In hydroponics, since there is no soil to act as a pH buffer, maintaining the right pH range is critical. Any pH change results in a response by the plants since they will not be able to easily control the pH around them.
— Graves 1983; Jones Jr. 2005; Resh 2013

Hydroponic vs. Soil Mediums:

Soil composition determines the pH of the root zone under normal growing conditions and acts as a pH buffer to maintain an adequate range Campbell, N. A. et al. 2002 (1). However, in hydroponics, maintaining this pH range is critical because there is no soil to act as a pH buffer. Therefore, any pH change results in a response by the plants since they will not be able to easily control the pH around them Graves, C.J. 1980 (2), Jones Jr., J.B. 2005 (3), Resh, H.M. 2013 (4).

Traditionally, the ideal hydroponic fertilization solutions used in greenhouse plant production are obtained by monitoring solution pH and electrical conductivity (EC) Kupers, G. et al. 1992 (5), Morimoto, T. et al. 1992 (6). The EC value only indicates the total strength of the solution so it is limited in its ability to manage the nutrient balance in hydroponic solutions Parks, S. et al. PDF 2011 (7).

Ideal Hydroponic pH ranges are different than soil pH ranges:

Hydroponics: 5.5 – 6.5
Soil: 6.2 – 7.2

The pH level will vary from plant to plant, but typically most plants prefer a slightly acid growing environment. A pH range of 5.5 to 6.3 is ideal for hydroponic or soil-less culture systems using inert mediums; i.e. mediums with no nutritional value (perlite, coco coir, expanded clay, rockwool, vermiculite, etc) or no medium at all (aeroponics, deep water culture, NFT etc.) Hussain, A. et al. PDF 2014 (8). Nutrient deficiencies can occur below pH 5 and above 7.5, due to the pH affecting the availability of some nutrients Parks, S. et al. PDF 2011 (7). Figure 1 gives you a good idea of the availability of nutrients for plant growth as pH changes. 

Figure 1: pH and nutrient availability in hydroponic systems:

                                            Pavel. 2016. 

                                           Pavel. 2016. 

Monitoring and adjusting pH:

As with soil, pH measurement of water samples is an important part of a hydroponic water testing program. The right pH is crucial to the health of the plants. It is adjusted when the nutrient solution is added to the water. Therefore, carefully maintaining the proper strength of the nutrient solution through the plants growing stages is critical Christie, E. 2014 (8):

  1. Start hydroponic seedlings with a diluted concentration of approximately ¼ strength.
  2. As plants grow, the nutrient strength increases to full strength. The nutrient solution must be replaced every two to three weeks.
  3. The pH of the nutrient solution will have a tendency rise (from acid to alkaline state) as the plants absorb the nutrients from the  solution. As a result, the pH needs to be checked periodically and adjusted if necessary.
  4. At the beginning, you should check pH on a daily basis. The pH can change at different rates depending on the type of  hydroponics system used, including the type of plant, its age, the growing medium used and weather.
  5. The pH should be initially adjusted, when mixing fresh nutrient solution, to pH 5.5  and allowed to ‘float up’ to no higher than  pH 6.3 before re-adjusting.
  6. When the plant is ready for flowering, the nutrient formula changes to a formula that favors flowering.
  7. Always adjust the pH of your nutrient solution AFTER adding all additives and nutrients to the mix.
  8. If the pH shifts drastically, this could be a sign that your plants are under stress (root rot, heat stress, insect attack etc).

The pH can changed at different rates depending on the type of hydroponics system used, including the type of plant, its age, the growing medium used and weather. The optimum pH range for soilless culture nutrient solution is between 5.8 and 6.5. Reviewing pH ranges in Figure 2, a pH of 6 is within the ideal range for most plants Hussain, A. et al. 2014 PDF (9).

Figure 2:  Ideal growth pH ranges for selected hydroponic plants:

 Hydroponic Food Production by Howard M. Resh Woodbridge Press, 1987

Hydroponic Food Production by Howard M. Resh Woodbridge Press, 1987

Hydroponics and water pH Test Kit Instructions:

  1. Nutrients must be added to the water before checking and adjusting the pH of your solution. Due to their chemical composition, fertilizers tend to lower the pH of the water.
  2. After adding nutrient and mixing the solution, check the pH.
  3. Use the above pH range guide for selected hydroponic plants (Figure 2).
  4. The kit contains a Sample Collection Vessel (with blue cap) and a ruler. Using the ruler, measure the Water (W) line on Sample  Collection Vessel.
  5. Tear off a 1-inch strip of the pH paper and insert in the water; then, immediately remove from the liquid. The color will change  instantly.
  6. Hold paper against the color chart shown in the inside lid of the box and match colors for soil pH.
  7. If the pH needs to be adjusted, add the appropriate pH adjuster*. Begin by using small amounts of pH adjuster until you become  familiar with the process.
  8. Recheck the pH and repeat the above steps until the pH level is brought to the desirable level.  This is a key step so, after  practicing few times, you will know what to do in the future. 

*Several types of chemicals are available to adjust pH. The most popular are phosphoric acid (to lower pH) and potassium hydroxide (to raise pH). Both are relatively safe, although they can cause burns. We recommend “diluted” pH adjusters found in most hydroponic stores. “Concentrated” adjusters are hard to manage and can cause difficulty in making pH adjustments.


References:

  1. Campbell, N.A. and J.B. Reece. 2002. Biology. 6th ed. San Francisco, CA: Benjamin Cummings
  2. Graves, C.J. 1980. The Nutrient Film Technique. Horticultural Reviews 5: 1-44
  3. Jones Jr., J.B. 2005. Hydroponics: A Practical Guide for the Soilless Grower. 2nd ed. Boca Raton, FL: CRC Press
  4. Resh, H.M. 2013. Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial  Hydroponic Grower. 7th ed. Boca Raton, FL: CRC Press
  5. Kupers G., van Gaalen J., Gieling T.H., van Os E.A. Dirunal changes in the ion concentration of the supply and return water of a  tomato crop grown on rockwool. Acta Hortic. 1992;304:291–300
  6. Morimoto T., Nishina H., Hasimoto Y., Watake H. Sensor for ion-control—An approach to control of nutrient solution in  hydroponics. Acta Hortic. 1992;304:301–308
  7. Parks, S., Murray, C. 2011 Leafy Asian vegetables and their nutrition in hydroponics. Horticulture Australia Limited and Ausveg
  8. Emerson, C.C.. 2014. Water and Nutrient Reuse Within Closed Hydroponic System. Electronic Theses & Dissertations. Paper 1096. Georgia Southern University
  9. Hussain, A., Iqbal, K., Aziem, S., Mahato, P. Negi, A.K. 2014. Review On The Science Of Growing Crops Without Soil (Soilless  Culture) – A Novel Alternative For Growing Crops. International Journal of Agriculture and Crop Sciences. IJACS/2014/7-  11/833-842