Answer: 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. I hope this helps with your project!
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