Photo: 46% DAP

Phosphorous

We typically use Diammonium Phosphate, or DAP, with an analysis of 18-46-0.  DAP contains nitrogen in a non-burn form that works great as a plant starter. 

Phosphorous is very important for crop establishment and is important for root growth among other things.  Phosphorous is more slowly available than the other 2 macronutrients.  Phosphorous also ties up in the soil and the phosphorous soil level determines how much of the applied phosphorous will be available to the plant.  At very low levels, around 5lb of soil P per acre, it takes 30lb of P to increase the soil level P 1lb per acre.  At a medium soil level P, around 25lb of P per acre, it takes 10lb of P fertilizer to increase the soil level P 1lb per acre.  When P reaches the saturation point, every unit of fertilizer added increases the soil level P.  This explanation of soil level P has not considered the amount that crops or animals draw off and that should be considered when trying to predict future phosphate soil levels. 

If you are interested in a more in depth explanation, you may like this link:

www.spectrumanalytic.com/doc/library/articles/p_basics