**Phosphorus** (P) is a macronutrient and, after nitrogen (N), typically _the_ limiting factor for plant growth. Potassium (K) is also often added in fertilizers.
P is an important plant macronutrient, making up about 0.2% of a plant's dry weight. It is a component of important compounds, including nucleic acids, phospholipids and ATP. Consequently, plants cannot grow without a supply of this nutrient. P is also involved in controlling key enzyme reactions and in the regulation of metabolic pathways (Schachtman _et al_., 1998).
Certain plant species actively increase root densities around phosphorus-rich patches (Snapp, 1995), because it is critical to optimize it's uptake.
**References**
**-** Schachtman _et al_., _Plant Physiol_ (1998); **116** (2): 447-45).
**-** Snapp, _Plant and Soil_ (1995); **177** (2); 211-21