pH and Compost

What is pH?
pH Factor means potential Hydrogen and measures soil’s acidity or alkalinity. The scale runs from 0 to 14 where 7 represents a neutral soil. Numbers below 7 indicate the degree of acidity, and numbers above 7 indicate the degree of alkalinity. The pH scale is logarithmic and each jump is a ten fold increase. A soil with a pH of 4 is 10 times for acidic than one with a pH of 5 and 100 times more acidic than one with a pH of 6.

Composting and the pH Factor
The microorganisms in compost operate best under neutral to acidic conditions with the pH in the range of 5 to 8. In the beginning stages of decomposition, organic acids are formed. Conditions that are acidic are best for the growth fungi and breakdown of lignin and cellulose. As composting goes on, the organic acids become neutralized. “Mature” compost generally has a pH between 6 and 8.

If there is not enough oxygen (anaerobic), the acid accumulation can lower the pH to 4.5 and limit microbial activity – thus slowing down the composting process. Aeration is needed to return the compost pH to an acceptable range.

Compost and Plants
Most garden plants do best in a slightly acidic to neutral soil – around pH 6.5 to 7. Compost can modify and stabilize pH. When compost is added to the soil, it acts as a chemical buffer and increases the plants tolerance to pH. Adding compost to a slightly alkaline soil will make it more neutral.

pH, STA Certification, and Aguinaga
Each month, Aguinaga sends a sample of compost to the US Composting Council where the pH and other properties are tested. By passing these lab tests every month, the US Composting Council awards us with the Seal of Testing Assurance (STA Certified.)