Mislead By Olsen P?
Many farmers have an obsession with the Olsen P levels of their soil. For them, maintaining or elevating Olsen P is just about the only reason for soil testing! Yet P (Phosphate) is just one of many nutrients required by the growing plant!
If P is deficient, then obviously, if the goal is to maximise productivity, an adequate level of plant available phosphorus is necessary in the soil. The Olsen P test gives a useful indication of such levels but no more than that - it is not an irrefutable determinant of plant growth response. In some cases (neutral-alkaline pH) Olsen P under-estimates plant available Phosphate, while in other cases (acidic pH) it over-estimates these levels. Olsen P also does not remain constant throughout the year as some farmers believe or expect - the same paddock sampled at different times will give different Olsen P readings. Low Olsen P readings do not necessarily indicate a problem, in times of rapid plant growth the Olsen P value can drop considerably: this is a result of plant roots up-taking large amounts of P from the soil - more Phosphate fertiliser is not required.
Clearly then, the Olsen P test result in isolation can be misleading, particularly if all one does is simply compare it to a rather arbitrary desired level (15,20,25,30 etc).
Further, up to 70% of the Phosphate in NZ soils is present in organic form. This Phosphate is released as the organic matter decomposes. The Olsen P test gives no indication of how much organic P can be made plant available. On the other hand, if soil biological activity including earthworms, fungi and bacteria is optimal and adequate environmental conditions in terms of moisture, temperature and available calcium prevail, significant amounts of the plants P requirement can be supplied from organic origins.
Take for example: assume the total Phosphate in the soil measures 1000kg/ha and 50% is present in organic form:
Inorganic P 500kg/ha + Organic P 500kg/ha = Total P 1000kg/ha
If say 5% of the organic P is mineralised, and it’s P released in the next 12 months, then the organic P can release 25kg/ha of Phosphate for plant use! That’s the equivalent of applying about 275kg/ha of Superphosphate.
Why would anyone apply Phosphate fertiliser if it’s not necessary? That does not make economic or environmental sense!
As opposed to taking into consideration just your Olsen P results, rather spend some time and money on good soil testing and find out what you already have and what other nutrients may be limiting your productivity.
