Phosphorus is one of three major elements required by plants for healthy growth, flowering and fruiting. It remains relatively immobile in soil and resists leaching from heavy rain or watering. Australian soils are often, however, deficient in phosphorus.
In soils with low pH, phosphorus is less soluble and therefore less available to plants. This limits growth. Cold weather can cause a temporary deficiency. Symptoms include poor growth and leaves that turn blue-green but not yellow (as in nitrogen deficiency). Oldest leaves are affected first. Here, in the brassica seedlings at TAFE, the leaves are a red-purple colour.
This phosphorus deficiency can be corrected by planting out these seedlings in a soil with plenty of organic matter. Decomposted animal manure, particularly poultry droppings is rich in phosphorus.
ref: Wikipedia and Gardening Australia, Judy McMaugh pg.10
Excellent detail once more. These seedlings where produced in an experimental totally organic growing media, there was an unfortunate lack of phosphorus in the original batch that produced these results. Seaweed concentrate was used to improve their performance, slight increase in phosphorus, before being planted out.
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