En la determinación del pH en suelo hay que considerar el tiempo de lectura y si realizar las medidas en reposo o agitando la mezcla suelo:agua . Este estudio comparó la medición de pH en el sobrenadante de la mezcla suelo:agua (1:2,5) luego de 15 minutos de reposo, con la alternativa de medir durante la agitación y en menor tiempo. Los procedimientos de medición de pH fueron: i) 3 minutos de agitación + 15 minutos de reposo y medición en el sobrenadante sin agitar (protocolo del laboratorio de INIA La Estanzuela); ii) lo mismo con medición en agitación de la mezcla durante 1, 2 y 3 minutos luego del reposo; iii) medición con agitación de la mezcla durante 1, 2 y 3 minutos sin reposo previo. Los valores de pH durante la agitación inicial fueron superiores a los del sobrenadante luego del reposo. Estas diferencias disminuyeron de 5,38 % a 3,91 % con tres minutos de agitación. No hubo diferencias en el valor de pH a los tres minutos de agitación con o sin reposo previo de 15 minutos. Las lecturas durante la agitación inicial de tres minutos, con tres repeticiones por muestra, presentaron un CV= 1,41 %. Medir el pH a los tres minutos de agitación sería una medición precisa y con una exactitud aceptable.

Whwn measuring the titratable acidity to pH=7 (AT_pH7) the soil is placed down with a buffer solution of pH=7 and the amount of acidity released is quantified with alkali. There is no comparative information about different analytical methods used in Uruguay. The AT_pH7 was measured by three methods: a) the mixture calcium acetate:soil was stirred and allowed a 16 hours rest, the extraction was filtered, and titration at pH = 7 was controlled by electrode (AT_pH7_fil); b) the same as the previous method, but after a 16 hours rest the supernatant was titrated unfiltered (AT_pH7_sob); c) four successive extractions with barium acetate buffered to pH = 7 were done, the extract was filtered and titrated to pH = 7 using phenolphthalein (AT_pH7fen) as an indicator. The AT_pH7fen values were considered as reference. When comparing AT_pH7_sob with AT_pH7_fil the first were considered as reference values. The fit between AT_pH7_fil and AT_pH7_sob was satisfactory. The average difference was 0.36 meq.100g-1 but differences from-1.00 to 1.70 meq.100g-1 were observed. There was no bias in the differences. Some sources of acidity may have remained in the sediment and were not evaluated when titrating the supernatant. The AT_pH_fil overestimated the average AT_pHfen values in 0.73 meq.100g 1. The average values of AT_pH_sob and AT_pHfen did not differ. The samples had the same classification according to their base saturation regardless of the method used. Neither the measurement of pH = 7 with electrode instead of phenolphthalein, nor the longest extraction time, or the use of calcium acetate as extractant introduced significant errors of interpretation. The AT_pH_fil method had acceptable precision and accuracy.