Buckwheat was chosen because previous studies have shown it can absorb concentrations of phosphorus beyond its own needs. So excess phosphorus could be left in the soil from buckwheat plant residue.

Phosphorus (P) is an essential nutrient for optimum plant growth ... Therefore, this legal definition of “high” P soils is different than the agronomic interpretation of high or excessive soil test P ...

Long-term use of fertilizer and manure nutrients on the Delmarva Peninsula has led to an enrichment of agricultural soils with phosphorus (P). Consequently, many soils in Delaware are now considered ...

Manure is rich in nitrogen, phosphorus, and potassium-key nutrients that plants need to grow. However, when applied in excess, these nutrients can accumulate in the soil to levels that are harmful ...

But, the potent formulas can also cause problems if you use too much or add it to soil that doesn't need it. So, if your plants are showing signs of phosphorus deficiency, get your soil tested.