Soil Phosphorus (Part 2) – Phosphorus Cycle
Phosphorus (P) is an important soil nutrient in NZ. There are several reasons for this, but a couple of the most
important are firstly, P only occurs in low levels in most of the parent materials from which our soils are formed, and
secondly; P plays a critical role in the metabolism of all living creatures on earth. The naturally occurring soil P
levels are insufficient to achieve and sustain high levels of primary production. Thus for many decades now, P
supplementation has been applied in the form of phosphate fertilisers.
Though P plays such a pivotal role in primary production (and in the wider economy - since most goods exported
from NZ are still derived from the primary sector), it is surprising that knowledge as to how P functions in the soil is
not generally well understood, either by farmers and growers, or by those who support and advise them. Very
often, soil P discussions reduce down to just a couple of considerations: What is the Olsen P soil test result? How
much fertiliser P do I need to order?
Whilst understandable, such an approach can be misleading. As in most things, a holistic big picture overview of
soil P and the P cycle will pay dividends to those with an inclination to “dig a bit deeper”. The payoff for doing so
will be that they become smarter, more intelligent producers, who are in a better position to make good farm
management decisions about both their soil P status and their P fertiliser requirements.
Figure 1: Phosphorus Cycle
Figure 1 outlines a simplified overview of the P cycle, illustrating the various forms in which P occurs in soils and
some the processes which transform P from one form to another. Soil is a dynamic system. As a result, at any
point in time (and depending on soil properties and climate etc), some P is likely to be present in most of the
various forms outlined in the figure.
Plants derive the P they require from the soil solution – the water present within the soil which enables soil
chemical reactions to take place. The amount of such soluble P that is available at any point in time is generally
quite low, perhaps < 1% of the soil P reserves, and much less than the P that the plants require for a season of
growth. Consequently, whenever plant roots uptake P from the soil solution, additional P needs to be supplied to
the soil solution from elsewhere in the soil P reserves i.e. to replenish what the plant has used.
It is important to understand both the nature and the quantity of these soil P reserves. The main soil P reserves
are: P that is in the process of weathering from the primary mineral P in the soil (held within the mineral structure of
the parent material of the soil); P that is loosely held on the soil mineral surfaces (of the various components which
comprise the soil); P that comprises secondary compounds (that are constantly forming and dissolving in the soil)
and organic P (comprising the P that has been assimilated into both living and dead organic materials).
The amount of P in soil solution is in equilibrium with the other forms of P in the soil reserves, and at any point in
time, usually about 5 - 10% of this reserve soil P is available to replace the P lost from the soil solution with the
balance (roughly 90%) of the soil P reserves stored in forms which are not immediately available (cf Figure 2).
Figure 2: Soil Phosphorus Availability
The Olsen P test, along with other similar laboratory tests (Resin P, Mehlich, Bray etc) endeavour to measure plant
available P i.e. the P in soil solution and the immediately available P held in the soil P reserves. As each laboratory
test has its strengths and weaknesses, using the Olsen P test in combination with at least one other test is a helpful
way to gain a greater confidence in the test results i.e. one test can confirm the other etc.
The P cycle figure also illustrates areas where P is lost from the system. Not much P is lost by leaching but
significant losses can occur via erosion i.e. when the particles of soil, and the P they contain, are physically
removed from the soil by major precipitation events. Animal grazing and excretion also impacts the P cycle i.e. P
deposited along tracks (dairy) and in stock camps (dry land).
Dave McKie
Soiltech Soil Scientist