free oxygen gas and hydrogen. The hydrogen is bonded with carbon dioxide to form sugars the plant can then use to grow. The water is often added by itself or as part of a nutrient solution, and carbon dioxide is either naturally present in fresh air or is added to the garden as a gas.
In a natural setting, plants use nutrients to grow to maturity, then when they die they fall to the forest floor and return their nutrients to the soil and new growth uses the nutrients to grow to maturity. Compost is useful in the garden because it is made from the decomposed building blocks of other plants (manure composts are plants processed through an animal first), and as such tends to have at least a little of all the required nutrients. When an animal dies in the forest, the scavengers eat the fats and meats, and the plants eat the remaining blood and bones—blood tends to be high in nitrogen, bone high in phosphorus.
In a garden, the previous year’s plants have often been cleared away and are not returning their nutrients to the soil. Even if they were, the nutrients removed along with the harvested portion of the plant would eventually mean a loss of nutrients in the system. In container gardens, the growing medium may be new and sterile, without any nutrients at all. The missing nutrients are added to the system in the form of fertilizers.
Fertilizers provide the nutrients needed for plant growth. The first three are known as primary nutrients and are so important that they are listed on the front of nutrient packaging:
Nitrogen (N) is needed to make plant cells and the chlorophyll (the green in leaves) required for photosynthesis. Nitrogen compounds make up between 40 and 50% of the dry matter of plant cells. It promotes large, healthy foliage, absorption by roots and proper plant development. Nitrogen deficiency is the most common nutrient problem. Growth nutrients commonly contain elevated levels of nitrogen
Organic nitrogen breaks down over time to become a form available to plants. Synthetic nitrogen forms can become available to the plant quickly and are often made with an easily dissolved salt. Nitrogen-deficient leaves will contain relatively little chlorophyll and tend to be pale green to yellow in color. Nitrogen is mobile in plants, and this enables it to be moved from older growth to young growing tips when supplies are short. This mobility of nitrogen explains why deficiency symptoms appear first in the older lower portions of the plants, working their way up to the growing tips.
Phosphorus (P) is required for photosynthesis and root development and assists in blooming. It is also used to form nucleic acid, which is an essential part of living cells. Phosphorus compounds are used in respiration and the efficient use of nitrogen. It is important throughout the life cycle of the plant, but use is elevated during the flowering stage. Bloom and flowering nutrients often contain elevated levels of phosphorus.
Phosphorus deficiencies usually manifest as a generalized under-performance of the plant—leaf development is stunted and bud size is reduced. Leaves may develop a bluish tint. Phosphorus assists in nitrogen uptake, so symptoms of phosphorus deficiency are often similar to those of a nitrogen deficiency.
Potassium deficiency often shows up as a yellowing or browning of the leaf edges and curled-over leaves, followed by yellowing spots in the interior of the leaf face. Discolored spots may appear on the undersides of leaves. Potassium is mobile, so deficiency symptoms show first on lower leaves as flecking or mottling on the leaf margins. Prolonged deficiency results in cell death along the leaf margins and the plants can show signs of wilt. These symptoms first display in older leaves and continue to work up through to the newer leaves if not corrected. Growth, root development, disease resistance and bud size are reduced.
The next three nutrients after the primary nutrients are called secondary nutrients: calcium (Ca), magnesium (Mg) and sulfur (S). Calcium-magnesium supplements can be used if needed, but sulfur deficiencies are rare since sulfur appears frequently in both synthetic and organic nutrients. There is some debate over whether or not silica (Si) should be considered a nutrient, but since it is helpful to plant structure, it can be treated as such under most circumstances. Silica supplements are available to boost silica levels.
The final group of nutrients are known as micronutrients. They are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo), nickel (Ni) and zinc (Zn). Micronutrients are only needed in small quantities when compared to the other nutrients. They may be added as a separate additives, or included as part of a nutrient line.
One benefit of using a particular nutrient line is that by following the manufacturer’s schedule, the plant should receive enough nutrition to grow. If you’re designing your own nutrient regimen, you should ensure there are sources for each of these nutrients. Regardless of the exact sources of a garden’s nutrients, they can make the difference between an average garden and an impressive one.
Source: Maximum Yield "Growing 101: Basic Plant Nutrition" November 26, 2018 https://www.maximumyield.com/growing-101-basic-plant-nutrition/2/1260