PH management and production in the greenhouse

Improper 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. PH management For general greenhouse production, a pH of 6.2 to 6.8 is considered ideal for mineral soils, and 5.8 to 6.2 are recommended for peat or bark media. Of course, this and production depends upon the crops grown. In grow media, cucumbers flourish at pH 5.8 - 6.0 while tomatoes requires a pH of 5.5 - 6.5 in the greenhouse for healthy plants. irrigation source. Alkalinity levels indicate lime. Calcium sulfate, or gypsum has a Except for nitrates, potassium, and total the relative ability of the media to resist neutral effect upon pH and can be used soluble salts, the availability of most or neutralize the effects of acids. The as a calcium source when no pH effect is fertilizer elements is affected to some higher the alkalinity level, the greater required. extent by the pH of the media. Calcium and magnesium become more availa-ble 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. What is pH? PH measures the relative concentration of hydrogen ions (H+) to hydroxide ions (OH-). The greater the number of H+ the amounts of acidification that will be re-quired to produce a desired pH drop. Lime Many horticultural soils require the addition of lime to correct its pH and to supply calcium and magnesium. Lime directly affects the alkalinity of the soil medium. The amount of lime needed 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. 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 volume of peat, propor-tionately less lime is needed. For example, if a cubic meter of pure peat required 7 kg of lime to raise the pH to 6.0, then a 50% peat, 50% perlite mixture would require only half as much. ions in relation to OH-, the more acidic the solution becomes. The greater the ratio of OH- ions 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. 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 Most potting soils today are amended with dolomitic limestone to supply Alkalinity Unlike pH, alkalinity is a measure of a solution’s ability to neutralize acids. It is the alkalinity level that determines how your irrigation water will influence the pH of the growing media. Most people who are familiar with pH would 5 + OHadequate calcium and seldom used alone In horticulture we magnesium and to buffer since it is very quick traditionally used a pH reading to make the acid content of peat. acting and short lived. It is sometimes used amendments to our growing media and in emergencies to raise pH after the irrigation water. But while pH is a good plants have been potted, or as a partial indicator of the availability of dissolved component of the lime additives. fertilizers in the root zone, it is not at all For greenhouse and nursery soils, the good at predicting the effect that a given calcium carbonate or dolomite lime used irrigation source will have on the pH of should be rated as 65 mesh or better, the media. which means that 65% of the particles are For that, you must also know the buffering small enough to pass through a 100 mesh It is always best to catch pH problems capacity or alkalinity of the media and the screen. This is often called pulverized early, before drastic steps are needed.

PH Manag ement an d Pr oduct ion 4 the amount of modifying action needed expect that high pH irrigation water would are: soil temperature, root vol-ume, tend to raise the media pH over time. leaching fraction, the buffering capacity of However, if the water has very low both the soil medium, and the irrigation alkalinity, it may not have a significant source. effect on media pH despite its high initial It is always best to catch pH problems count. By contrast, if we used irrigation early, before drastic steps are needed. water with a high alkalinity level, say When it becomes necessary to raise or 200 ppm bicarbonate a greater effect on lower the pH in the crop, it is wise to media pH can be expected. start conservatively to avoid over-shooting The higher the alkalinity of the irrigation the tolerance range in the other direction. water, the faster the root media pH will Wild pH swings can be worse than the change. Rain wa-ter sources of irrigation original problem. water normally contain little or no Raising pH bicarbonate. Well water sources often Heavy leaching tends to reduce salts range from 50-150ppm. Although some and raise pH provided the water pH is bicarbonate alkalinity is good, higher higher than the soil. This also will help levels can be toxic to some plants and in to remove any pH-related toxic levels of these cases, well water must be mixed minor elements such as manganese or with rainwater to bring the bicar-bonate iron. Hydrated lime has been used to raise into the desired range. If insufficient pH in existing crops by dissolving 1 kg bicarbonate is available naturally, then per 100 li-tres fresh water. After leaving potassium bicarbonate can be added. the mixture overnight, the clear solution Both limestone and bicarbonate raise pH is drenched onto the crop. This method by the production of carbonate. Over does not work very well with hard water time, high alka-linity water sources will sources. Potassium bicarbonate dissolved tend to increase pH, while low alkalinity at 1 kg per 100 litres will add about 600 sources will decrease it, depend-ing ppm bicarbonate to water plus 400 ppm on the rate of leaching and fertilization potassium. However, it is best not to and the nature of the fertilizers used. raise the pH too quickly due to the posGrowers with very low alkalinity levels sibility of ammonia release. Lower rates have sometimes found it useful to provide of bicarbonate (60-180 ppm or 1-3 kg. a modest background level of bicarbonate per 1000 litres) with each watering are of (60-100) ppm. By then adding small effective at raising pH over time. additions of acids to achieve the desired pH, they can dose nutrient solutions with Lowering pH sufficient buffering capacity to change or Iron sulphate dissolved at 1-4 kg per 100 stabilize the pH of the root media. litres has been used where pH is too high. The solution should be acidified enough to Changing Media pH dissolve all sulphate. Ammonium sulphate Understanding the relationship of alkalinity will lower pH slowly but very effectively to pH makes it easier to change the media due to the action of nitrifying bacteria. pH when required. Regular pH testing of The crops treated should be ammonium the root media is necessary to monitor tolerant, the pH should not be less than the condition of the root media and to 6.2, and the temperature should be above identify the need for amendment. 15oC. Several factors come into play when Fine ground elemental sulphur at 5 g per attempting pH alterations. Variables that 15 cm pot or 15 g per 2-3 gallon container can influence the rate of pH change and is recom-mended by some sources as a gradual way of bringing down pH. It is the action of certain soil bacteria that create the change, usually in about 10-15 days. Acid additions are the most common method of reducing pH. They work best if used as a regu-lar component of the feed solution to prevent the media Plant roots are exposed to the substrate solution. The root hairs, extensions of the root epidermal cells, from becoming too basic. Reducing the feed solution to are the interface between the plant and the substrate around pH 4, depending on the solution. Roots are very sensitive to pH. The acidity or basicity of the substrate solution determines how well the roots can take up nutrients and how well the alkalinity, can be effective for producing a more rapid change, plant can grow. but some nutrients may be tied

Hand held pH meters are available that gives an accurate reading of the media and solution in your greenhouse. This is an essential tool every greenhouse manager should keep at hand. Keeping record of readings is essential for tracking problem areas in the production unit as a whole.

up. Most growers use a laboratory analysis or a trial and error method to calculate how much acid to add to a solution to get a desired pH.

Residues

Wherever chemicals are used to raise or lower pH in irrigation water they may leave deposits on leaves, particularly when used in misting systems. Precipitates caused by sulphuric and phos-phoric acids are more of a problem than nitric acid, but nitric acid is more hazardous to handle and can elevate EC levels at high injection rates.

Testing pH

All commercial horticultural businesses, regardless of size, should have a reliable pH meter. Fol-low the instructions included to preserve the accuracy and life of your instrument. Solution sam-ples can be read directly, or after a few hours of settling time. Dissolved CO 2 in water supplies can cause slightly lower readings until the sample has come to equilibrium with the air. Freshly mixed samples of media should be watered and allowed to stand for 24 hours before a reading is taken to release some of the lime and fertilizers. The preferred method for testing media pH is to obtain several representative samples of a crop and to measure each separately. Multiple measurements will give greater accuracy in reading, and will show the degree of variability of pH from pot to pot. A saturated media extract or a 1:1 soil to distilled water ratio is fine for measuring pH.

Testing Alkalinity

If you have regular solution analysis performed you can usually request an alkalinity or bicar-bonate test also. Do-it-yourself alkalinity test kits are also available for low cost from scientific companies. The effects of both pH and alkalinity are important to the nutrition and root health of your crops. Understanding these principles will help to take some of the guesswork out of managing pH in the media and solution. By: J van Heerden