This is the first in a series of articles discussing the Dynamic Plant Nutrition System. The goal of this series is to give a practical, working knowledge of the science of plant nutrition.
Many factors determine the nutrition of a plant. These factors are dynamic: they are continuously changing. The four main factors that we will be concerned with are: substrate (media), water, fertilizer program and plant species. Together these factors make up the substrate solution.
To begin the series we will review a few properties that influence the dynamic plant nutrition system.
pH: The pH is a measure of the concentration of hydrogen ions (H+) in a solution in comparison to hydroxide ions (OH-). The pH is measured on a scale of 1 to 14, with 1 being the most acidic (lots of H+, few
OH-), 7 being neutral (H+=OH-), and 14 being the most basic (few H+, lots of OH-). All four factors of the substrate solution influence the overall pH and the ability of plants to take up nutrients. This is because pH determines whether nutrients are soluble (in a form that can be taken up by plants).
Managing pH is challenging because not all nutrients are soluble at the same pH, not all plant species take up nutrients at the same pH and pH can change over time. While nitrogen, potassium, calcium and copper are soluble (can be taken up by a plant) over a wide pH range in peat-based substrates, other nutrients are only soluble in narrow pH ranges. Magnesium is most soluble at
a pH above 6.5. Therefore, when a pH of substrate solution is below 5.5, we might find a deficiency of magnesium in the plant tissue. Phosphorous, iron, manganese, boron and zinc are most soluble at a pH below 6.5. Therefore, when the pH of the substrate solution is above 7, we may have deficiencies of phosphorous, iron, manganese zinc, or copper in
the plant tissue.
The opposites are also tru
e, when the pH of the substrate solution is high, magnesium may be at toxic levels in the plant tissue. When the pH is low, phosphorous, iron, manganese, boron and zinc may be at toxic levels in the plant tissue.
Future articles will discuss recommended pH ranges for specific plant species.
Alkalinity: It is just as important to know the alkalinity of irrigation water as it is to know the pH. This is because alkalinity of irrigation water makes the substrate solution pH dynamic -it makes the media pH change over time. When alkalinity is high, the more water that gets added to the substrate solution, the higher the pH changes. The longer the crop season, the higher the pH climbs. Alkalinity does not mean alkaline (alkaline means that the pH is over 7). Alkalinity is a measurement of bicarbonates and carbonates in a solution that has the ability to neutralize acids. Recommended alkalinity range is between 40-120ppm CaCO3.
High alkalinity causes a buffering effect. This means that the solution resists change of pH. The higher the alkalinity, the more acid is necessary to correct a high pH. For example, if you have a pH of 7.5 and an alkalinity of 50 ppm, it will take a small amount of acid to bring your pH to an ideal pH of 6.2. If you have a pH of 7.0 and an alkalinity of 300, it will take a much larger amount of acid to bring the pH to an ideal pH of 6.2. It is important to know your alkalinity at the beginning of a crop so that you can make a plan to reduce alkalinity before substrate solution pH rises.
Electrical Conductivity (E.C.) is a measurement of the amount of dissolved salts (ions) in a solution. Ions are electrically charged molecules, [anions (⁻) and cations (⁺)]. The higher the number of ions, the more electrical current the solution can conduct, and the higher the E.C.
E.C. tells growers the amount of ions, but not specifically which ions are in the sample. While a full laboratory analysis is recommended to determine the exact composition of water and fertilizer solutions, an E.C. test can be performed quickly as part of an in-house testing program. It is important to know the E.C. of your media solution because a high E.C. indicates over-fertilization and a low E.C. indicates under-fertilization. A high E.C. can lead to nutrient toxicity and increase chances of some root diseases. A low E.C can lead to nutrient deficiencies and stunted growth.
In the articles to come the four major factors of the substrate solution will be discussed. Future articles will also review testing the substrate solution and how to interpret the test results and make corrections.
Want to learn more now? Outstanding, easy-to-understand resources can be found at:
· Substrate pH and Water Quality by Douglas A. Bailey, Paul V. Nelson, and William C. Fonteno www.ces. ncsu.edu/depts/hort/floriculture/plugs/ph.pdf
· The Pour Thru Method website: www.pourthruinfo.com
· Understanding pH Management for Container-Grown Crops by Paul R. Fisher and William R. Argo; Meister Publications
