How can this be? How can the plant continue to grow? The answer to the apparent riddle is that there is in most fertile soils a large pool of what can be called retained P or “fixed” P that rapidly goes into, and out of, solution.
To help understand this concept, consider what happens when sugar is added to a bucket of water: as the sugar is poured into the bucket and stirred, it dissolves and the solution remains liquid.
Sugar can be added until the solution becomes saturated, at which point any further added sugar will not dissolve and will collect on the bottom.
Now imagine that the bucket is half full of undissolved sugar. The water will be saturated with sugar, and if hummingbirds drink half the liquid – hummingbirds love drinking sugar water – the remaining water will still be saturated with sugar.
Consider what happens when the bucket is once again topped up with water. The solution will no longer be saturated because the added water dilutes it, but this is only a temporary condition because, even without stirring, some of the solid sugar on the bottom of the bucket will dissolve and go into solution until the water is once again saturated.
The process of hummingbirds drinking followed by topping up can be repeated over and over, and as long as there is still sugar on the bottom of the bucket, the water will continue to saturate with sugar.
As the solid sugar supply is depleted it will take increasingly longer for the solution to become saturated, and eventually the bucket will run out of sugar and the water will become increasingly dilute.
Alternatively, if there are no hungry hummingbirds, water can still be lost by evaporation.
In this case, although there is less water, all the sugar will remain; therefore some of the sugar that was in solution will have to precipitate back out as solid sugar crystals.
The sugar in the bucket behaves in a similar way to the pool of retained – or “fixed” – P in the soil. There is a supply of retained P in contact with the soil solution P. The retained P will continue to dissolve and go into solution until the soil solution becomes saturated. As the water content increases or decreases P will continue to dissolve or precipitate accordingly.
It is a bit more complicated in that hummingbirds cannot selectively suck the sugar out of the water and plants can do that with P to a degree, but the principle is the same. That is why a crop can take up more P than is in soil solution at any given time.
Many salesmen will tell you that all the P from soluble P fertilisers becomes fixed and is lost. This is not true.
Most of the soluble fertiliser P added to soils remains like the sugar, solid but available. Only a small portion of the fertiliser P will be converted into truly fixed and unavailable P.
So despite what the salesman says, the practical implications of soluble P fertilisers becoming rapidly insoluble in the soil is good for farmers. Some things to consider:
- Retained P is not locked up and lost
- Because there is very little P in the soil solution at any one time leaching of P, as occurs with nitrate, is not a problem in New Zealand
- Once the retained P tank is filled up the amount of retained P is much, much greater than the amount of P removed, and hence the timing and amounts of maintenance P fertilisation are not critical to keep the system running
- Because soluble fertiliser P added to the soil quickly changes to an insoluble retained form, it does not matter which soluble form of P is used as fertiliser, despite what the salesman says – the cheapest is therefore the best
- Rock phosphate is not a soluble form of phosphorus. On acid soils the best quality rock phosphate is about half as good as a soluble fertiliser on a percent P basis. Poorer quality rocks are of less to no value as fertiliser
- The Olsen soil test does not measure the P in solution which would be of little use. Instead the Olsen test measures how much insoluble P (retained P) is available to come into solution (how much sugar is in the bucket) and is therefore a useful diagnostic tool, and
- As with other nutrients, clover has a higher P requirement than grasses. When P is deficient, clover is the first thing to go and with it the foundation of the clover-based pastoral system.
Editorial note – Soil P retention is sometimes incorrectly called P fixation. It is the most misused concept in applied soil science. The topic was first discussed in Fertiliser Review No 1 and 2. The message is worth repeating.
