Mobility within tree: High (symptoms in older leaves first)

Uptake Form: H₂PO₄^- (dihydrogen phosphate), HPO₄^2- (hydrogen phosphate), PO₄^3- (phosphate)

Function: ADP, AMP, basal metabolism

Deficiency/Toxicity Symptoms:  Phosphorus deficiency causes dark green colour on expanding leaves, inhibit growth, and reduce budding, blossoming and fruiting.  Stone fruits with low P will mature early and have poor quality.  Severe deficiency to cause visual change in leaves is rare in fruit trees. Excess P is usually due to an antagonistic relationship with other metals such as Zn, Cu, Fe or Mn.  Analytical results are therefore needed to interpret a problem since visual symptoms can be variable. There are several tests for P availability; however, P-Olsen testing is recommended for the calcareous alkaline soils of Okanagan Valley.

Phosphorous Sources

Supply of P from soil minerals and mineralization of organic matter often provides sufficient P for tree fruit crops. Phosphorous availability is soil is highly controlled by P minerals in soil and soil pH. Maximum P availability is between soil pH of 6.5 and 7.5.

Ammonium phosphate fertilizers are the most common choices for P fertilizers for replants because of their high P availability when applied to the root zone.  The most used ammonium phosphates are the granular monoammonium phosphate (11-52-0) or the soluble 10-52-10.  Rock phosphate and triple superphosphate (0-45-0) are granular forms, which are less soluble and are less available to trees when surface broadcast.  Liquid formulations suitable for fertigation include ammonium polyphosphate (10-34-0) or phosphoric acid (0-54-0).  It should be noted that phosphite based fertilizers are not a source of P to the tree.  Phosphites are used to help move whichever nutrient is attached to it into the plant. Common P fertilizer sources in BC are shown in Table 17. 

Organic waste materials, such as composts, with high known P (near or exceeding 1 %), can be applied at rates of 4-5 tonnes/ha rototilled into the planting row.  With any incorporated granulars or composts, make sure the fertilizer is mixed well with planting hole soil or peat mixtures.  Reduce the rate by half on extremely coarse textured gravelly soils to avoid root burning.