GREENHOUSES I SHADE NET I HYDROPONICS I AQUAPONICS
Ph is most important in greenhouse farming (Pic: Hannah)
An Explanation of pH Management
I
mproper management of media pH commonly results in poor growth and reduced plant quality in greenhouses and nurseries. The pH or soil reaction has a primary influence on the solubility and availability of plant nutrients. Many crops have a narrow range of pH tolerance. If the pH of the soil medium falls above or below this tolerance zone, they may not grow properly due to nutrient deficiency or toxicity. For general greenhouse production a pH of 6.2-6.8 is considered ideal for mineral soils, and 5.8-6.2 is recommended for peat or bark media. Of course, this depends upon the crops grown. Poinsettias are tolerant of variable pH, while seed geraniums are very particular, since pH 5.7 or below can result in iron toxicity problems. Except for nitrates, potassium, and
total soluble salts, the availability of most fertilizer elements is affected to some extent by the media pH. Calcium and magnesium become more available as the pH increases, but the opposite is true for iron, manganese and phosphorus. A one unit pH drop can increase the solubility of manganese by as much as 100 times and the solubility of iron by as much as 1000 times. pH explained The pH measure is a relative concentration of hydrogen ions (H+) to hydroxide ions(OH-). The greater the number of H+ ions in relation to OH- the more acidic the solution becomes. The greater the ratio of OHions to H+, the more basic the solution becomes. PH is measured on a scale of 1-14. A pH of 7 indicates that the H+
and OH- ions are in balance. A reading below 7 means that there are more H+ ions and a reading above 7 indicates more OH- ions. pH H+ OHIn horticulture we have traditionally used a pH reading to make amendments to our growing media and irrigation water. But while pH is a good indicator of the availability of dissolved fertilizers in the root zone, it is not at all good at predicting the effect that a given irrigation source will have on the media pH. For that, you must also know the buffering capacity or alkalinity of the media and the irrigation source. Alkalinity levels indicate the relative
12
Undercover farming I January/February 2022 I Volume 19 No 1 11
