What you need to know about pH

In fertigation fed crops the pH of the water supply and, even more importantly, the pH of the substrate/soil has a major impact on overall success.
pH is the degree of acidity, reflecting the concentration of H + ions in a solution, and is expressed in a logarithmic scale from 0-14.  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 to 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, ammonical nitrogen, sulphates, and phosphates leads to lower pH.  While conversely, Nitrate fertilizer produces a higher substrate pH.

Meanwhile, the pH of the water supply is important as this directly impacts the solubility of fertilizers. In addition it can cause precipitation in systems, leading to blocked drippers, which causes additional problems.
  
Bicarbonate levels are key

However, 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.

So 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 pH substrate effect can be compensated.  However, it is better to prevent a rising pH by taking out bicarbonate using an acidifying product such as Nova PeKacid as the starting point.  

By Scott Garnett
Senior Agronomist