a large potato grown using phosphorus fertiliser with other potatoes in the soil in the background

What Role Does Phosphorous Play In Potato Production?

Potatoes need adequate nitrogen, phosphorus, and potassium to grow. Each nutrient plays a role in plant growth and the quality of your potato tubers. For example, phosphorus is vital for developing a strong root system and for promoting good tuber development.

What is phosphorous fertilizer?

Fertilizer contains the vital nutrients nitrogen (N), phosphorus(P), and potassium (K). The percentage of each by weight is denoted in the NPK formula. A balanced formula, such as 10-10-10, contains equal amounts of nitrogen, phosphorus, and potassium. A formula with a high middle number is considered a phosphorus fertilizer.

Bone meal (2-16-0.6), rock phosphate (0-30-0), and bloom boosters (0-44-0) are common phosphorus fertilizers. When looking for fertilizer high in phosphorous, check that the middle number is at least twice as high as the first and last numbers.

You can find phosphorus fertilizers in the gardening section of hardware stores or at gardening centers. Phosphorus fertilizers may be either chemical or organic and come in both granular and liquid formulas.

How does phosphorous affect the growth of potatoes?

Phosphorus plays a vital role in converting energy from the sun to food, fiber, and oil explains the Intermountain Farmer’s Association (IFA). Phosphorus is needed to perform photosynthesis which controls the metabolism of sugars, cell division, cell enlargement, and the storage of energy. In the case of potatoes, this means larger and healthier tubers as the plant stores its excess energy in the tubers. Phosphorus encourages good root formation, spurs early shoot growth, and improves the quality of fruits. Phosphorus also helps plants adjust to cold temperatures, lessens moisture loss, and improves plants’ ability to absorb and transport water and other nutrients say IFA. Phosphorus helps your potato plants fight disease, too.

Do potatoes need phosphorous fertilizer?

Potatoes need adequate phosphorous to promote a robust root system, improve plant health, and to form large, healthy tubers. While they need phosphorus at planting time, they may also require additional phosphorus during the bulking-up period. Potatoes bulk up (or form large tubers) about 70 days from planting, depending on the cultivar and whether you are growing early mid-season or late potatoes.

Tubers begin to set on shortly after the potato plant blooms and start bulking up several weeks later. If your soil lacks phosphorus, or you didn’t apply enough phosphorus at planting time, you may need to apply supplemental phosphorus fertilizer during the growing season.

 At what stage should phosphorous be applied to potatoes?

Applying phosphorus, typically as part of a balanced fertilizer, is generally recommended in the spring at planting time. Still, an additional application is sometimes needed when the potatoes begin to bulk up.

Some recommend calculating the total phosphorus needs of your potatoes for the season (1.5 pounds per 1,000 square feet) and dividing it into three equal applications: 1) at initial planting, 2) when new growth appears, and you hill your potatoes the first time, and 3) shortly after the potatoes bloom and begin to form tubers.

How do I apply phosphorous fertilizer to my potatoes?

Apply phosphorus fertilizer by side dressing the potatoes following the manufacturer’s recommended application rate and working it into the soil with a garden hoe or other garden tools. Many prefer to add phosphorus fertilizer when they hill their potatoes.

If you choose a liquid-based fertilizer, apply it when you water your potatoes after hilling them.

How much phosphorous do potatoes need?

According to HOSS, potatoes need about 1.5 pounds of potassium per 1000 square feet during the growing season, but they don’t need it all at once. Instead, HOSS recommends dividing the amount into three equal amounts and applying them separately during the season.

  1. Apply one-third of the phosphorus fertilizer at planting time.
  2. Make a second application when your potatoes break through the soil or at the first hilling.
  3. Apply phosphorus fertilizer one more time when you hill them the second time or after the plants have bloomed and tubers are beginning to form.

How do I know if my potatoes need more phosphorus?

Many gardeners use the health of their potatoes as a guide to when and if to apply phosphorus fertilizer during the growing season. Watch for these signs that your potatoes need more phosphorus.

  • Stunted plant growth.
  • Deformed, Twisted, or Curled Leaves
  • Dead or dying older leaves with brown spots.
  • Purple or reddish color on the lower leaves.
  • New leaves are small and look bluish-green.

What happens when potatoes get too much phosphorous?

Ironically, too much phosphorus can also cause stunted growth in potatoes, but there are other things to look for.

  • Leaf discoloration.
  • Leaf veins that are yellow or dark colored.
  • Browning on the tips of leaves, particularly on new growth.

Do you need to give potatoes supplemental phosphorus fertilizer?

Under normal conditions, applying a balanced fertilizer in the spring at planting time provides the phosphorus your potatoes need to grow and thrive. However, if your soil is low in phosphorus, you may need to give your potatoes a boost with phosphorus fertilizer when new growth begins and again a few weeks later when you need to hill them again or when new tubers begin to form. This will provide the phosphorus they need when the potatoes begin to bulk up.

How do you know if your soil is low in phosphorus?

The most reliable way to determine how much phosphorus is in your soil is by doing a soil test in the fall after the growing season is finished or in the early spring before you plant your potatoes. Most University Extension offices provide soil testing services for a reasonable fee. The soil summary report will tell you if your soil lacks phosphorus and provide instructions for correcting it.

Because soil changes over time, it is recommended that you test your garden soil every 3 to 5 years, or sooner if you suspect a deficiency, it is also beneficial to check and amend your soil pH. Follow the recommendations in the soil report to amend your garden soil before planting in the spring.

Summary

While potatoes do require adequate phosphorus to produce healthy foliage and large healthy tubers, most experts agree that providing them with phosphorus at planting time is all that is typically needed. Some choose to divide the amount of phosphorus required into three equal portions and apply them at planting, at the first hilling, and again with the final hilling or when the tubers are beginning to set on and begin to bulk up.