Relationship between fertility extremes and growing medium pH

The pH of a growing medium increases or decreases depending on the fertilizer used, the crop grown, and the water alkalinity. However, pH change is usually gradual. When fertility levels in the growing medium are either very low or excessive, the pH of the growing medium can change rapidly. When the fertilizer level is very low in the growing medium (measured as electrical conductivity (E.C.)), there is a rapid increase in the growing medium's pH. Likewise, when the E.C. of a growing medium becomes excessive because of a potentially acidic fertilizer, the pH drops. So why does this happen and is it a concern for the crop grown?

Low Fertility = Higher pH?
Typically, when a crop is grown in a growing medium where the overall fertility level is very low (E.C. is below 0.3 mmhos/cm), the pH tends to increase rapidly by approximately 0.5 pH unit. This can be seen in Figure 1 on days 8-10, when the pH rose from 5.8 to 6.5 as the E.C. dropped below 0.3 mmhos/cm. However, if the same growing medium with the same crop is fertilized more frequently so the E.C. does not drop below 0.5 mmhos/cm, the pH changes very little. The phenomenon of sudden pH climb due to low E.C. often occurs when growers feed at very low rates, withhold fertilizer to minimize plant growth or simply do not feed at all. It is believed that this occurs because the plant has few fertilizer elements to take up from the growing medium that can generate acid through the roots. In addition, the water alkalinity and limestone in the growing medium work together to also increase the pH of the growing medium.

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