Greenleaf Farms

Phosphorus is  “P” value (the second number in the fertilizer rating N-P-K).

Plants absorb most of their phosphorus from the soil solution as orthophosphate (H2PO4-), regardless of the original source of phosphorus.  Since orthophosphate’s negative charge prevents it from being attracted to the soil’s positive charge (cation), it will still react strongly in the soil, particularly if there are large amounts of iron and aluminum naturally,  but it will products that are insoluble which in short means it is unavailable to plants.

A major factor controlling these insoluble/soluble reactions is the soil pH.   If the soil ph is low or high, the  phosphorus solubility availability is low,  because the phosphorus needs the soil to be in the 6.0 to 7.0 pH range to be absorbed.  That means we then need something, another chemical,  to change the soil to the optimum level so it can absorb phosphorus:  enter lime.  Of course there is also bone meal and rock phosphate, but lime is the cheapest alternative out there.

The legal definition of available phosphorus in fertilizer is the sum of the phosphorus soluble in water plus that which is soluble in  citrate solution or (P2O5). This number can  vary from 0 to 100 percent. So when shopping, look for is a  number as high as possible, as that  is the more effective phosphorous source.

This is especially important for short-season, fast-growing crops,  crops with restricted root systems (tomatoes) , or when purchasing starter fertilizers.