From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1997, No. 8


Knowing that phosphorus is essential for plant growth is not enough. Knowing why it is essential and when it is limiting plant growth are what counts. An old physics principle...for each action is triggered a reaction... might apply in plant growth. It reminds us that a shortage of phosphorus will trigger some reaction and symptom.

Prevention of crop stress due to a shortage of phosphorus is not always possible. Early crop stress detection becomes a vital part of a nutrient management program. Knowing the primary functions of phosphorus and how crops react when phosphorus is in short supply is critical to early detection.

What are the basic functions of phosphorus for plant growth? Think of phosphorus as a type of plant growth regulator. It is the energizer in plant development and is essential in capturing and storing energy during photosynthesis. It becomes an organic compound called adenosine triphosphate or ATP. In this form, it becomes the primary provider of energy needed to power many plant processes...such as:

How does phosphorus help plants tolerate stress? The answers can be found in the functions of phosphorus in plant growth. The following soil and crop conditions serve as indicators of when phosphorus might be in short supply and how phosphorus helps plants cope with conditions associated with stress.

Soil acidity can be highly variable within any given field. Highly acidic soils restrict root growth and allow iron and aluminum to lock up phosphorus. Consider band placement of phosphorus when soil acidity cannot be adjusted. High pH soils also tie up phosphorus and many micronutrients.

Minimum tillage provides excellent soil protection against erosion and contributes to energy savings. However, availability of phosphorus, magnesium and zinc to seedlings can be reduced due to the cooler, moist seedbed created by more crop residue on the soil surface.

Extreme temperature changes can injure seedlings, such as wheat and canola, when soils test low in phosphorus. Research has documented the role of phosphorus for improved seedling vigor, winter hardiness and stand survival. Winterize fall seeded crops by insuring the soil reservoir is adequately filled with phosphorus and potassium.

Low soil test levels for phosphorus can slow plant growth by slowing nutrient movement from roots and use of carbohydrates for cell growth. Slow seedling development, short plants, small leaves, purpling of older leaves, and delayed crop maturity are typical symptoms. Later in the season, a crop can signal a shortage of phosphorus by delayed tassel emergence and higher moisture grain at harvest time for corn...lower quality and reduced harvest yield for cotton...slow regrowth rate for forages...poor grain fill for small grains...thick rinds and lower juice quality of citrus.

Stem and leaf diseases increase with crop stress. Some are related to inadequate nutrition. Applied phosphorus has been shown to allow crops to better tolerate diseases such as take-all root rot fungus and Septoria leaf blotch in wheat, downy mildew and blue mold in tobacco, Cercospora in soybean seed, and brown stripe disease in sugarcane. Defense against disease relies on genetic resistance with optimum plant health and nutrition.

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For more information, contact Dr. Noble R. Usherwood, Southeast Director, PPI, 655 Engineering Drive, Suite 110, Norcross, GA 30092-2837. Phone (770) 825-8070. E-mail:
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