Plant Nutrition 101

#3 Phosphorous

byRiley Holly

Plants use phosphorous in the form of H2PO4-, HPO4=, & PO43-. Phosphorous is approximately 0.2% of the plantís required nutrients.Phosphorous stimulates early growth, root formation, promotes seed formation and is important in photosynthesis.

A deficiency of phosphorous shows up as

*Slow growth, stunted plants

*Dark dull green leaves (an early indicator)

*Purple under leaves on some plants, on older leaves

*Delayed maturity

*Poor flower, seed, & fruit development

*Dying leaf tips

*Yellowing leaf margins

*Interveinal yellowing streaks or striping

*Poor (or no) fruit development

*Premature fruit drop.

Excess symptoms show up as

*Mimicking Zn & Fe deficiency

*Interfering with micronutrient absorption

The ideal soil pH for phosphorus utility lies between 6.5and7.5.

Although phosphorus is mobile in the plant, it is immobile in the soil due to its sorption (absorption or adsorption, binding to positive ions of Al (aluminum) & Fe (iron) in soils with pH<6.5, or precipitation to calcium phosphates at high pH (above 7.5).This sorption is less dependent on pH in fine textured soils with large surface areas (more positive ions available).

Because phosphorous so readily becomes immobile, it is important to apply at optimum times (during slow root growth and rapid vegetative growth).

In its commercial form, phosphorous is supplied as P2O5, which is approximately 44% phosphorous.With a 16-16-16 NPK fertilizer, a pound would yield 0.70 pounds (0.16 x 0.44) of phosphorous.

Organic fertilizers containing phosphorous are many. Some with the higher percentages are bone meal (23%), fish meal (6%), rock phosphate (3%), and cotton seed meal (2%).

Because leaf analysis can become expensive for the backyard gardener, it is important to watch plants for different symptoms.Corn is a good indicator of plant deficiencies and can be used as an alternative to leaf analysis.

Try planting a single corn next summer in locations where you would like to have an indication of nutrient deficiency.A phosphorous deficiency will show up as purplish-red streaks on the leaves.There are also inexpensive kits to test for N, P, K, & pH at nursery facilities.

Deficiencies first show up in older leaves due to its mobility in the plants.Avocados show stunted growth, lack vigor, and have an open canopy.

An excess of phosphorous results in weak stalks, poor seed development, and interferes with micronutrient absorption (Zn & Fe).

Fertilizing by banding at planting time is ideal. Place the fertilizer just beyond the roots and 2-4 inches below the surface, and then cover with soil.

Broadcasting phosphorous is not a satisfactory method as it will not leach down before it will be carried away by irrigation or captured by the positive ions.An alternate method is applying the phosphorous as a liquid in the irrigation water.If required, additional phosphorous should be added in cold weather.

Phosphorous should be applied during growth times because of the rapid sorption and precipitation causing its immobility.


Phosphorous deficient plants may remain darker green than normal plants and develop purple discoloration, first on the underside and later throughout.Plants grow slowly, stalks are thin and shortened and maturity is delayed.