Is pH of greenhouse soils measured adequately?

We measured the soil pH on fourteen soil samples collected from greenhouses under intensive fertilizer application. All the soil samples contained carbonate, but the pH values measured by conventional method were too low in view of carbonate equilibrium. We examined the effect of the soil to water ratio, shaking time and standing time and found that the pH gradually increased as the standing time was prolonged. It reached a maximum after 192 to 264 h of standing and then decreased. The maximum pH values were fairly close to those that are calculated from the thermodynamic equation based on carbonate equilibrium.