How to rig a fertiliser experiment
The results from fertiliser experiments are particularly noteworthy for their ability to get the results required, it is worth explaining how and why. For the full details underpinning this see the FFC Bulletin article “The Fundamentals of Soil Nutrient Management, Soil Testing and Fertiliser Recommendations” [3], and also “Understanding biological / organic fertilisers using kelp (Macrocystis pyrifera) as an example" [2].
A plants response to fertiliser does not just depend on the fertiliser, its growth response is to the total amounts of nutrients supplied from both the soil (or potting mix) and the fertiliser. There is an optimum amount of each nutrient that any given plant needs to maximise yield (or any other measurement). If there are excessive amounts of nutrients it will harm the plant and suppress yield, and if there are too few nutrients yield will also be suppressed. So, if a plant is grown in nutrient deficient soil and nutrients are supplied via fertiliser, the plant will grow more, potentially a lot more if the soil is really deficient. If the soil is around optimum, applying the same amount of the same fertiliser will have no effect on yield. If the soil is already in excess, applying the same level of fertiliser will cause a decrease in yield. So, the same application of fertiliser will result in increased yield, no yield change, and a decrease in yield depending on the nutrient level in the soil used.
Clearly to sell more fertiliser, the companies selling it will want to do tests in nutrient deficient soil not soil with excessive nutrients. To pick this kind of trick up, the person looking at the experiment’s results needs to know what the nutrient level of the soil that was used in the tests and they then need to know if that is deficient, optimum or excessive, and then interpret the results on that basis. In short, always take the results of fertiliser trials with a pinch of salt.
https://www.bhu.org.nz/future-farmi...nderstanding-biostimulants-and-biofertilisers
Also what is a bio-stimulant by definition?
