The Dutch publication Boerderij investigated the quality of soil sampling and soil analysis. Following this, it also investigated the quality of fertilisation recommendations given by different laboratories, based on the analysed soil samples. The results of this comparison are remarkable, not to mention disconcerting.
Boerderij had 7 soil laboratories analyse 1 soil sample. 5 of these laboratories are accredited by the Dutch Accreditation Council.
As it turned out, the Pw-number (a benchmark for the phosphate fraction that is soluble in water) of this soil sample varied between 37 and 75. Yes, you are reading this correctly: 1 laboratory found twice as much phosphate in the sample than the other. An enormous difference.
The labs also have different opinions on the soil’s ability to supply nitrogen. According to Eurofins Agro (in Dutch) this is only 45 kilograms per hectare. The other accredited labs put this number at 110 to 119 kilograms per hectare. Again, an enormous difference.
Naturally, this comparison is only a snapshot. You cannot simply conclude that one lab always comes back with higher numbers than the other. However, that would hardly be an extenuating circumstance; the chaos and unreliability would only increase.
To my amazement, a supplier of soil scans, who is a major consumer of soil analyses to convert relative values to absolute values, gave a laconic answer to our findings. ‘What else is new’, as far as he was concerned. He did add that labs, in his experience, do deliver consistent results. High is always high, Low is always low. He was fine working with that.
Of course, the question remains whether or not 50 years of fertilisation advice have resulted in the optimum quantity of minerals for the crops at the right time. How precise has the fertilisation process actually been in the past years?
Against the backdrop of precision farming with variable fertilisation in exactly the right spot, the expression ‘penny wise, pound foolish’ seems very apt, by getting your soil sampled one more time elsewhere could actually pay off. This to be sure that you’re not fine tuning on the last grams of fertiliser, where at the same time your global starting point is unreliable.