You have received your soil test results and found that you have high phosphorus in your soil. Excessive phosphorus in the soil may be due to where you live, especially in the coastal areas of South Carolina, where there are natural phosphorus deposits present in the soil. If that’s the case, ensuring you don’t add any additional phosphorus products to your soil is important.
In the Upstate and mid-state parts of South Carolina, phosphorus levels are generally in the low to normal range. If you live in those areas and have a high phosphorus level, you’ve added too much of a phosphorus-based product to your raised beds or inground vegetable garden. In that case, you’ve overdone adding organic matter, such as composted leaves, animal manures, mushroom compost, or added too much bone meal or fertilizers with a high phosphorus level. Normal phosphorus levels should range between 55 to 80 pounds per acre on a soil analysis.
Phosphorus is a large molecule that does not leach out of the soil, unlike nitrogen and potassium. When you have a high pH and excessive phosphorus, calcium and phosphorus react leading to an insoluble compound that is unavailable to plants, effectively locking up the nutrients.
Similarly, in a low pH soil, zinc and iron can react with phosphorus, again leading to locking up of these elements, and may lead to zinc and iron deficiencies.
So, what do you do? To help remove excessive phosphorus, plant a cereal fall cover crop, such as rye, oats, wheat, or barley. Before planting your garden next spring, the entire cereal cover crop must be physically pulled up by the roots and discarded to remove the plant-absorbed phosphorus. It should not be tilled into the soil as recommended for most cover crops, as it would add the phosphorus back into the soil.
Don’t add compost for at least the next two years because this will increase the phosphorus and calcium levels. Changing nutrient levels in raised beds will take several seasons. When adding organic matter to your raised beds, remember that more is not better. For more information on raised bed gardening, see HGIC 1257, Raised Beds.
Another option to reduce phosphorus, calcium, or boron levels is to grow brassica crops, as they are nutrient-dense plants. Brassicas include broccoli, collards, cabbage, cauliflower, kale, mustard greens, spinach, Swiss chard, and turnips. For more information on proper planting dates, see HGIC 1256, Planning a Garden.
Next summer, plant Solanaceae crops, such as tomatoes, peppers, Irish potatoes, or eggplant to further reduce the soil nutrient levels. To prevent disease issues, remember that all Solanaceae crops cannot be planted in the same area every year and must be on at least a 3-year crop rotation plan. For more information on crop rotation, see HGIC 1330, Crop Rotation.
Soil testing every year will help you follow the progress of reducing excessive phosphorus. For more information on soil testing, see HGIC 1652, Soil Testing.
In short, reducing phosphorus levels will take time, monitoring, and patience.