Plants need nutrients to grow and bloom. Plants that grow in containers have limited amounts of soil and use soil nutrients more quickly than plants that grow in the ground. Most commercial potting media contain enough fertilizer to nourish plants for a short time- less than a month in most cases. For the continued health of the plant, you can replenish depleted nutrients with fertilizers.
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Some fertilizers are formulated for specific purposes, such as lawn fertilizer and tree stakes, but selecting the correct fertilizer for other uses can be overwhelming. Each nutrient has its own role to play within the plant as well as contributing to work together in plant processes.
Symptoms of nutrient deficiencies are often similar and can mimic plant diseases. Soil or plant tissue testing is the best way to determine the reason for plant problems.
A soil test is required to properly determine the nutritional makeup and any deficiencies. The results will indicate nutrients naturally present in your soil as well as providing recommendations on how much of each nutrient to add because too much of a nutrient is as bad for plants as too little.
A soil test also checks the soil pH, which is important for the delivery of nutrients in the soil. Soil composition changes over time therefore testing should be repeated regularly, ideally every second year.
Understanding the Numbers
Most fertilizer formulas are labeled by the percentages of nitrogen (N), phosphorus (P) and potassium (K), present in its formulation and always given in that order across all brands. A 12-8-4 fertilizer formula contains 12 percent nitrogen, 8 percent phosphorus and 4 percent potassium by weight. Other secondary and micronutrients may be present as well.
Nitrogen is an essential element in every plant cell. It is a building block of all amino acids, enzymes, DNA and chlorophyll. Nitrogen stimulates plant growth and reproduction; foliage grows rapidly and deepens in color. Insufficient nitrogen causes stunted growth and yellowing of the foliage.
Nitrogen sources are not created equal. Organic nitrogen sources like composted manure and composted plants release their nitrogen gradually. They are excellent for building soil fertility.
Inorganic fertilizers are chemically formulated to provide nitrogen in the form of ammonium or nitrate that is available to plants immediately. Excess nitrogen washes away and may contaminate groundwater. This is especially a problem in soils that drain quickly.
Phosphorus is the essential nutrient for plant photosynthesis, the transfer of genetic material from one generation to the next, and the movement of nutrients within the plant cells.
It aids in the production of plant oils, sugars, and starches.
Phosphorus stimulates root growth and flower formation necessary for blooms and seed. Symptoms of phosphorus deficiency include fewer and smaller leaves, stunted growth, delayed maturity, and loss of resistance to disease. Organic compost and ground phosphate rock are the best sources of this essential nutrient.
Potassium is required to build proteins, helps with photosynthesis, and activates enzymes. Adequate potassium levels are essential for good food harvest and overall health. Potassium also regulates water use inside the plant. Without sufficient potassium, leaves may dry out, develop spots, and curl.
Organic compost and kelp meal are excellent sources of this integral nutrient.
Calcium, magnesium, and sulfur are important nutrients for plant growth and can be found in many fertilizer formulas.
Micronutrients such as boron, copper, chloride, iron, manganese, molybdenum, and zinc are also needed in small amounts. These nutrients may be included in the commercial fertilizer formulas and are available in well-formed plant or animal compost.