Phosphorus has a negative charge; however it sticks strongly to aluminium and iron. It does this particularly well if the soil pH is below 7, if you raise the pH with calcium it stick to the calcium ions.

 The plant cannot remove Phosphorus from either, therefore Phosphorus is most readily available at a pH of 6.5 .

Phosphorus is very immobile in soils, it doesn't move to the roots, therefore it is essential to create the best conditions to promote good root activity so that the comprehensive root system comes in contact with enough P. Soil porosity is important to ensure sufficient oxygen is present for respiration soils with organic matter present are more likely to have a strong mycorrhiza population, mycorrhiza live off sugars in the plants roots but help to pull in phosphates in return, this symbiotic relationship with plants is beneficial to both. 

As mentioned in the nitrogen report, phosphorus is needed to synthesise important molecules such as proteins and DNA which are responsible in cell development.

Phosphate Deficiency

The picture above shows the smaller plant deficient in phosphorus, exhibiting yellowing and eventual death of lower leaves.

In mild deficiency cases growth is checked and spindly, new leaves are dark green and reduced in size, lower leaves may become orange/green in colour. 

Severe deficiency will cause stunted plants with young leaves dark green and reduced in size. Lower leaves become orange with browning at the margins resulting in death of affected leaves. Flowering is delayed, bloom size reduced and fewer blooms on sprays. Petals may suffer some loss of colour.

Where there is an excess of phosphate certain micro-nutrient deficiencies may  be result e.g. iron, copper, manganese.

For excess water heavily in an attempt to leach excess, be aware of possible micro-nutrient deficiency.