Nutritional problems are very common in greenhouses and may go undetected for prolonged periods. Over- or under fertilisation will normally result in reduced plant vigor, which may make the plants more susceptible to diseases and insects. There are many good water-soluble fertilisers available and in recent years, improved guidelines for fertilisation programs have been developed. It is now possible for growers to test the growing medium throughout the production cycle. Because these measurements can be taken in the grow rooms or in your greenhouse, it allows growers to make immediate adjustments to their fertilisation practices. The two most important measurements that can be collected are pH and EC (electrical conductivity) of the growing medium. Regular monitoring of growing medium EC and pH will eliminate almost all problems associated with fertilisation. The importance of pH and EC, as well as a method to measure them, are outlined here.
pH (ACIDITY OR BASICITY). pH is a measure of how acid or basic the growing medium is. pH is expressed on a scale from 0 to 14, where 0 - 7 is the acid range and 7-14 is the basic range. A pH of 7 is considered to be neutral. From a scientific point of view, pH is a measure of the amount of hydrogen ions that are present in a solution. This has little practical relevance form a plant nutritional point of view, since hydrogen is not a plant nutrient.
pH is important, because it affects the availability of micronutrients in the growing medium. At a high pH, some nutrients that are essential for good plant growth become unavailable and the plants will start developing deficiency symptoms. Iron eficiency is probably the most common problem that occurs when the pH of the growing medium is higher than optimal. Iron deficiency symptoms include yellowing of the leaves, especially in between the veins, and normally occur in the younger leaves of the plants. Since many micronutrients cannot be redistributed within plants, symptoms can appear quickly if the pH of the growing medium is not within the optimal range.
Almost all crops prefer a slightly acidic pH and grow well if the pH is between 5.4 and 6.0. However, some crops prefer a slightly higher or lower pH.
EC (ELECTRICAL CONDUCTIVITY). Electrical conductivity (EC) is a measure of the total amount of salts, including fertiliser salts, in the growing medium. Since the vast majority of salts in fertilizer are macronutrients, EC can be used as an indicator of the presence of macronutrients in the growing medium but gives little or no information about the presence of micronutrients. Most crops prefer a growing medium EC of 2 to 3.5 mS/cm (1 mS/cm = 1 mmho/cm = 1 dS/m = 1000 µS/cm).
The preferred growing medium EC for salt-sensitive plants is 1 to 2.6 mS/cm, while heavy feeders prefer an EC between 2.6 and 4.6 mS/cm.