Phosphorus in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 34

tests are better than a larger application every few years. For cropping, adequate
levels of phosphorus should always be applied, as worthwhile net returns depend
on high yields. If phosphorus is the limiting factor to crop growth, the yield will be
substantially lower than achievable yields.
Note . Split applications of phosphorus are probably best done using foliar
fertilisers, as phosphorus fertilisers top dressed onto the soil surface remain close
to where they are placed, and do not easily move down into the root zone.
What happens if adequate phosphorus is not supplied?
If a paddock has a recent history of adequate phosphorus applications, it will
probably have a reasonable level of residual phosphorus that will be partly available
to a subsequent crop or pasture. This level can be enhanced by addition of
appropriate soil microbiology.
If the paddock is only lightly grazed, there may well be enough phosphorus for
one or two years, but if grazing pressure is high, production will be limited by lack
of available P, and gross returns will be lower.
If adequate phosphorus is not available for growing crops, then potential yield
will be lower than the maximum yield potential, and gross dollar returns will also
be lower.
Phosphorus toxicity
Excess phosphorus in plants will cause yellowing of leaves and ill thrift in the
plants. On rare occasions this is done deliberately, for example, when growing
'Virginia leaf ' tobacco to obtain a golden dried leaf colour instead of the normal
brown colour. In most farming enterprises, however, supplying too much
phosphorus is practically and economically undesirable.
Excess phosphorus intake by animals (including humans) will lead to nutrient
imbalance with calcium and magnesium, bone problems, sickness, breeding
difficulties, and has been linked to cancer.
Poor returns on $ invested.
Medium returns on
$ invested.
Good returns
on $ invested.
Too Low Medium Too High
PHOSPHORUS LEVELS IN SOIL
Figure 11 Plant response to applied phosphorus. In photograph, left arrow shows plants with phosphorus
applied, while right arrow shows plants without phosphorus. ( Soil Sense C-05)