Phosphorous is a mineral that is considered a key macro-nutrient in gardening. When you see a three-numbered guaranteed analysis ratio on a package of fertilizer, such as 10-10-10, this is a representation of the macro nutrients in a product. The second number in the ratio always stands for the amount of phosphorous, along with Nitrogen and Potassium, respectively. Macro nutrients exist in soil to varying degrees and are taken up as nutrients by plant roots when dissolved in water. Phosphorous plays several key roles in plant growth.
Understand that phosphorus is an essential nutrient to fuel the plant's life support system, photosynthesis. It helps in the process of transforming sunlight captured by the chloroplast cells into chemical energy, such as simple sugars. This is the process that fuels plant growth and allows the plant to cope with environmental and cultural stress.
Be aware that phosphorous plays a key role in the formulation of plant lipids or oils, simple sugars and starches. It effects how rapidly the plant grows in combination with nitrogen. Phosphorus is the most important macro-nutrient for root tissue development and the production of buds and flowers. Fertilizer designed for flowering plants will nearly always have a significant amount of phosphorous, as will starter fertilizers that seek to quickly support new root establishment.
Know that phosphorus in fertilizer can come from either organic sources or from synthetic chemical formulations, and even can be a mixture of the two in some products. Common forms of phosphourous in fertilizer include bone meal and superphosphate.