Phosphate Fertilizers Phosphate fertilizers are usually low in toxicity because a large portion of the phosphate is precipitated in the soil before it can reach the plant roots. The concentration of phosphorus in soil solution at any one time is very low. No limit is normally set for the safe rate at which phosphates may be applied with, or near, the seed of field crops. Diammonium phosphate is more toxic than other phosphate fertilizer - see above under nitrogen fertilizers. Sulphate of potash-magnesia has approximately the same toxicity per unit of potassium as muriate of potash.
Potassium nitrate is one of the safer sources of potassium.
Boron is particularly toxic, and should not be banded. Guidelines for Safe Rates of Nutrients Applied at Seeding Fertilizer toxicity varies widely, depending on the amount of soil moisture. To ensure completely safe rates of banded fertilizer for all seeding conditions would require extremely low rates of application. In most cases, it is advisable to use lower rates of fertilizer at seeding than those listed in Table 12. If fertilizer requirements are high, it may be better to broadcast most of the fertilizer required and to band only a small portion at seeding.