Not all phosphorus-based fertilizers are the same. For this reason, ensuring sufficient and timely phosphorus nutrition for potatoes can be difficult or prohibitively expensive.
If two fertilizers contain the same total amount of elemental phosphorus, but it is contained in different compounds that react differently in the soil, fertilizers can vary greatly in how much and at what stage of the growing season will be available to plants. This reality is frustrating for many potato growers and leads to frequent crop fertilization errors.
To ensure growers get what they paid for by investing in crop nutrients, all major commercial phosphate fertilizers are labeled with the percentage of phosphorus (P2O5) that is actually available to plants. Phosphorus available to plants is calculated by measuring the solubility of a phosphate fertilizer in both water and ammonium citrate, a weak acid. It mimics organic acids that occur naturally in the soil or are secreted by plant roots.