Soil Balancing, Part 4 (Phosphorus and Potassium)

November 6, 2023

Phosphorus and potassium are both key minerals for growing healthy crops.

Phosphorus is the energy mineral. It is common to refer to certain crops as having a certain number of days till maturity – for example, 100-day corn. The truth is that crop maturity is not limited by time, but by energy, which is where phosphorus comes in – it is involved in all of energy transfer throughout the plant.

While it is certainly important to have adequate phosphorus levels in the soil, we also need to point out that phosphorus is a biological mineral – meaning that without biology, plants don’t have good phosphorus uptake. Phosphorus release is largely dependent on biology and mycorrhiza fungi.  The possible partial exception is when soil pH is below 7.

In soil, we like to see a minimum of 75 ppm phosphorus, but ideal is more like 125 ppm. Phosphorus ppm should also be equal to potassium ppm.

Excessive phosphorus, while not as big a problem as excessive levels of calcium, magnesium, or potassium, can still have a negative effect, especially as the level goes upward of 400-500 ppm. It is especially interesting that zinc levels generally follow phosphorus levels. If phosphorus levels are low, soil zinc levels tend to be low. If phosphorus is excessive, zinc levels climb as well. And that is where part of the problem is. As zinc levels climb, copper does not necessarily follow, which creates an imbalance in zinc to copper ratios, which should 2:1, respectively.

Potassium is a growth mineral. While not the only growth mineral, it has a huge impact on the quality of plants and can keep them from lodging.

Potassium is also a fruit fill mineral – to have crops that have exceptionally large fruit with really good flavor and disease resistance, make sure your soil has adequate levels of potassium. A critical note here is that potassium does not release well in high pH soil or tight, high magnesium soil, or soil that is too dry.

Potassium is one of the 3 major base saturation minerals. It should be at 4-5% of the soil’s base saturation, with a minimum of 100 ppm or 200 pounds per acre, whichever measurement your soil report uses.

How do you get phosphorus and potassium? There are multiple sources, with compost perhaps being the best if you need both. Any manure should have plenty of phosphorus and potassium. There are also fertilizers that have both.

Boron and sulfur will generally increase the availability of both, as do cover crops. In fact, you could almost say that any cover crop increases their availability so long as the cover crop is feeding biology.

One last thing on potassium. Excessive potassium will either push out calcium and magnesium, or increase pH so it needs to be kept at balanced levels in the soil.