Answer: Although plants make their own "food" through photosynthesis, they need certain nutrients to help them do this. You probably noticed that Miracle-Gro, as well as other fertilizers, have 3 numbers on the container. These numbers refer to the percentage of nitrogen (N), phosphorous (P), and potassium (K) in the fertilizer. These 3 elements are referred to as macronutrients because plants need them in fairly large (i.e., macro) amounts to thrive. How these elements interact is complicated (and probably Mother Nature's secret!) but in general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers (and eventually fruit), and potassium keeps the root system healthy.
Other nutrients that plants need, but in lesser amounts, are: calcium, magnesium, sulfur, boron, chlorine, copper, iron, molybdenum, and zinc. If the plant is not receiving the appropriate mix of nutrients, the deficiency will show up in a variety of ways. For example, yellow leaves on older growth is a sign of nitrogen deficiency.
The pH of the soil--how acid or how alkaline it is--affects how plants take up nutrients. Even though the nutrients are present in the soil, if the soil is too acidic or alkaline these nutrients become "locked up" in complex molecules that plants can't use. That's why people apply lime (to raise pH) or sulfur (to lower pH) to their soils.
Seeds have stored energy (enough for them to develop a root, a shoot and a set of leaves). Once the first pair of true leaves develops, the seedling is ready to begin the conversion of sunlight into carbohydrates for growth (photosynthesis). It's at this point that fertilizer application is most useful, and it is not necessary to use fertilizer prior to this stage of growth. Hope this information helps you!
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