What You Need to Know About pH

The pH is the degree of acidity, reflecting the concentration of H⁺ ions in a solution, and is expressed on a logarithmic scale from 0 to 14. While a pH of 7 is neutral, a low pH of 0 is very acidic while a high pH of 14 is very alkaline.

Most edible crops grow well at a pH of between 5 and 6.5.

Why is pH Important?

The pH of the substrate influences nutrient uptake by the plant. For example, a high pH limits the uptake of iron, which generally leads to leaf yellowing and production loss.

Substrate pH can be manipulated by the nitrogen source.

The use of urea, ammoniacal nitrogen, sulphates, and phosphates leads to lower pH and conversely, nitrate fertiliser produces a higher substrate pH.

The pH of the water supply is important as it directly affects the solubility of fertilisers. It can also cause precipitation in systems leading to blocked drippers, which causes more problems.

Bicarbonate Levels are Key

The amount of bicarbonate in the water is key as this reflects the buffering capacity. The less bicarbonate in the water the weaker the buffer capacity — which is preferable. The pH of soft water (containing low levels of bicarbonate) fluctuates easily, while for hard water (containing a high amount of bicarbonate) the pH is stable and more difficult to manipulate.

To judge irrigation water properly it is important to know the pH as well as the amount of bicarbonate. High amounts of bicarbonate in the irrigation water will increase the pH of the substrate during long crop cycles.

By choosing acidifying nitrogen sources, the we can compensate for the pH substrate effect.