UPDATE: How available is phosphorus in soils?

Phosphorus is often in short supply 

Phosphorus is an essential nutrient limiting ecosystem processes in a wide range of natural and managed land systems. Biological processes which are affected by phosphorus include plant growth and the response of ecosystem productivity to increasing carbon dioxide concentration which has far reaching consequence for food and fiber production, climate mitigation, and the functioning of the Earth system.

Human land use has changed soil phosphorus 

Humans intentionally add phosphorus in form of organic material (like manure) & mineral fertilizer as well as remove phosphorus by harvesting biomass. On top of that land management has lead to accelerated soil losses. . In a new study, several global datasets describing these drivers were combined with a process model for soil P dynamics to reconstruct the past and current distribution of P in cropland and grassland soils. The study provides the simulated size of different soil P pools for the period 1900–2018.