Phosphorous is essential for root production and plant growth. Many potting mixes have lime in them. The lime is there to counter the acidity of the peat moss often used in potting mixes. However, I have recently discovered–with help from my friends at Texas Plant and Soil–this can cause a serious problem. Lime is basically calcium (Ca) which is a positively charged ion and it reacts readily with negatively charged phosphorous (P) fixing the nutrient in the soil and making it unavailable to the plant. We lime the soil to get rid of acidity. If the soil is too acid, iron will also tie up the phosphorous. Phosphorous is most available to a plant at a pH of 6 to 7. However if your soil is too acidic at time of planting, it is too late to lime because you will not get the nutrient to the plant. Lime at least four months before you plant. As far as the lime in the potting soil is concerned, we quit using peat moss and use partially decomposed hurricane Ike mulch while it lasts. This keeps the mix less acidic and is a sustainable resource for now.