Converting available phosphorus (P) to total phosphorus (TP) requires understanding the distinction between the two and typically involves a specific chemical analysis, as they represent different forms of phosphorus in soil or water samples. Available phosphorus refers to the fraction of phosphorus that is readily accessible for plant uptake, often measured by extracting with specific reagents like Bray, Olsen, or Mehlich solutions. Total phosphorus includes all forms of phosphorus in the sample, including organic and inorganic forms, both available and unavailable.

Here’s a general outline of how to measure total phosphorus from a sample:

Method 1: Laboratory Analysis

Method 2: Using Available Phosphorus Data (Approximation)

If you have available phosphorus data and need an approximation for total phosphorus, consider the following empirical relationship (note that this can vary based on soil type and local conditions):

Important Notes:

• The exact conversion can vary widely based on the specific soil or water chemistry, and a precise measurement often requires the full laboratory digestion method.
• Always validate empirical approximations with local agronomic or environmental data to ensure accuracy.

For accurate and reliable results, it is highly recommended to use a laboratory analysis method that measures total phosphorus directly, rather than relying solely on empirical conversions from available phosphorus.

Thank you @Qasim Ali

Orthophosphate is the oxidized form of phosphorus that exists in different pH-dependent states (H2PO−4, HPO2−4, PO3−4). It is the most bioavailable form of phosphorus and is commonly found in dissolved forms in natural waters. The determination of total phosphorus (TP) in an aqueous sample is based on digestion of the sample to convert phosphorus compounds into orthophosphate, which can then be determined based on spectrophotometry.PO4 are three times the concentration of those reported as P. For example, analytically if you measure 0.06 mg/L as PO4, that's equivalent to 0.02 mg/L as P. To convert PO4 to P, divide by 3. To convert P to PO4, multiply by 3. Total phosphorus represents all phosphorus contained in the ingredient, including non-available phosphorus that is mostly bound to phytate. Practically, Total phosphorus is measured by first digesting the sample to convert all the other forms to orthophosphate, then the orthophosphate is measured by the ascorbic acid method. The test measures both dissolved and suspended orthophosphate as the sample is not filtered