Understanding Soil Nutrients – Phosphorus

This is the second entry for the Understanding Soil Nutrients Series. Today we will focus our attention on a very important soil nutrient – phosphorus. If you missed the first entry to this series, here it is:

Understanding Soil Nutrients – Nitrogen

Why Is Phosphorus Important To My Vegetable Garden Plants?

Plants need phosphorus for strong root growth, fruit, stem and seed development, disease resistance, and general plant vigor. Phosphorus doesn’t move in the soil as easily as nitrogen does so you don’t have to add it as frequently. Depending on where you live in the country, your soil may have plenty of phosphorus, but it may be unavailable to plants. Phosphorus availability depends on warm soil temperatures, pH range, and the levels of other nutrients, such as calcium and potassium, in the soil. Deficiency symptoms include stunted plants with dark green foliage, reddish-purple stems or leaves, and fruits that drop early. Rock phosphate and bone meal are good sources of phosphorus.

Phosphorus (or P) is generally noted for its ability to promote a strong root system in plants. Most transplant or starter solutions contain higher levels of phosphorus to help the plant grow stronger root systems. Before adding any phosphorus, or phosphates, to your soil, make sure you complete a thorough soil test (For more on soil testing, please check out How To Properly Test Your Soil and How To Take A Soil Sample For Testing).

When using fertilizers, check the N-P-K ratings on the back of the container. This will tell you what percentage of the product contains phosphorus. Phosphorus is always the middle numeral in the N-P-K rating, demonstrated in the picture below:

The Middle Number Represents The Percentage of Phosphorus

The best times to add phosphorus to the soil is just before planting – especially with transplants. Use composts and well-aged manures as amendments to the soil, as these have good levels of available phosphorus for plants to use. Be sure to monitor phosphorus and pH levels in your soil to make sure you are not applying more than what is needed.

8 Comments on Understanding Soil Nutrients – Phosphorus

  1. I have discovered through your website my greenbeans have too much nitrogen! We turned under grass to make a new garden this spring, also adding leaves. Then planted. Maybe two weeks had gone by. Now my greenbeans are beautiful, lush and green, with no blooms! Is there anything I can do? Thankyou

    • Hi Danna, I have never heard of adding nitrogen to green beans before – usually there is no need to. Green beans are legumes which have the ability to fixate nitrogen in the soil on their own so adding nitrogen is usually not needed. Nitrogen has the tendency to become lost very easily in soil because it “moves around” due to leaching, water run off and other factors. I wouldn’t add any more nitrogen to your green beans. It may take a little time for the nitrogen to settle out, then maybe the green beans will start producing. Also make sure there are no pests or diseases that could be causing the blooming issue, and take a soil test of the area to check the nutrient and pH levels.


  2. hi i am growing a good size patch of sweet corn it is about 10-12 weeks old 200 mm high it looks a bit yellow rather than lush green would that be to much nitrogen or not enought

    • Hi Malcolm, thank you for visiting Veggie Gardener. Corn generally requires quite a bit of nitrogen in order to fluently produce. The best advice that I can give you is to perform a soil test in your garden area to assess how much available nitrogen is in your soil. Also, nitrogen leaches from the soil fairly rapidly due to rain run-off and other factors, so for growing corn, nitrogen needs to be supplemented in most cases. If you use a nitrogen fertilizer, using one pound of fertilizer per bushel of corn produced is a general rule of thumb. Make sure to follow the fertilizer manufacturer’s recommendations for usage.

      The leaves turning yellow could also be a sign of over-watering, especially if the yellowing leaves are predominately found near the bottom of the plant. So make sure the plants are not receiving too much water. Generally, about 4 liters (approximately 1 gallon) of water per week is a good place to start. Of course, this depends on how much rainfall you receive as well.

      I hope this helps to answer your question. If you have any more comments or questions, please feel free to contact me.

  3. How much phosphorus is too much? I was taking the phosphorus levels with my N-P-K soil test and i have some soil with 64lb A/6′ soil (H). Will that kill my plants?

    • Hi Anna – I have read that 300 lbs/A is considered a very high level of P in soil. 64 lb/A is not that bad, and can be very good for growing plants that need more phosphorus in soil, such as root crops (potatoes, carrots, onions, turnips, etc.).

      If you want to balance the nutrients out more then adding compost over time would be best. It may take several years for them to balance out, but really, 64 lbs/A is not that high.

      To answer your question, too much P should not kill your plants. If there is more available than what the plant needs it will simply not use it. The only real issue with high levels of P in soil is the extra phosphorus ending up in the watershed. Phosphorus can create a lack of oxygen and promote algae growth in streams, rivers and lakes. This is one of the reasons for the “Dead Zone” in the Gulf of Mexico.

  4. I live in florida and I do a lot of composting. I grow peppers in pots all year long, but I’ve been having probs with the pepper plants lately. I grow all kinds of peppers and they’ve been struggling with yellow leaves, bud drop, small fruit are among the many probs I’ve been having. I think because I recycle my dirt I need a boost of P. By adding blood meal to the compost bin as it brews, would you think that would be the best way to take care of the prob?

  5. Hello,
    My plants have a lot of green but not much vegetables, what is wrong?

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