AVRDC Non-Circulating Hydroponic System

By Hideo Imai

SUMMARY

Thee AVRDC soil science unit developed a noncirculating hydroponics system which requires neither aeration, nor any check of pH, EC, temperature or nutrient. Once the facility is established, the only input required is the addition of nutrients once every three weeks. Composition of nutrient solution need not change for each crop.

During 1986, soybean, sweet pepper, large-fruited tomato, melon, and cucumber were successfully grown during the summer. Yields of the latter three were 3.1 kg (average fruit weight: 150 g), 2.34 kg, and 4.72 kg/plant, respectively.

INTRODUCTION

Hydroponic plant culture can be superior to soil culture in many respects: (1) land is unnecessary, only water must be available; (2) No hazard from continuous cropping; (3) chemical free product; (4) shortened growth period; and (5) high yield and better quality.

Modern hydroponic systems are fully computerized and automatically control pH and electric conductivity (EC) adjustments, air and solution temperature measurement and adjustment and if necessary, automatic nutrient analysis and compensation, and a C02 supplier. Various systems to provide aeration and nutrient have been developed: pumping, circulation, flowing, misting, dripping, etc.

While the demand for a substantial increase in vegetable production in the humid tropics has arisen worldwide, it is well known that vegetable production is very low in the humid tropics. There are many reasons for this shortfall: the wide distribution of infertile and problem soils; the high rate of disease and insect infestation; poor soil and water management; the lack of simple but effective technologies to overcome these constraints; and probably the worst, the lack of funds and trained personnel.

Hydroponics would appear to be the very system to overcome many of these constraints. However, the utilization of hydroponics is still limited to institutes involved in scientific research and some cash vegetable crops because of the costly facilities required, their high running cost and the need for highly educated personnel. Therefore, if a hydroponic system which does not require either costly facilities, high running cost or highly educated personnel could be successfully developed, it would contribute to a substantial increase in vegetable production in the humid tropics.

The objective of this research project is, therefore, to develop a hydroponic system which requires neither costly facilities nor intensive care, and is easily operable by farmers in the tropics.

CURRENT HYDROPONICS AND THEIR CONSTRAINTS

Hydroponic systems are generally categorized into either the batch system or flowing or circulating system. The first type is commonly referred to as water culture, while the other is named according to the medium used as a sand, gravel, rockwool, vermiculite, peat moss, etc., culture. Whether or not the system adopted is successful depends on the extent to which it can meet the following requirements:

1) avoid a great fluctuation of nutrient concentration in the culture solution;

2) maintain the solution pH in the range of 5 to 7.5;

3) avoid a sharp increase in solution EC (below 2mmohs);

4) maintain an adequate solution temperature; and

5) provide a continuous oxygen supply.

In the batch system, pH and EC adjustment, and nutrient and water compensation are frequently required. The circulating system, which is generally installed with automatic pH and EC adjustment equipment, requires more space and facilities, and also has a very high running cost.

In summary, the major constraints of present hydroponic systems are the necessity for daily control of solution pH, EC, nutrient concentration, and the continuous supply of oxygen.

PRINCIPLES OF NONCIRCULATING HYDROPONICS

1) Background

The concept of noncirculating hydroponics was developed through the cultivation of tomatoes on beds raised one meter high. Although water consumption of these tomato plants grown on high beds was only one-sixth to one-tenth of those grown on low beds only 25 cm high, the former produced a higher fruit yield than the latter.

In addition, the root systems were totally different from each other. Plants grown on the low beds were totally different from each other. Plants grown on the low beds were less tolerant to water stress than those grown on the high beds. This suggests that the root systems of plants are easily adaptable to its water environments. If this is true, then two root systems with different functions -- one for oxygen uptake and another for water and nutrient uptake -- can be developed from the same plant. Thus, the most serious constraint in hydroponics, that being aeration, can be overcome.

2) How to Develop Diverse Root Systems

a) Seedlings

The seedlings must be very healthy with active rooting. To minimize transplanting shock, soil should not be used as the seedling media. Instead, smoked rice husks (SRH) and vermiculite or their mixture arehighly recommended as appropriate seedling media.

b) Netting

To separate roots responsible for oxygen uptake from roots for water and nutrient absorption, a net is set at 12 to 15 cm below the top cover where plants are held (see fig 3. 1). The roots of the plant are initially forced to grow inside a wide open area between the top cover and the net. After the roots reach the net, they start growing laterally and branching vigorously. Thus, a sizable portion of roots spread above the net before passing through to reach the nutrient solution below. The roots above the net require two to three times as much oxygen as those below the net and since their main function is aeration they are called "O roots."

The success of this noncirculating hydroponic system is dependent on the speed and quantity of O roots developed. The net setting is a key technique for this system and it performs two important roles: first, in the separation of the O roots from the water nutrient roots (WN roots) and secondly, in the acceleration of lateral root growth and branching.

WATER MANAGEMENT

Water management is another essential technique to properly develop the two different types of roots. During the period from one to two weeks after transplanting, the water level of the solution must be kept 5 to 10 cm above the net to immerse the seedling roots completely. (The period of immersion and the water level change based upon the growth stage of the seedlings.) The water level should be decreased as plants grow and ultimately be set at 1 to 2 cm below the net until harvest.

In the noncirculating system, however, WN roots should elongate only to a depth of 15 cm into the solution due to the limited amount of dissolved oxygen. The amount of water absorbed by plants increases greatly with plant growth, hence the water level recedes sharply daily from the fruit enlarging stage to harvest. WN roots which have stopped elongating because they have reached the nutrient solution start growing once again as the water level recedes. Although the roots below the net elongate, the length of the roots immersed in the solution remain almost the same. The portion of the roots which is increased between the net and the solution are principally O roots.

We have confirmed that after the roots were allowed to elongate by decreasing the water level for one week, and then water was added to return the water level to the original height, that within a couple of hours the tomato plants started to wilt and never recovered. The tomatoes were suffocated by the water. When WN roots are exposed to air, they take only 2 to 4 days to convert to O roots. Therefore, when crops with vigorous top growth (especially tomato) are cultivated by this system using a small container under high population density and high temperature, it is recommended to install a simple water sensor, or some other device to automatically water and thereby keep the water level constant.

3) Nutrient Supply, pH and EC Control

a) Nutrients

Unlike current hydroponic systems, the final goal of our noncirculating system is to simplify all of the procedures as much as possible. Thus, the following principles were tested.

  1. A nutrient solution with the same composition is used for all crops.
  2. The frequency of replenishment of the nutrient solution and/or nutrient supply (fertilizer) is dramatically reduced.
  3. A locally available fertilizer mixture is used as the sole/major source of nutrient.
Regarding principle (1), various types of crops have been grown satisfactorily with the same nutrient solution in different seasons, i.e. tomato (processing and fresh market), melon, cucumber, strawberry, sweet pepper and vegetable soybean. Therefore, the same nutrient solution can be used for most vegetables.

The key to principle (2) depends on the container size, that is, the amount of nutrient solution stored. When, for example, 24 tomato plants were cultivated in a 200 x 150 x 50 cm3 large container (1500 kg of solution), fertilizer must only be applied every three weeks during the first month after transplanting, after which time frequency is increased to once every two weeks until harvest. When a 50 x 40 x 20 cm3 small container (40 l) is used, however, fertilizer must be added once a week, and in summer cultivation, twice a week.

Concerning principle (3), compound fertilizers produced by the Taiwan Fertilizer Co. have been tested (No. 1 [N-P2 o5-k2-0=20-5-10]) and No. 43 [N-P2 05 -k2 O-MgO=15-15-15-4] mixture). Although acceptable yields were obtained in tomato, melon and cucumber with this fertilizer (containing the compound fertilizer mixture plus micronutrients), this fertilizer mixture was apparently inferior to a nutrient solution specially prepared at AVRDC. (See fertilizer section which follows for the composition of the AVRDC solution).

b) pH and EC

Since the variation in the pH of the solution during one month of cultivation is within one unit, practically no pH adjustment is needed with the AVRDC noncirculating system. In circulating systems that constant pH adjustments suggests aeration or solution circulation (oxygen supply to the roots immersed in the solution) accelerates root growth into the solution, and consequently, a substantial increase of root mass in the solution results in a sharp increase of nutrient uptake by plants (especially N), hence resulting in a great change in pH. On the other hand, in the noncirculating hydroponic system, root elongation is limited only to a depth of 15 cm into the nutrient solution due to the limitation of the oxygen supply, thereby minimizing both nutrient fluctuation and change in pH.

Also, no control of EC is required in the noncirculating system.

ESTABLISHMENT OF A NONCIRCULATING HYDROPONIC SYSTEM

The minimum requirements for this system are as follows: 1) Container: Three container prototypes -- one suitable for commercial production and two for home production.

For commercial production, containers should be made of bricks or a similar material. It is desirable to construct the container as deep as 50 to 60 cm below the surface in order to keep the solution temperature low. The bottom must not necessarily be constructed of bricks. According to our experience, a 0.3 mm plastic sheet is strong enough to avoid leakage of nutrient solution from the bottom for a year.

For home production, a plastic container (any shape) available locally can be used. This is sufficient for two melons, cucumber, tomato plants and four sweet peppers etc. per container.

2) Net: This is the most important component of the physical structure of the system. As is illustrated in figs. 1b and 1b, wood is used only for the skeleton of the rack to strengthen the structure. Light materials such as polystyrol are used for both the top cover and sides to reduce the weight and to keep out heat and light. The bottom of the frame is netted. With this structure the O roots are forced to grow inside an open area between the top cover and the net. The polystyrol used for the top cover and sides also aids in the development of O roots by maintaining the level of humidity in the open area relatively high.

For home production, since the rack floats on the surface of the nutrient solution, it is entirely made of polystyrol, thereby automatically adjusting to the water level.

The netting used in this system is the type commonly used for the cultivation of tomato and Chinese cabbage seedlings. The mesh size of the net is important and the net with 3mm x 2.5 mm is recommended for most vegetables. As previously explained, the net plays two important roles. In order to support healthy plants throughout the growth period, the most important technique is how and where to net.

3) Seedling bag: Seedlings prepared in SRH or vermiculite and placed in the nutrient solution are generally strong enough to minimize transplanting shock and to shorten the period of establishment. During summer cultivation however, seedlings sometimes do not grow well in a greenhouse due to high temperature which causes a sharp decrease in the amount of dissolved oxygen in the solution and a great respiration loss of carbon assimilates. The seedlings are damaged before their roots reach the net and start vigorous elongation and branching. To avoid this hazard, we have developed a net bag seedling culture.

A bar, 8cm in diameter and 15 cm in length is made from the same net that is used for the netting. It is packed fully with vermiculite or SRH and seeds are grown directly on it. They are then cultivated with the culture solution for 3 to 4 weeks. Since after 3 to 4 weeks many roots have already passed through the net bag, the seedlings are transplanted together with the net bag and allowed to grow until harvest. Initial growth retardation in the summer cultivation is completely overcome by this net bag seedling culture.

a) Fertilizer: As mentioned during discussion on nutrients above, a commercial fertilizer mixture tested was inferior to the AVRDC solution which was specially prepared. A solid compound fertilizer and BASF (Foliar Nitrophoska) are also used in this system.

Simple net house: For commercial production, the AVRDC noncirculating system should be placed inside a simple net house to prevent insect and disease infection. An iron pole is the best material to use as a framework for the net house. PVC pipes reinforced with cement, also provide a strong structure. Although bamboo may be easily available locally, is not strong enough to resist typhoons. The roof consists of a clear plastic sheet. The sides are completely surrounded with a fine net to keep undesired insects out and desired ones such as bees, in. The construction cost using iron poles for a simple net house of 18 x 3 x 2.5 m was about U.S. $1,400.

AUTOMATIC WATER (OR SOLUTION) SUPPLY DEVICE

a) Floating ball valve

This device is commonly used in the water storage area of a household toilet. In the system, three different root systems, each with different functions, are developed, namely, O roots, nutrient roots (N roots), and water roots (W roots).

The nutrient solution is kept in a plastic container (50 x 15 x 15 cm3) and put inside a large wooden box. A plastic basket with a net at its bottom was used as a netting rack. Initially, the nutrient solution is applied, not only inside the container but also outside it, within the wooden box up to the level at which seedling roots are completely immersed in the solution. Due to the net at the bottom of the basket, first, roots develop uniformly in the basket, then pass through the net into both outside and inside the solution container as the level of solution diminishes. A floating ball is set at 3 to 4 cm below the top of the container, in order that when the solution level recedes several cm below the present level, water is automatically supplied from a water reservoir to return it to the desired level. Thus, the solution outside the container is gradually diluted with water and finally, turned into pure water. At the same time, the nutrient solution is successively supplied as needed into the container; consequently, N roots are separated from W roots.

This work has been undertaken solely at AVRDC and will be formally reported in the AVRDC 1986 Progress Report. (Ed. note: I am not familiar with AVRDC, don't know who they are, or where this article came from, I am ashamed to say.)

There are two advantages in this system:

  1. Because water is mainly absorbed through W roots, uptake of the nutrient solution by plants is much less than in other noncirculating systems, and hence, decrease the frequency of the nutrient supply.
  2. Since both W roots and N roots are floatiny on the surface of the solution, sufficient oxygen can be also supplied to these roots; therefore, plants show very vigorous growth even under summer greenhouse cultivation.
b) Water Sensor

A precise, but inexpensive, water sensor is now available. One Taiwan-made set costs only US $40. A Japanese-made product is also on the local market for US $40. The solution level can easily be controlled by this sensor at any height desired. This system has already successfully produced more than 3 kg of large fruited tomato (average weight 130 g) during midsummer cultivation.

GROWTH PERFORMANCE AND YIELD OF SOME VEGETABLES USING NONCIRCULATING HYDROPONICS

a) Vegetable Soybean

Plants which produce more protein than carbohydrate require considerable oxygen for their growth. Soybean accumulates nearly 40% of its protein from its seeds, and yield loss by flooding is enormous because of its high demand for oxygen.

In noncirculating hydroponics, however, soybean growth was very vigorous throughout the growth period, and no flood damage was apparent. It was noted that nodules were formed only on O roots. Vegetable soybean cultivar G 9038 was transplanted on 12 March 1986 and harvested on May 5, 1986. Thirty 10-day-old seedlings were transplanted to a wooden container (90 x 120 x 40 cm3, holding 400 l of nutrient solution, hereafter referred to as nutrient container) while 56 plants were grown in a large concrete container (2 x 1.5 x 0.6 m3, 1500L, referred to as large container) which was placed in a simple net house.

When fertilizer was supplied at R5 stage, the nutrient solution accidentally leaked out and this resulted in the drying up of fully developed leaves in the large container. Although soybean yield was higher in the medium container than in the large container, seed number per pod was extremely low because mutual shading of leaves came to be very serious at R4 stage due to vigorous top growth.

b) Cucumber

Cucumber has been considered to be very sensitive to all external factors and difficult to grow in hydroponics. The optimal range of solution pH is narrow (from 6.5 to 6.7) while its nutrient uptake is great. This causes great fluctuation of both pH and nutrient concentration.

In the noncirculating hydroponics, however, cucumber proved to be one of the easiest crops to cultivate. It is highly recommended to use the seedling bag in summer cultivation to avoid transplanting shock. Cucumber growth with the seedling bag was better than that without it. In the first trial, 48 seedlings were transplanted to the large container on 14 March 1986 and fruits were ready to be harvested one month after transplanting. During the first two weeks of harvest, cucumbers produced 256 fruits with a total weight of 23.3 kg and the average fruit weight was 91 g. Yields decreased sharply from six weeks after the first harvest because 48 plants were transplanted to only 33 containers, thereby mutual shading of leaves hastened their senescence. After most of the lower leaves fell out, the plants recovered completely and produced fruit actively again. Thus, even at five months after transplanting, about 15 kg of fruits were harvested. In the second trial, the number of plants transplanted was reduced to half of the previous trial. 24 seedlings cultivated in a net bag were transplanted to the large container on 8 May, 1986 and fruits were ready to be harvested three weeks later. Since mutual shading was minimized, the plants yielded more than 110 kg of cucumber during the 100 days of cultivation.

c) Melon

Similar to cucumber, melon developed very long, but less branched roots which were characterized by bin and voluminous root hairs. Due to the root hairs, the roots floated on the surface of nutrient solution even without a net. But the net was very effective to develop O roots, hence this is one of the most promising crops for cultivation in noncirculating hydroponics. In an early summer trial (transplanted on May 8, 1986 and harvested several times from July 2 to August 20), 24 plants produced 38 fruits which weighed 56 kg, in 100 days cultivation. The average fruit number and fruit weight per plant were 1.58 and 1,480 g, respectively. Similarly, 24 melon plants which were transplanted on 17 July 1986 and harvested on 8 September yielded 25 fruits with the total weight of 41.8 kg. The duration from transplanting to harvest was 53 days.

Melon was also grown in the small container (40 x 30 x35 cm3) in a greenhouse at midsummer successfully. Although air temperature inside the greenhouse went up to 45 degrees C, the small container provided two plants with enough space and nutrient, consequently, each plant yielded two fruits with an average weight of 1.75 kg. Melon requires high temperature and light intensity for healthy growth and is also very susceptible to fusarium wilt, gummy stem blight and powdery mildew. Cultivation in net house and sparse planting are highly recommended.

d) Tomato

Tomato is one of the most popular vegetables being cultivated in hydroponics. Tomato plants have tremendous rooting ability, and easily root directly from stem whenever placed in high humidity conditions. In summer, however, yields decrease sharply due to high temperature and flooding in the tropics and cultivation by hydroponics was far more difficult than cucumber or melon. Fortunately, this difficulty can be overcome by: 1) using net bag for seedling preparation; 2) avoiding a great decrease in water head (more than 5 cm from the pre-set) by a simple system of automatic water supply; and 3) minimizing fluctuation of nutrient concentration in the solution.

In the summer of 1986, we succeeded in harvesting nearly 3 kg of large-fruited tomato using the AVRDC noncirculating hydroponics system. The average fruit weight was 130 g, but the largest one exceeded 200 g.

e) Leafy vegetables

Several leafy vegetables have been grown using the small container with different netting systems. Growth rate of these vegetables was very fast and they were ready to be harvested within one month after sowing. Only basal fertilizer was necessary.

Courtesy of the Hydroponic Society of America. Used by permission.

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