Choosing a soil laboratory

All soil testing laboratories are not the same. You will often find that two laboratories will give totally different results and recommendations on the same soil sample. How do you choose? The two things that you must look for in choosing a laboratory are the analytical methods used and the quality control system in place. 

There are many different methods used to determine the amount of available nutrients and they do not always give the same description of the soil. Some methods have been developed for use under a limited range of soils. For example, the Troug method for analyzing phosphorus works well on acidic soils, but gives false results on alkaline soils.  There are many regional laboratories that target growers in a fixed area with one general soil type. They choose the simplest, quickest analytical methods that will give reliable results on that soil. If you send a sample from another area with a different soil type, the results may be misleading.

 Other laboratories, such as Agro Services International, do not target any one soil type and must therefore use methods that are applicable over a wide range of soils. These methods are more tedious than those used by regional laboratories, but they must be used in a laboratory that accepts samples from a wide geographic range or from regions with variable soil types.

 How do you know if your laboratory uses suitable methods? Ask them if their methods have been calibrated with the soil types that you are growing on or if calibrations were done with soils completely different from yours. You may be surprised to discover that many laboratories cannot answer that question.

 Even if your laboratory uses suitable methods, you need to know if there is a strict quality assurance program in place. Errors can occur and the laboratory must have procedures that immediately detect if something has gone wrong. For example, at Agro Services International, all of our instruments are regularly checked and calibrated using certified analytical standards. Also, in every batch of analyses,  we include standard samples, that is, samples with known nutrient levels. If the results from these standards do not match the known values, ALL results are thrown out and the entire process is checked until the source of error is found and corrected.

 Remember that at the end of the day, the purpose of soil  analysis is to predict the nutrient needs of crops growing on your soil. If you are getting clear responses to a nutrient, but your laboratory says you should not have problems with that nutrient, it is time to look for a new laboratory.

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