Plant Nutrition of Greenhouse Crops
pH Management of Greenhouse Crops
Nutrient deficiencies and toxicities are often related to high substrate pH due to the use of poor-quality irrigation water. However, initial symptoms of nutrient deficiencies or toxicities are not always visible until they become a problem that increases costs and reduces quality (and, therefore, profitability). Monitoring the pH and soluble salts of growing substrates gives the grower the ability to correct issues before they become problems that damage crops. The pH of the growing substrate or media affects the availability of nutrients, especially micronutrients. Most nutrients are available at a pH range of 5.4 to 6.2 in soilless substrates; however, for each plant species there is an optimal pH range
Measuring pH
The most common nutritional problems occur in greenhouse crops when pH of the growing medium is outside the optimum range. Medium-pH is a measure of the acidity (low pH = acid) or basicity (high pH = basic, also called alkaline) of the growing medium. The pH of a growing medium is important because it affects a chain of events affecting plant health. Several nutrients are affected by medium-pH, but the most important are phosphorus and most micronutrients, especially iron, manganese, copper, zinc, and boron (which decrease in solubility at high pH), and molybdenum (which increases in solubility at high pH). The optimum range for most crops growing in a soilless medium is 5.4 to 6.2. Media pH above 7.0 results in reduced micronutrient and phosphorus solubility. Figure 18.1 shows optimal availability for many nutrients at corresponding pH levels.
Substrate Extraction Methods
As in the case of measuring EC, pH can be measured by collecting or “extracting” pore water from the container substrate and measuring the leachate with a pH meter.
pH Meters
A number of inexpensive meters are available which can be used to monitor pH and EC separately (with two instruments) or together (with one instrument). Pocket or portable meters are small in size and convenient. There are also non-portable, desktop models which are more expensive and offer more precision. When selecting a pH meter, look for an accuracy of ±0.1 pH unit and a range of 1 to 14. The best pH meters are those calibrated with at least two standards, such as a pH 4 and pH 7.
Managing Substrate pH
Growing media that are too acidic or too alkaline can cause many problems with plant growth as discussed above. Therefore, maintaining pH in the proper range for the desired use of the growing media is critical for successful production of greenhouse crops.
Lowering Growing Media pH
Growing media pH can be lowered by various methods. A moderate reduction can be achieved by switching to acidic fertilizer. More substantial corrections can be accomplished through the applicationof acid fertilizers, drenches of iron sulfate or sulfur, or acidification of the irrigation water.
Selecting a Fertilizer to Correct pH. A high media pH can be corrected by switching to a more acidic fertilizer. The most acidifying fertilizers are ammonium sulfate and monoammonium phosphate (MAP), followed by diammonium phosphate (DAP). Less acidifying fertilizers are urea, ammonium nitrate, and anhydrous ammonia.
Iron Sulfate Drench. Drenches that can be used to lower the growing media pH to the desired level include iron sulfate, aluminum sulfate, and flowable sulfur. Once the pH level has been corrected the drenches are discontinued. At that point remedial or preventative action is taken to ensure that a future pH decline does not occur. This may include switching to a more basic fertilizer if fertilizer was the problem. If the problem stems from high water alkalinity, continuous injection of acid into the water system will control the problem.
Acidification of Water. Using a fertilizer injector to add acid to the irrigation water directly reduces the alkalinity. A hydrogen ion from the acid will combine with a bicarbonate and carbonate to form carbon dioxide and water. Different types of acids can be used, including: sulfuric, phosphoric, nitric, and citric. Typically, a grower will add enough acid to reduce the pH of the water to 5.8. Acid selection, rate, and methods of injection were covered in Chapter 16, Irrigation Water for Greenhouses.
Raising Growing Media pH
When pH is only moderately low, switching to a more basic fertilizer generally solves the problem. When the pH decline is more severe, that is, when it falls below 5.4, a growing media drench with flowable limestone (calcium carbonate, CaCO3) or potassium bicarbonate may be needed. As previously mentioned, a starter lime charge can be added based on the growing media components used in the mix and the desired starting pH.
Flowable Lime and Potassium Bicarbonate Drenches. Consider substrate drenches with either flowable lime or potassium bicarbonate to raise the pH. Several factors affect the choice between flowable lime versus potassium bicarbonate. Both materials are incompatible with all types of water-soluble fertilizer and other chemicals and need to be applied by themselves as a soil drench. Flowable lime has a more predictable and stable effect on medium-pH, without increasing medium-EC. Potassium bicarbonate is easier to apply; however, and should be used on flood floors or when applied through low-volume drippers.
Adding Limestone to Growing Media. While there are several post-planting strategies to alter substrate pH, pre-plant adjustment with limestone affords growers a great opportunity to accommodate species-specific requirements in the greenhouse. As previously mentioned, plants vary in their pH requirements. Many greenhouse crops grow well at a pH from 5.8 to 6.2. However, there are a number of species that require lower or higher substrate pH values. Sphagnum peat moss is the primary component for most greenhouse substrates. With a pH of 3.0 to 4.0, it is too acidic to use without amending for production. When other components such as perlite, bark and/or vermiculite are added to peat to produce the substrate mix, the pH will still be too low and will then need to be raised so it is within a range acceptable for plant growth. For soilless substrates used in containerized crop production, adjusting the pH of growing substrate is accomplished by adding limestone.
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