An adequate P supply is essential for successful crop production. Growers using organic production techniques are encouraged to use on-farm resources or products receiving minimal processing. Crop producers using compost and manures as a N supply generally apply more P than is removed in the harvested crop, leading to long-term P accumulation. Rock P is generally insoluble until it reacts with an acid source in the soil; which greatly limits its usefulness. Microbial additives and mycorrhizal fungi may help solubilize a small portion of the soil P, but they perform best in low soil P conditions where deficiencies are common. Cover crops may help acquire soil P, which is subsequently released when it is killed, but they provide no additional P to the soil. Careful soil analysis should be used to avoid P deficiency leading to loss of yield and quality, and also avoid excessive P accumulation, which can pose potential problems with water quality.