Legumes
Much is known about how to grow the more profitable hydroponics crops – tomato, cucumber, pepper, strawberry, lettuce and cut flowers – because, after all, they' re profitable. People have done extensive work on making them more so.
Legumes can also be grown hydroponically, but there is far less experience.
Legumes have great possibilities for human food due to their double function as both grain and vegetable and because they are a rich source of proteins.
Legumes are an important group of plants that have a symbiosis with nitrogen-fixing bacteria. The bacteria, which form colonies inside nodules in their roots, provide nitrogen in ammonium form, which is then transported by the xylem toward the leaves to synthesize amino acids and proteins and; on the other hand, the plants return carbon compounds synthesized during the photosynthesis, necessary to obtain cellular energy to stay alive.
Advantage: Hydro
The intensive use of nitrogen fertilizers in traditional soil-based agriculture brings with it the risk of leaching nitrates into the soil and water. This practice also inhibits the action of nitrogen-fixing bacteria. The biological nitrogen fixation represents an economic and ecologically clean alternative to excessive chemical fertilization.
Also, hydroponics systems don't pose significant pollution risks because they are closed systems and consume smaller quantities of fertilizers to prepare the nutrient solution. The use of nitrogen-fixing bacteria in soilless culture using media or substrates can reduce the consumption of nitrogen fertilizers, decreasing the production cost to prepare the nutrient solutions.
Soilless culture is a good alternative for producing legumes of excellent quality and health, with less consumption of fertilizers, saving water at the same time.
Another advantage of the hydroponics legumes production is that it provides earlier harvest than traditional agriculture due to the fast and vigorous growth of the plants. While the green bean yield is around five to six tons per hectare in soil, soilless culture using inert substrates can produce around ten to fifteen tons per hectare.
Trials here at the Centro de Investigación de Hidroponía y Nutrición Mineral of the Universidad Nacional Agraria La Molina, Lima, Peru, show it is possible to hydroponically produce several legumes, including beans and green beans (Phaseolus vulgaris L.), and peas (Pisum sativum.)
The water stress and the salinity are two natural conditions that occur frequently in many countries of the world, mainly in developing countries. Hydroponics saves significant water and, through the use of inert substrates and the application solutions with optimum concentrations of essential nutrients, avoids increasing salinity.
Legume hydroponics and other crops high in proteins, vitamins and minerals, could be a good alternative in fighting childhood undernourishment where there is high risk of nutritional deficiencies, as occurs in Africa and Latin America.
Why nitrogen matters
Nitrogen is the mineral element that plants require in the greatest amounts. It is a constituent of many organic compounds necessary for normal plant growth and development. Nitrogen is necessary to build amino acids, proteins, coenzymes, nucleic acids, vitamins, chlorophylls and cytochromes. Nitrogen deficiency inhibits plant growth and produces smaller plants. Leaves show pale and yellow green colour, due to a strong decrease in the chlorophyll content.
Plants take up nitrogen through their roots as inorganic forms like nitrate (NO3-) or ammonium (NH4+). The main nitrate source in soil is the mineralization and nitrification of organic matter. Through this process, which requires time (usually several months) to develop, organic nitrogen is converted into ammonium which is then oxidized through nitrification to nitrate.
Mineralization and nitrification are not necessary on hydroponics crops because the nitrate and ammonium are added directly through chemical fertilizers. The main fertilizers that provide nitrate and ammonium are: ammonium nitrate, potassium nitrate, calcium nitrate, magnesium nitrate, mono-ammonium phosphate, bi-ammonium phosphate, ammonium sulphate. Also there are some commercial formulae that contain all the nutrients in optimum concentrations that the plants need.
In traditional agriculture, urea and ammonium nitrate are often the source of nitrogen. In organic agriculture ammonium and nitrate are provided by incorporating organic matter from such sources as compost, manure, humus, island guano and algae into the soil. These organic sources have to decompose and mineralize to free the nutrient minerals that form part of the organic matter before the mineral nutrients can be taken up by the plants through their roots.
There are many commercial stock solutions available to prepare nutrients for organoponics production. In nutrient solution, the nitrate and ammonium ions and certain amino acids and organic acids are available and taken up by the roots. These then are are distributed through the xylem to the different parts of the plant, where they will be assimilated into different organic compounds.
Nitrogen fixation
Legumes mainly obtain nitrogen through symbiosis with micro-organisms called nitrogen-fixer bacteria. The biological nitrogen fixation explains the fixation of atmospheric nitrogen (N2) into ammonium (NH4+,) which then is assimilated by the plant to form important nitrogen compounds.
Biological nitrogen fixation occurs in legume plants that produce grain and forage. The main grain legumes are: peanut (Arachis hypogaea), chickpea (Cicer arietinum), soybean (Glycine max), lentil (Lens culinaris), lupine (Lupinus sp), bean (Phaseolus vulgaris), pea (Pisum sativum) and caupi (Vigna unguiculata). Among the forage legumes are alfalfa (Medicago sativa) and clover (Trifolium sp).
Nitrogen-fixing bacteria live and develop into nodules, special organs located into the roots of the host plant. Nodulation is the result of the infection caused by the bacteria when they penetrate into the roots. Symbiosis among legumes and bacteria is not obligatory; legume seeds germinate without any bacteria association and plants can remain without forming symbiosis during their life cycle.
The more common type of symbiosis occurs between members of the Leguminosae family and the soil bacteria of the Rhizobium, Bradyrhizobium and Azorhizobium gender, all of them commonly named rhizobia. The Rhizobium generally forms nodules in warm-region legumes (alfalfa, bean, clover, lentil bean); Bradyrhizobium forms nodules in symbiosis with plants from tropical and subtropical regions (soybean, lupine, caupi, peanut). Legumes need their own specific rhizobia to nodule and nitrogen fix. For example, the rhizobia that nodule in soybean cannot do it in alfalfa, and vice versa.
In hydroponics systems
Nitrogen-fixing bacteria can be used in soilless culture, but only in systems that use media or substrates, not in water systems such as root floating or NFT. In natural conditions, nitrogen fixing bacteria can be found in some natural media like quarry and river sand. The legume plants that grow in these natural media can present nodules in their roots without inoculating bacteria to the system. This has been observed in several trials we made where there were with and without bacteria infestation treatments. Growers can use inoculants to infest the roots, commercial products that introduce selected bacteria into the soil or substrate. In this way there is a symbiosis between the bacteria and legumes, fixing atmospheric nitrogen and, consequently, increasing the crop yield. This implies the use of a renovated natural resource (atmospheric nitrogen) and the decreasing use of the chemical nitrogen fertilizers with less cost and pollution in soils and waters.
Inoculants are made up of one or more rhizobia families in an inert material (solid or liquid) that serves them as support and protection. They are sold in little plastic bags that contain a mix of peat moss with calcium carbonate to neutralize the acidity of the media. The peat moss, previously sterilized, has high water retention power and protection capacity for the rhizobia in adverse conditions. Also, there are liquid inoculants to infest seeds. Each little bag weight 200 grams (0.4 pounds) in a peat moss base, and each vial contain 150 ml (liquid inoculants). The doses will depend of each crop: for example, to infest 50 kilos of soybean seeds it will be necessary to use only one little bag, whereas two bags are required for 25 kilos of clover seeds.
Before sewing, the seeds are mixed with the inoculants. which adhere to or are absorbed by the seeds. Once the seeds have been sown the media must be watered daily with water only until they germinate. Then the seedlings must be watered with nutrient solution, the first week with the half doses and then with the complete doses until harvest. Using innoculants it is generally possible to get five to six harvest of green beans during a two and a half month growth period.
Nitrogen-fixing bacteria require anaerobic conditions to fix the atmospheric nitrogen into ammonium, a condition that is generated inside the nodules. But the lack of oxygen decreases the root's breathing. For this reason it is very important to avoid flooding the media, otherwise it could affect the nutrient availability in the root cells and, therefore, the nutrients source for the bacteria.
Nutrient solutions for legumes
In comparison to other crops, the nutrient solution for legumes doesn't vary a lot. But the formula must have more emphasis on the concentration of nitrogen and molybdenum. The nitrogen concentration must be neither higher nor lower; an optimum concentration for legumes is 100 parts per million. Other crops need at least 180-200 ppm.
Nitrogen is a constituent of many compounds in vegetable cells, therefore, the nitrogen deficiency inhibits plant growth.
Molybdenum is a component of the enzymes nitrate reductase and nitrogenase. Nitrate reductase reduces the nitrate into nitrite during its assimilation by the vegetable cell, then the nitrite is reduced to ammonium, necessary to build amino acids, which are precursors of proteins. The enzyme nitrogenase from the nitrogen-fixing bacteria transforms gaseous nitrogen into ammonia and then the ammonia produces ammonium. Therefore, the lack of molybdenum can affect these two processes. Legumes require smaller quantities of molybdenum, but it is necessary to maintain a good concentration in the nutrient solution (between 0.05 and 0.1 ppm.)
In several trials carried out here in Peru, different nitrogen levels have been proven (0, 50, 100, 150, 190 and 250 ppm) in green bean and bean crops. The results demonstrate that the higher the nitrogen concentration (190 and 250 ppm) in the nutrient solution, the higher yield the plant will produce, but with low or poor production of nodules in the roots. On the other hand, with an intermediate nitrogen level (100 and 150 ppm), the yields are also higher, but formation of nodules in the roots is strong and efficient. Lower nitrogen levels (less than 50 ppm) bring lower yields and nodule production.
These results demonstrate that it is possible to produce legumes hydroponically without affecting the biological nitrogen fixation, but it is necessary to support the nitrogen fertilization with intermediate nitrogen levels to stimulate growth and the production of nodules in the roots.
Also we have proven different levels of phosphorus (20, 40 and 60 milligrams per liter) and potassium (200, 300 and 400 milligrams per liter.) The legume production requires significant quantities of potassium; an optimum concentration in the nutrient solution must be between 250 and 300 milligrams per liter. Higher levels of potassium (more than 300 milligrams per liter) can produce calcium and magnesium deficiencies. The phosphorus is important because it stimulates the flowering and the seeds formation inside the fruit. The phosphorus plays an outstanding part in the rooting and flowering stages
The nutrient solution we tested to produce such hydroponic legumes as beans, soybeans, green bean, chickpea, etc. has the following concentration in milligrams per liter or ppm:
K220 ppmFe1.50 ppm
N130 ppmMn0.80 ppm
Ca*150 ppmB*0.50 ppm
S*70 ppm*Zn0.15 ppm
Mg*45 ppmCu0.10 ppm
P35 ppmMo0.10 ppm
*The nutrient solution concentration includes the quantities that water contains. The water we use contains approximately: 100-120 mg Ca/L, 15-20 mg Mg/L, 32-35 mg S/L, 0.3 ppm B.
If the water to prepare the nutrient solution has low levels of calcium (less than 80 mg/L), magnesium (less than 12 mg/L) and boron (less than 0.3 mg/L), the formula must be adjusted, adding calcium nitrate, more magnesium sulphate and boric acid. A third stock solution C must be prepared with calcium nitrate because if mixed with sulphate or phosphate there will be precipitation, mainly of micronutrients.
The electric conductivity of the nutrient solution must be between 2.0 and 2.3 mS/cm and the pH between 6.0 - 6.5. Everyday plants must be watered with the nutrient solution; the approximate quantity is 100 to 150 milliliters per plant, according to the weather conditions, media and age of the plants. Over watering must be avoided because it can produce flooding of the media, affecting the root breathing and plants can dead.
Yield
The green bean media yield in Peru is about 5,020 kg per hectare. With soilless culture this yield is 100 percent or more else higher (see Table 1); for this reason hydroponics is a profitable technique, and would be a good option in developing countries where there are scarcity of fertile soils and water.
Alfredo Rodríguez-Delfín is a regular contributor to The Growing Edge. He is biologist and hold a master scientiae in soil fertility from Universidad Nacional Agraria La Molina, Lima, Peru
REFERENCES
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