How to Reduce High Phosphorus Levels in Soil

Excessive fertilization with artificial or organic materials can lead to toxic soil that no longer supports plant growth. This is a common problem in raised beds full of organic matter but it can happen in any garden. Today I will look at both the cause of high phosphorus in the soil and the methods you can use to reduce the level.

Is High Phosphorus a Common Problem?

For us seasoned farmers, this is a new problem that I have never heard of until I arrived 10 years ago. How important is the problem? Auburn University reports that “more than 40 percent of Alabama garden soils have very high or excessive levels of phosphorus even though phosphorus is naturally low in all Alabama soils”. Alabama is no exception, making this an important issue that should be a warning to all farmers.

In the past, artificial fertilizers were not very popular and gardeners did not have easy access to bagged compost. That has changed, causing farmers to poison their soil in an attempt to improve it.

Think about this. If you add 2 pounds of 13-13-13 fertilizer over 100 square feet and grow top crops like cabbage, peas, potatoes, tomatoes, and corn, only 27% (0.5 lb) of phosphate is used by the plants . The remaining 73% (1.5 lb) is left behind in the soil.

What Causes High Phosphorus in Soil?

To understand this issue it is important to understand how nutrients move through the soil. What happens when you add 10-10-10 fertilizer to the soil?

Nitrogen is highly soluble and rain or irrigation water quickly transports it to lower soil levels. Nitrogen is also used in large quantities by everything that lives in the soil, including bacteria and they use the nitrogen source quickly. The rest is washed to lower the level of the soil where plants have difficulty reaching it.

Potassium is also highly soluble in water and can escape with nitrogen without being heavily trapped in clay and organic matter due to high CEC values ​​that attract positive ions. Therefore, it moves slower in the soil than nitrogen, but it still moves faster.

The movement of phosphorus in the soil is very different. It is not very soluble and is easily converted into insoluble forms when it combines with other ions. When phosphorus fertilizer is applied to the surface of the soil, it usually stays there and moves into the soil a fraction of an inch each year.

How does this affect your garden? When you fertilize your garden, nitrogen and potassium move quickly into the soil, while phosphorus stays on top. Later in the season you add another fertilizer and the same thing happens. Over the years you apply more fertilizer and phosphorus continues to accumulate in the soil.

Not only does phosphorus accumulate, but it tends to stay close to where most plant roots are. Before long the soil contains too much phosphate and becomes toxic.

Excess soil phosphorus reduces the plant’s ability to take up the necessary micronutrients, especially iron and zinc, even when soil tests show that there is an adequate amount of those nutrients in the soil.

The same happens with organic fertilizers that include manure and compost. Such materials have high levels of phosphorus compared to nitrogen. Plants use nutrients in a ratio of about 3-1-2, but manure and compost usually have a ratio of 1-1-1. They contain much more phosphorus than nitrogen, making the problem of phosphorus accumulation even worse.

The use of large amounts of organic matter in raised beds makes it easier to demonstrate phosphorus toxicity than in actual soil.

Symptoms of high Phosphorus levels

Excess phosphorus can cause iron and zinc deficiencies, especially in acidic soils. This can cause intervein chlorosis which many people attribute to iron deficiency, but if phosphorus binds iron, adding more iron will not solve the problem.

Iron deficiency in plants shows yellowing between the veins of the leaves while zinc deficiency shows as white tissue. Both are most visible in the smallest tissue and can occur at the same time.

“Annual bedding plants and newly planted shrubs can show symptoms. Acid-loving plants such as azaleas and blueberries, which grow in neutral and alkaline soils, can be killed if there are too many.
amount of phosphorus is used”.

Adding zinc or iron to high phosphorus soil is not helpful because both are quickly converted into unavailable forms. Foliar sprays of any kind can help a stressed plant but this is only a temporary fix.

High Levels of Natural Phosphorus

Some soils have naturally high levels of phosphorus. In such areas it is very important not to add more fertilizers.

Reducing Phosphorus Levels

Many fertilizer nutrients can be removed by water but that is not effective for phosphorus due to the low solubility of the salts that are formed (calcium and magnesium phosphate salts).

The first step is to stop adding extra phosphorus. This includes artificial fertilizers and anything other than a number between zero and all natural fertilizers such as manure and compost.

Another option is to replace the soil. Unfortunately, this is laborious and expensive, but it works.

A slower option is to plant cover crops that are high in phosphorus. This includes rye, oats, wheat, barley and brassicas (broccoli, collards, cabbage, cauliflower, kale, mustard greens, spinach, Swiss chard, and turnips). These plants will remove excess phosphorus and incorporate it into plant parts.

It is important that at the end of the season these plants are removed completely, including the root system. By removing the plant you also remove phosphorus. Plants can be fertilized and used in other parts of the garden that need more phosphorus.

Cover crops work, but they are a slow repair process. It may take several years before levels drop enough for some crops.

Let Nature Take Care of Things

What happens if you do nothing? Over time the environment converts phosphorus into “rock phosphates” which are not very soluble and do not bind other nutrients, except in acidic soils.

It is not clear how fast this process is, but one estimate is that soils with phosphorus levels between 150 to 200 ppm will likely have problems in 3 to 5 years. The higher levels will take much longer. These values ​​vary greatly according to soil types.

Prevent the Phosphorus Problem

There are things you can do to prevent the problem.

• If you use a synthetic fertilizer, use one with zero or a very low middle number unless you are deficient in phosphorus.
• Do not add more than five or two inches (5 cm) of manure or compost each year. This value will probably not cause you a problem.
• Do not add too much organic fertilizer such as fish emulsion or seaweed because, like other organic matter, they are high in phosphorus compared to nitrogen.
• Vermicomposting has a high level of phosphorus, so don’t use too much.
• Do not use bone meal – it has no place in the garden.