P-Al is the result of the determination of the (semi) soil supply of phosphate. This is done with ammonium lactate (Al) at a low pH. P-Al indicates how much phosphate can be replenished. The laboratory result is given in mg P2O5/100 gram. The unit of measurement is often kg P/ha.
The extensive soil analysis Fertilization Manager by Eurofins Agro gives insight in the phosphate status of the soil.