Sources of error in advisory soil tests

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Abstract
Soil tests discussed are for pH, calcium, potassium, and phosphate. The variation between values of a soil sample tested at different laboratories was found to be much greater than when duplicate tests were made in the same laboratory on the same day. This was due partly to a factor that was constant between laboratories and partly to a factor that fluctuated. The fluctuating factor must have been caused by long-term variation within a laboratory, and this component of variation must normally be included when the precision of a soil test value is estimated. Estimates of the sizes of these components and their relationship to the soil test value are made.

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