greenhouses I shade net I hydroponics I aquaponics
Delayed 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.
pH Management Crucial for Success
M
any crops have a narrow range the solution becomes. The greater the of pH tolerance. If the pH of ratio of OH- ions to H+, the more basic the soil medium falls above or the solution becomes. PH is measured below this tolerance zone, they on a scale of 1-14. A pH of 7 indicates may not grow properly due to nutrient that the H+ and OH- ions are in balance. deficiency or toxicity. A reading below 7 means that there are more H+ ions and a reading above 7 In general greenhouse production a pH of 6.2-6.8 is considered ideal for mineral soils, indicates more OH- ions. and 5.8-6.2 is recommended for peat or In horticulture traditionally a pH reading bark media. But, this largely depends upon is taken to make amendments to growing the crops grown. Poinsettias are tolerant media and irrigation water. But while of variable pH, while seed geraniums are pH is a good indicator of the availability very particular, since pH 5.7 or below can of dissolved fertilizers in the root zone, result in iron toxicity problems. it is not at all good at predicting the effect that a given irrigation source will Except for nitrates, have on the media potassium, and total Alkalinity levels indicate pH. Therefor you soluble salts, the the relative ability of the must also know the availability of most buffering capacity or media to resist or neutralize fertilizer elements alkalinity of the media is affected to some the effects of acids. The and the irrigation extent by the media higher the alkalinity level, source. Alkalinity pH. Calcium and the greater the amount levels indicate the magnesium become of acidification that will relative ability of the more available as the media to resist or be required to produce a pH increases, but neutralize the effects desired pH drop. the opposite is true of acids. The higher for iron, manganese the alkalinity level, the and phosphorus. A one unit pH drop can greater the amount of acidification that will increase the solubility of manganese by as be required to produce a desired pH drop. much as 100 times and the solubility of iron by as much as 1000 times. Lime But what is pH? Many horticultural soils require the addition of lime to correct pH and to pH measures the relative concentration supply calcium and magnesium. Lime of hydrogen ions (H+) to hydroxide ions directly affects the alkalinity of the soil (OH-). The greater the number of H+ medium. The amount of lime needed ions in relation to OH- the more acidic
16 Undercover farming I
January/February 2021
I Volume 18 No 1
depends upon these factors: the type of lime, the fineness of the lime particles and the pH, acid content, and buffering capacity of the media. Most potting soils today are amended with dolomitic limestone to supply adequate calcium and magnesium and to buffer the acid content of peat, a prime constituent of many potting soils. Other types of limestone include calcium carbonate, which supplies only calcium, and hydrated lime, which reacts very fast, but is not as long lasting. Hydrated lime is seldom used alone since it is very quick acting and short lived. It is sometimes used in emergencies to raise pH after the plants have been potted, or as a partial component of the lime additives. For greenhouse and nursery soils, the calcium carbonate or dolomite lime used should be rated as 65 mesh or better, which means that 65% of the particles are small enough to pass through a 100 mesh screen. This is often called pulverized lime. Calcium sulfate or gypsum has a neutral effect upon pH and can be used as a calcium source when no pH effect is required. The amount of lime to use depends upon the peat source. If the peat is highly buffered (acidic), then more lime is needed to have the same effect. Perlite, vermiculite and most other aerating materials have little effect on pH, but since their addition decreases the 17
