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Fertilizing your plants in hot weather

The recent bout of hot, dry weather has turned our grass brown, curled our tomatoes and made our maple leaves crispy around the edges. You might think that fertilizing these stressed plants is the answer.  However, this could be exactly the wrong thing to do!  Here are some general guidelines for using fertilizer this summer.

  • Avoid applying fertilizers when plants are dormant or under heat and drought stress.
  • Fertilizers should be incorporated into the soil to prevent losses. If you cannot physically incorporate a fertilizer at this point in the season, it may be best to stick with a water-soluble fertilizer.
  • Fertilizers are not accessible to plants in very dry soils.
    • Water the soil before applying fertilizer, allowing water to soak into the ground.
    • Water again immediately after fertilizing.
    • Avoid washing granular fertilizer into hard surfaces like sidewalks, driveways, and streets.
  • Liquid fertilizer is a good option if a plant is showing nutrient stress and needs nutrients right away. 
  • Slow-release fertilizers dissolve with watering and release nutrients in small doses over time.
  • A soil test every 3 to 5 years will help you understand the health of your soil.

The three numbers on fertilizer containers are the fertilizer analysis. They indicate the percent of nitrogen, phosphorus and potassium in the fertilizer, and are always listed in the same order. A 100-pound bag of 10-20-10 fertilizer contains 10 pounds of nitrogen, 20 pounds of phosphorus, and 10 pounds of potassium. This equals a total of 40 pounds of nutrients. The rest of the fertilizer, or 60 pounds in this example, is a carrier or filler such as sand, perlite, or rice hulls. A complete fertilizer is one that includes all three elements.

All parts of a plant need nitrogen for growth—the roots, leaves, stems, flowers and fruits. Nitrogen gives plants their green color and is needed to form protein. A lack of nitrogen causes the lower leaves to turn yellow and the whole plant to turn pale green. On the other hand, too much nitrogen kills plants.

Phosphorus is needed for cell division and to help form roots, flowers and fruit. Phosphorus deficiency causes stunted growth and poor flowering and fruiting.

Plants need potassium for many of the chemical processes that allow them to live and grow. A potassium shortage shows up in various ways, but stunted growth and yellowish lower leaves are common symptoms in many plants.

For more information about plant care, visit www.extension.umn.edu.

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