Module 5 Introduction
In this module, you will complete readings and exercises that will allow you to diagnose common problems in plant nutrient deficiency and learn to manage pests as it relates to designing a functional aquaponics system. You will begin by learning about plants and their growth requirements. At the end of this module, you will write a quiz that covers this content.
Upon completion of this module, you should be able to
Plants are photoautotrophic organisms, which means that they have the ability to synthesize organic molecules and grow using mineral salts and light. The process of synthesizing food from carbon dioxide and water using light as a source of energy is called photosynthesis. Oxygen is released as a byproduct of this reaction. As a part of the biosphere, the plants are playing a major role serving as a food source for the rest of the living organisms on the Earth. The plants maintain oxygen level and prevent accumulation of carbon dioxide in atmosphere mitigating greenhouse effect.
In order to grow, the plants need certain conditions including light, enough moisture, appropriate temperature, and minerals salts. The important role of agronomists is to provide plants with necessary conditions in order to ensure their optimal growth.
In aquaponics, the plants become a critical component removing salts and toxic compounds like ammonium, which will otherwise accumulate in water due to fish metabolism and activities of microorganisms to maintain good water quality. On the other hand, the plants will benefit from carbon dioxide released by fish and bacteria increasing their yield. In the end, it will be the plants, which make aquaponics a truly sustainable food production system.
When you grow plants in soil-less culture instead of soil, there are some detrimental consequences and benefits. Soil, as a greenhouse substrate, is not desirable because it can increase risks for pests and diseases, and has inconsistent properties. Soil-less culture eliminates the disadvantages of soil. Water can provide nutrients at a much higher and consistent rate than soil, and therefore, soil-less crop production is preferable in a protected environment.
There are several techniques which can be attributed to soil-less culture. They can be divided into three groups depending on the specific root environment: water, air, or inert substrate, like rockwool or coconut coir. Recall from Module 1 that the most important techniques include Deep Flow Technique (DFT) (also referred to as Deep Water Culture (DWC)), Nutrient Film Technique (NFT), Ebb and Flow, Aeroponics, and substrate culture. Remember, contrary to common opinion, aquaponics is not just a soil-less technique, but rather, is a concept based on recycling nutrients. Various soil-less techniques can be successfully used in aquaponics, including those listed above.
On the other hand, the constant presence of water using techniques, like DFT, can contribute to a high humidity environment, especially if you are using aeration to supply dissolved oxygen and are growing in an environment with low air circulation.
|Required Reading In this reading, you will study the major differences between soil and soil-less crop production, basic plant biology, water quality for plants, plant selection, plant health, pest and disease control, and plant design. Somerville, C., Cohen, M., Pantanella, E., Stankus, A., & Lovatelli, A. (2014). Small-scale aquaponics food production: Integrated fish and plant farming. Rome: Food and Agriculture Organization of the United Nations. Read “Chapter 6: Plants in Aquaponics” (pp. 83-102)|
Aquaponics systems are conducive to many different plants, including leafy vegetables; vine crops, such as tomato, cucumber, pepper, and egg plant; berries; culinary herbs; medicinal herbs; flowers; aquatic plants etc. Over 60 different species and varieties of greenhouse crops were used in a study conducted by the researchers from Crop Diversification Centre South in Brooks, Alberta (Savidov, 2004). This study demonstrated that there are no limits in choosing crops for aquaponics. However, if you are seeking to make money in your operation, it is best to grow something that is going to generate the highest level of income per unit area per unit time. Considering this, culinary herbs are an excellent choice. Herbs such as basil, cilantro, chives, parsley, portulaca, and mint will generate more income than fruiting crops, such as tomatoes, cucumbers, eggplant, and okra. On the other hand, one should always consider the existing demand on the market as it is limited for high value crops like culinary herbs and considerably higher for conventional vegetables. Fruiting crops have longer culture periods and are also prone to more pest and disease problems. Lettuce and micro-greens are also excellent options for aquaponics as a large portion of the harvested biomass is edible.
Growing plants in water (hydroponics and aquaponics) is not an environment that is ideal for every type or variety of plant. For example, substrate culture will be more appropriate for vine crops or berries grown in aquaponics. Many fruiting crops were studied in aquaponics trials at CDC South and proved to be very successful when grown on coconut coir and charcoal (biochar) as soil-less substrates (Savidov, 2011). The hydroponic or aquaponic crops, which are grown for their roots, such as carrots or beets, are often incapable of forming a tap root in water. Therefore, they also need a soil-less substrate, such as perlite or coconut coir, as a growing medium.
As the development of aquaponics as a technology has progressed over the years, it became evident that aquaponics crops may not just be as successful as crops in a conventional production system, but exceedingly more productive compared to hydroponically or soil-grown crops (Savidov, 2005). One should remember, that it is not the source of nutrients—synthetic fertilizer or fish manure—but the type of plant, which determines the most successful technique.
It is important to remember that large, fast-growing crops like cucumbers, tomatoes, or okra have higher demand for nutrients, compared to leafy vegetables. Therefore, they require higher inputs of nutrients deriving from fish per square metre.
|Required Reading For a more in depth overview of some common aquaponic plant choices see “Appendix 1” (pp. 169-181) in Somerville, C., Cohen, M., Pantanella, E., Stankus, A., & Lovatelli, A. (2014). Small-scale aquaponics food production: Integrated fish and plant farming. Rome: Food and Agriculture Organization of the United Nations.|
Plant requirements for growth can be divided into two groups: climate factors and mineral nutrition. Let’s begin with the climate factors, which include temperature, humidity, light, and carbon dioxide content.
Optimal temperature is one of the most critical factors in aquaponics crop production as many greenhouse crops, such as cucumber, tomato, and pepper are tropical plants and will not tolerate temperatures lower than 16oC without a decrease in yield. On the other hand, some leafy vegetables, like cabbage family plants, can tolerate considerably lower temperatures, but are more sensitive to a heat shock. The diagram below illustrates temperature requirements of cold climate (broccoli) and warm climate (maize) plants (Hatfield & Prueger, 2015):
Therefore, it is advised to have a target greenhouse temperature of 20-24°C. This temperature range will be appropriate for both cold and warm climate plants. Temperature differential between days and nights works as a signal for plants to reproduce. Therefore, it is advised to keep the temperature difference of 2-4°C between night and day during the reproductive period of some plants like tomatoes.
Air temperature is more important than water temperature when it comes to aquaponics. However, if fine air temperature regulation is a concern in your growing situation, the water can mitigate some of the negative impacts of variable air temperature on your plants. As one example, the deep flow systems at Lethbridge College allow the plants to thrive even though the air temperature in the greenhouse gets up to 40°C; however, when the researchers try and grow using NFT under the same conditions, the plants show definite signs of heat stress and wilting. The water volume of the deep flow system in this case is acting as a temperature relief for the plants just like the cool earth is to soil-based plants growing in the heat of summer.
In general, plants can tolerate higher air temperature better than they can tolerate high temperature in the root zone.
Humidity levels fluctuate with change in air temperature and plants are also constantly transpiring, which adds water vapour to the air. Humid air or dry air can contribute to a number of problems for your plants as outlined in the table below (Peery, 2016):
|Humidity too Low||Humidity too High|
|Stunted plants||Increased foliar disease|
|Smaller leaf size||Nutrient deficiencies|
|Dry tip burn||Increased root disease|
|Increased infestation of spider mites||Edge burn (guttation)|
Vapour pressure deficit (VPD) is more accurate when determining water loss from the plant. VPD is simply the difference between the vapour pressure inside the leaf compared to the vapour pressure of the air. Low VPD means lower transpiration rate leading to decrease of the nutrients absorbed by the plants, while higher VPD leads to the plant stress due to the faster water loss and leaf desiccation. VPD is important to know because it is used to determine if air exchanges are needed and if air temperature needs to be increased in order to hold more moisture. Using the table below, aim to keep your relative humidity in the yellow range depending on the air temperature you’re growing in.
Air recirculation is an important management tool, which prevents variability of environmental conditions in the greenhouse. It is recommended to have air circulation calculated on the basis of double area size. For example, air circulation should not be less than 6,000 CFM for a 3,000 square foot greenhouse.
Light is not a limiting factor in summer; however, the lack of light in winter can negatively affect the crop yield. Therefore, supplemental light is necessary to produce maximum yield during this season. Plants respond only to a certain spectrum of sunlight called Photosynthetically Active Radiation (PAR). PAR is measured in micromoles of photons per square metre per second. Additional PAR of 100-150 micromoles of photons per square metre per second should be provided for higher yields in winter season. High Pressure Sodium lights (HPS) are most common, although grow LED lights are becoming more and more popular thanks to more availability for growers due to decreased prices.
Elevated CO2 level in the atmosphere is a growing concern for environmentalists; however, high yields of greenhouse crops cannot be achieved without enriching air in a greenhouse with CO2 as it is a main source of carbon for plants. Therefore, CO2 is supplemented in commercial greenhouses with the target level of 500-1000 ppm depending on a season. Liquid CO2 is a preferable source as it does not contain toxic flue gases like ethylene. However, CO2 burners are still used in many greenhouses.
Aquaponics provides a unique advantage over other greenhouse operations as CO2 needed for plants is produced naturally by fish and bacteria as a part of the greenhouse ecosystem in aquaponics.
The plants in an aquaponics grow bed are not different in their selection of nutrients from the plants grown on soil and in hydroponics. Plants require 16 essential elements to grow. As the table below illustrates (Troeh & Thompson, 1993), carbon, oxygen, and hydrogen exist in the highest concentrations in the plants’ tissues together, comprising 95% of total elements.
|ppm, dry wt.||%, dry wt.|
Macronutrients are essential nutrients that are present in relatively large quantities. Micronutrients are essential nutrients that are present in relatively minute quantities.
|Carbon* (C)||Chlorine (C)|
|Oxygen* (O)||Iron** (Fe)|
|Hydrogen* (H)||Manganese (Mn)|
|Nitrogen (N)||Boron (B)|
|Potassium** (K)||Zinc (Zn)|
|Calcium** (Ca)||Copper (Cu)|
|Magnesium (Mg)||Molybdenum (Mo)|
*From CO2 and H2O ** Must be supplemented
The plants are capable of absorbing nutrients by both roots and leaves. Carbon, oxygen, and hydrogen are supplied by water (H20) and carbon dioxide gas (CO2). The others are absorbed by the plant roots from the culture water as minerals in ionic form. For example, most metals are absorbed as positively charged ions or cations (molybdenum is an exception), whereas nonmetals such as phosphorous, sulfur, and boron, are absorbed as negatively charged ions or anions. Nitrogen is an exception as it can be presented by both cationic (ammonium) and anionic (nitrate) forms. There are special protein molecules called “transporters,” and membrane pores, by means of which the nutrients can move across the cell membrane. Electrochemical gradient is playing an important role in minerals uptake.
Macronutrients and micronutrients play important physiological roles. For example, macronutrients, such as carbon, nitrogen, oxygen, hydrogen, and sulfur, are necesasary for protein biosynthesis. Phosphorous is an essential component of nucleic acids and ATP, a molecule that stores energy. Metals, like calcium and potassium, play an important role in the ionic balance in plant cells. Magnesium is a part of chlorophyll, a molecule, which plays a pivotal role in trapping solar energy. Most of the microelements, such as iron, manganese, zinc, copper, and molybdenum, are part of enzymes—biological catalysts—necessary for most processes in normal plant cell functioning. It is important to remember that none of the plant nutrients can be replaced with other nutrients. Therefore, lack of just one nutrient can cause stagnation and death of the plant even if the rest of the nutrients are in sufficient amount. Liebig’s Law of Minimum states that the growth of the plant is determined not by total amount of available nutrients, but by the scarcest nutrient, which, thus, becomes a limiting factor.
Nitrogen is the single most important nutrient in plant nutrition and a major limiting factor in most agricultural systems. Higher plants possess an intricate mechanism for acquiring nitrogen from their environment. Two forms of mineral nitrogen that are essential for plant nutrition are ammonium and nitrate. Plants absorb two H+ ions per one ion of nitrate (NO3=), which drives pH up in your water. Remember that pH is the number of H+ ions available in solution, such that the greater the number of H+ ions the more acidic the solution and the lower the pH. The plants are basically reversing the drop in pH (increase in H+ ions) caused by the nitrification process (conversion of ammonia to nitrate). Thus, the plants in aquaponics continuously remove an excess of acidity that is produced in the nitrification process. This process is the most important mechanism of pH control in aquaponics. Plants use the nitrate for protein biosynthesis in a process opposite to nitrification that was carried out by microorganisms.
The macronutrients and micronutrients must be in balance in order for plants to grow well. High or low levels of any given nutrient may affect how the plants use the other nutrients. For example, “excessive amounts of potassium may interfere with the uptake of magnesium or calcium, while excessive amounts of either of the latter nutrients may interfere with the uptake of the other two nutrients” (Rakocy et al., 2006, p. 10).
Research is showing that a wide range of organic compounds in the root environment support healthy plant development. The microbial decomposition of the organic matter in the aquaponics system creates a wide spectrum of soluble organic compounds. However, only a few of them, such as low molecular weight organic molecules like amino-acids, sugars, plant hormones, and vitamins, can be directly absorbed by the plant roots. Most of the larger organic molecules, such as proteins, have to be broken down to smaller molecules and minerals in order to be absorbed by plants due to smaller size pores. Some examples of organic compounds present in the grow bed are vitamins, enzymes, coenzymes, amino acids, and hormones. Organic compounds:
Some aquaponic systems may have high organic loads, which leads to competition between the plants and aerobic microorganisms populating plant roots for oxygen; therefore, maintaining a high dissolved oxygen level is important. Fast growing plants with extensive roots will take a lot of oxygen from the water source. A low dissolved oxygen level results in a decrease in root respiration, meaning the plant absorbs less water and does not take in as many nutrients. Root cell tissue dies and the plant grows poorly. Plant root pathogens develop when dissolved oxygen is low and carbon dioxide is high.
Factors Affecting Nutrient Uptake
Various factors affect nutrient uptake: plant species and variety, environmental conditions, and composition of the medium.
First, you must consider the plant species and variety. The plants have to transpire from 200 to 1,000 litres of water to make one kilogram of dry matter. This water contains dissolved nutrients. Nutrients are retained and water is lost. Thus, the factors, which promote transpiration, such as environmental factors like temperature and solar radiation or genetic factors, for example, varieties with larger leaves or faster growing crops, will also affect nutrient uptake. Higher humidity, on the contrary, will suppress transpiration, therefore negatively affecting nutrient uptake. Vapor Pressure Deficit or VPD, is an important characteristic calculated from temperature and relative humidity. The higher VPD, the faster plants transpire enhancing nutrient uptake. Higher level of carbon dioxide in air will positively affect nutrient uptake through enhancing growth rate.
Composition and pH of the growing media are important factors of nutrient uptake. Low pH promotes uptake of nitrate, one of the most important ions in plant nutrition, as well as other anions. Slightly acidic pH, 5.5-6.5, also helps keep most nutrients in dissolved form. On the other hand, more alkaline pH favors cations uptake including metals and ammonium.
Mineral composition of nutrient solution often determines uptake of individual nutrients due to antagonistic interactions. Most common antagonisms include calcium uptake inhibition by excess of potassium, potassium and calcium uptake by sodium in saline soils, and magnesium uptake inhibition by calcium. That’s why it is important to keep certain nutrient ratios in the nutrient solutions. The remarkable advantage of aquaponics is that the more favorable balance between the nutrients establishes while the system matures (Savidov, 2005).
The plants need optimal temperature in the root zone. It means that nutrient uptake will be inhibited when the temperature is too low (usually lower than 10°C) or too high (above 28°C for most crops).
Another important factor is oxygen concentration in the root zone. The nutrient uptake requires metabolic energy in most cases. Lack of oxygen in the root zone leads to decreased metabolism in root cells and negatively affects nutrient uptake. That’s why DO level in root zone should maintained above 1 ppm.
Genetic factors of nutrient uptake:
Composition of the medium:
Crops are healthy when the water quality is good and there is a healthy population of nitrifying bacteria present in the system. There are several ways to monitor plant health including nutrient analysis of plant tissue, visual monitoring, leaf temperature, growth rate, etc.
Lack of nutrients in water leads to various physiological disorders in the plants, which are expressed in decreased growth rate and visual symptoms of the nutrient deficiency including colour of the leaves, curling, bud and flower abortions. It is important to avoid nutrient deficiency before visual symptoms appear as it will lead to decrease of yield. That’s why it is necessary to monitor nutrient content in water and in plant tissue on a routine basis.
The plants develop a mechanism to mobilize most important macronutrients including nitrogen, phosphorous and potassium from older tissue for the need of younger, actively growing tissue. However, other nutrients, most importantly, iron and calcium, cannot be immobilized. Therefore, the symptoms of iron and calcium deficiencies will be evident in younger leaves and, conversely, one should first examine older leaves to find the symptoms of nitrogen, phosphorous and potassium deficiencies.
The table below shows the mobility of various nutrients, which will help to identify deficiency of a specific nutrient.
Interveinal chlorosis is one of the most common symptoms of nutrient deficiencies (see the typical deficiency below).
(http://keys.lucidcentral.org/keys/sweetpotato/ (Links to an external site.))
Unfortunately, several nutrients including iron, zinc, and manganese cause similar symptoms of deficiency making it difficult to properly identify the deficient nutrient.
The following image shows various nutrient deficiencies in bean plant leaves only.
Below are the brief descriptions of macro- and micronutrient deficiencies.
Plants often show nutrient deficiencies symptoms during first months of aquaponics operation due to lack of some nutrients at the initial stage. Iron and calcium deficiencies are usually most common. Magnesium deficiency was also observed in a pilot-scale aquaponics facility at the Crop Diversification Centre South in Brooks. At this stage, supplemental nutrients may be required. While system matures the symptoms will gradually disappear as the content of nutrients will build up and beneficial microflora in rhizosphere will be formed. Use of digested solid fish waste as a supplemental nutrient source will compensate for the lack of some nutrients in liquid fish effluent.
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