There are three main groups of nutrients that affect pH: The inorganic ions (e.g., H+ and K+), which contribute positively to pH. The ratio of these ions to the total nutrient solution is called the ionic strength, and high ionic strengths cause acidity. Proteins, which contribute negatively to pH. The most important proteins for buffering in soil are legumes (clovers, vetches, etc.).
Soils with high protein content have a lower pH. Organic matter, which contributes positively to pH.
To increase or decrease the pH of my soil, you can use organic and inorganic acidifying agents to alter pH. Organic acids such as acetic acid and phosphoric acid have low concentrations of H+ and are slow acting. Inorganic acids such as hydrochloric acid and sulfuric acid have high concentrations of H+ and are rapid acting.