Essential Nutrients

Phosphorus (P)

Form used by plants:

H₂PO₄^- (when soil pH <7) and HPO₄^2- (when soil pH>7)

Important functions:

• Involved in energy transfer in plant cells

• Component of nucleic acids, coenzymes, phospholipids, and phytic acid

• Stimulates seed development and root formation

Ideal foliage range for apple leaves:

0.10-0.40%

< 0.08% may indicate P deficiency but in most cases, some other factor may be limiting P availability to crop. It is usually pH limited (P most available with soil pH between 6-7.0).

Crop requirement:

Range between 5 and 20 pounds of phosphorus per acre per year, or the equivalent of 10-45 pounds of P₂O₅ per acre per year

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Ideal soil range:

10-20 ppm (Olsen test should be used in our area)

< 10 ppm is considered low

20-40 ppm high

> 40 ppm is considered to be excessive

 

Soil tests are not usually a good indicator of phosphorus stats as related to crop response.  Availability of this nutrient is largely controlled by soil pH.  As the pH of acid soils is increased or that of alkaline soils is reduced toward neutrality (soil pH 6.0-7.0 is optimum), the levels of soil P tend to increase even though no P fertilizer has been added. At soil pH levels below 6.0, P is generally in an unavailable form because of reactions with soluble iron, aluminum, or manganese, or their hydroxides.  At pH levels above 7.0, phosphorus fixation occurs thus making it unavailable for plant uptake. 

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Best indicators:

Leaf analysis provides the best means currently available for determining the status of phosphorus in fruit trees (tree Fruit Nutrition). Extremely high levels of P on the soil or in the leaves may be associated with zinc deficiency.  Soil tests for P correlate well with cover crop and sod growth. 

Mobility in plant:

Remobilized within plant

Deficiency symptoms (rare in WA state):

• Older leaves are affected first and may be small, bluish green on the margins or main veins or undersides of leaves having purple pigmentation.  Older leaves drop early.  Flowering is reduced.  Fruit quality may be affected. Delayed maturity.

• Addition of fertilizer P may be advantageous in nurseries and replanted orchards.

Excess problems/Interactions with other elements:

• High leaf P levels may indicate zinc or copper deficiencies.  When the P:Zn ratio is 150:1 or more, Zn deficiencies are possible.  Ratios of 50:1 or lower indicate adequate supply, providing foliar sprays of Zn compounds have not contaminated leaves.  P:Cu ratios are less definitive.  It is estimated that ratios of 200:1 may indicate Cu deficiencies.

• Phosphate will enhance the absorption of molybdenum.

• Nitrogen fertilization can increase P concentration in plants by increasing root growth, by increasing the ability of roots to absorb and translocate P, and by decreasing soil pH as a result of the absorption of ammonium (NH₄⁺) and thus increasing the solubility of fertilizer P.

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