JP6754605B2 - Hydroponic cultivation method of asparagus - Google Patents

Description

The present invention relates to a hydroponic cultivation method of asparagus, and more specifically, asparagus is cultivated by setting the liquid level of the nutrient solution at a position where the root immersion rate satisfies a specific numerical range. Regarding hydroponic cultivation method of gas.

Asparagus is a perennial herbaceous plant with rooting properties and low water resistance. Therefore, asparagus is generally grown in open-field or house-grown areas in well-drained land, and young shoots that germinate from bulbs and grow to the desired length are edible. Harvested as. Multiple bulbs are gathered at the tip of the rhizome of asparagus, and young shoots germinate from each bulb.

Harvesting of young shoots occurs mainly from spring to summer, but asparagus in this period requires a large amount of water, so a lot of labor must be spent on irrigation. Furthermore, the cultivation of asparagus required a large amount of fertilizer, and the additional fertilizer work was also difficult.

In addition, asparagus is originally a crop that can continue to harvest young shoots from the same strain for 10 years or more. Moreover, the longer the cultivation period, the greater the number of bulbs at the tip of the rhizome, which improves the yield of young shoots, and in addition, the thicker the young shoots, the higher the commercial value.

However, as disclosed in Patent Document 1 and Non-Patent Document 1, asparagus releases a large amount of crop growth inhibitory substances called allelopathic substances. In soil where asparagus is cultivated, the concentration of allelopathic substances increases as the cultivation period becomes longer. Therefore, in the actual field, it is very difficult to cultivate asparagus for a long period of time and continue harvesting, and it has been required to establish a technique for cultivating the same strain for as long as possible.

As a technology to solve the problems of labor required for irrigation and topdressing and the problems of allelopathic substances, in recent years, a cultivation method of growing asparagus using an aqueous solution (nutrient solution) in which nutrients necessary for cultivation are dissolved without using soil has been adopted. It is being actively considered. For example, in Non-Patent Document 2, after growing asparagus to some extent using a rock wool cube while giving a nutrient solution, asparagus is placed on the rock wool slab together with the rock wool cube, and the nutrient solution is further given. A method for cultivating asparagus (hereinafter, this may be referred to as a rock wool cultivation method) is disclosed.

However, asparagus has a characteristic that the roots become huge as the cultivation is continued because the asparagus stores nutrients in the roots (mainly storage roots), and this characteristic is impaired by the method of Non-Patent Document 2. That is, in the method of Non-Patent Document 2, although the problem of allelopathic substances is solved, there is a new problem that rock wool inhibits the enlargement of roots, and as a result, there is a demand for a longer cultivation period. It was still unfulfilled.

Furthermore, Non-Patent Document 2 merely suggests that the rock wool cultivation method may be able to increase the germination rate of young shoots as compared with the case of cultivation in soil, and asparagus is cultivated in a nutrient solution. At that time, no specific means for improving the germination rate was examined.

Japanese Unexamined Patent Publication No. 2005-176748

Satoru Motoki, "Use of activated carbon to avoid allelopathy in asparagus continuous cropping disorder", Horticultural Research Vol. 5 (2006) No. 4, P437-442 Sachiko Matsubara, "Plant Growth by Rockwool Cultivation and Soil Cultivation of Asparagus", Okayama University Faculty of Agriculture Academic Report 78, P11-16, 1991

Therefore, an object of the present invention is to provide a method for hydroponic cultivation of asparagus, which can improve the germination rate of young shoots.

According to the present invention, a hydroponic cultivation method for asparagus is characterized in that the position of the liquid level of the nutrient solution is set so that the asparagus root immersion rate is greater than 20% and 80% or less. Provided.

A hydroponic cultivation method of asparagus in which asparagus is cultivated using the nutrient solution and young shoots are germinated is a preferred embodiment of the present invention.

A method for hydroponic cultivation of asparagus in which the temperature is maintained at 20 ° C. or higher at least when young shoots germinate is a preferred embodiment of the present invention.

The method of hydroponic cultivation of asparagus in which the root immersion rate is the root immersion rate of all asparagus strains is a preferred embodiment of the present invention.

In the present specification, "hydroponic cultivation of asparagus" means cultivation of asparagus using an aqueous solution (nutrient solution) in which nutrients necessary for cultivation are dissolved, not soil, and rock wool is not used. In that respect, it is different from the rock wool cultivation of Non-Patent Document 2 described above.

The "root immersion rate" is based on the longest root (called the longest root) of the stored roots of asparagus at the start of hydroponics, and the length of the longest root immersed in water. It means the ratio (%) of the roots. "The root immersion rate is 100%" means a part where the longest root starts to be generated from the rhizome, that is, the boundary between the rhizome and the longest root and the position of the liquid level of the nutrient solution coincide with each other.
As shown in FIG. 1, the bulbs are located above the boundary between the rhizome and the longest root, and in the present invention, the liquid level of the nutrient solution is set at a position where the root immersion rate is less than 100%. By doing so, contact between the bulb and the nutrient solution is effectively avoided.
When the liquid level is located above the boundary between the rhizome and the longest root, the root immersion rate is expressed as "100% or more".

According to the present invention, there is provided a hydroponic cultivation method of asparagus capable of improving the germination rate of young shoots. The reason why such an effect can be obtained in the present invention is not clear, but the present inventors consider it as follows. As described in Non-Patent Document 2, asparagus was considered to be unsuitable for cultivation in a nutrient solution because of its lack of water resistance and permanence. Therefore, the cultivation method using nutrient solution has not been studied so much, and it has not been known what kind of asparagus is unsuitable for cultivation with nutrient solution.

However, as a result of repeated research by the present inventors with great efforts, asparagus tends to be difficult to germinate when the bulbs come into excessive contact with the nutrient solution (particularly water in the nutrient solution). It turned out that this is the reason why it is called "low water resistance". On the other hand, regarding roots, it was also found that there was no problem even in a watery environment.

Based on the above-mentioned new findings, the present inventors diligently examined the conditions under which the roots could sufficiently absorb nutrient water while surely cutting off the contact between the bulbs and the nutrient solution, and found that the roots were found at the start of hydroponics. By setting the position of the liquid surface so that the immersion rate of the bulb is within a specific numerical range, sufficient nutrient water is absorbed from the roots without bringing the bulbs into contact with the nutrient solution, and as a result, germination occurs with a high probability. This was successful, and the present invention was completed.

It is the schematic explaining the name of each part of asparagus. It is a figure which shows an example of the hydroponic cultivation system suitable for carrying out the hydroponic cultivation method of this invention, (A) is a schematic diagram which shows the structure of such a system, (B) is in the said system. It is a schematic diagram which shows the state which put the seedling or the root stock and the nutrient solution in one of the cultivation containers of. It is a schematic diagram which shows the fixing state of the seedling in Example 1-2 and Comparative Example 1-2. FIG. 5 is a schematic diagram showing a state of planting of root stocks in Example 4.

The present invention is a method of hydroponically cultivating asparagus by setting the liquid level of the nutrient solution at a predetermined position. Asparagus is divided into an underground part and an above-ground part as shown in FIG. 1, and the underground part consists of a rhizome and a root. The rhizome has multiple bulbs, and young shoots germinate from these bulbs. In the rhizome of asparagus cultivated for multiple years, there are traces of past stems behind the rhizome in the direction of travel, and bulbs are present at the tip. Roots consist of storage roots and absorption roots. The storage root is a thick root generated from the rhizome and mainly plays a role of anabolic product storage, but also absorbs nutrient water. Absorbing roots are thin roots that develop from storage roots and absorb nutrient water.

On the other hand, the above-ground part consists of young shoots, parent stems, side branches, flowers, artificial leaves and the like. In the present specification, young shoots and parent stems are distinguished by the presence or absence of side branches. That is, the stem after germination from the bulb and before the side branch emerges is the young shoot, and the stem after the side branch emerges is the parent stem.

The present invention targets all varieties of asparagus for cultivation. Specifically, green asparagus, such as Welcome, Super Welcome, Green Tower, Mary Washington 500W, Paul Tom, Halquital, HLA-7 (Gainrim), UC157, UC800, Axel, Kanda White, Green FLET'S, etc .; Purple Asparagus Examples of gas, such as Harumurasaki and Purple Passion, can be mentioned. In addition, as will be described in detail later, white asparagus can be cultivated by shading the young shoots.

FIG. 2 is a diagram showing an example of a hydroponic cultivation system suitable for carrying out the hydroponic cultivation method of the present invention, (A) is a schematic diagram showing an example of such a system, and (B) is a schematic diagram. , It is the schematic which shows the state which put the seedling or the root stock and the nutrient solution in a cultivation container. Hereinafter, the present invention will be described in detail with reference to FIG.

The system 1 shown in FIG. 2 includes a cultivation container 2, a pump 3, and a tank 4, and the nutrient solution stored in the tank 4 flows in the direction of the arrow “←” through a hose, a pipe, or the like and circulates. ing.

As shown in FIG. 2 (B), in the present invention, asparagus seedlings or root stocks 5 that have been grown to some extent in advance are cultivated. Specifically, the roots 6 that have been grown to a maximum length of 10 cm or more, preferably 10 to 15 cm in advance are placed in the cultivation container 2 and hydroponically cultivated. If the longest root is too short, there is a risk that various settings such as the flow rate of the nutrient solution and various instruments such as the cultivation container must be changed during hydroponics. If the longest root is too long, the work of fixing to the cultivation container 2 may be complicated. As long as the longest root 6 has a length within the above range, the seedling or root strain 5 to be cultivated may be one in the first year of cultivation or may be one in the second year or later of cultivation. The growth until the longest root 6 reaches the above numerical range may be carried out by a known method. For example, when a seedling or root strain of the first year is to be cultivated, it is sown on vermiculite containing water. It is sufficient to grow until the longest root reaches the above numerical range while controlling the conditions such as temperature, moisture and light.

In the present specification, the "seedling" means asparagus having not only the underground part but also the above-ground part, and the "root stock" has the above-ground part cut off and has only the underground part. It means asparagus that is not in the state. In the present invention, either seedlings or root stocks may be cultivated.

The nutrient solution 8 used in the present invention is an aqueous solution in which components that directly and indirectly promote the growth of asparagus are dissolved, and for example, phosphorus, nitrogen, potassium, calcium, magnesium, sulfur, iron, boron, manganese, etc. It is an aqueous solution in which necessary elements such as zinc, copper, molybdenum, and chlorine are appropriately selected and contained. The specific composition may be appropriately determined according to the variety, cultivation conditions and the like.

In the present invention, when the asparagus seedlings or roots 5 and the nutrient solution 8 are placed in the cultivation container 2, the position of the liquid level 8a of the nutrient solution 8 is such that the root immersion rate of the asparagus seedlings or roots 5 is 20. It is important to set it so that it is greater than% and less than 100%, preferably greater than 20% and less than 80%, and particularly preferably 50% or more and 80% or less. By setting the position of the liquid level 8a of the nutrient solution in the above range, it is possible to sufficiently absorb the nutrient water from the roots while avoiding the contact between the bulb 7 and the nutrient solution 8 as much as possible, and as a result, as much as possible. This is because it is possible to germinate with this strain to increase the yield.

When a plurality of asparagus seedlings or root strains 5 are present in one container, the immersion rate of roots of 80% or more of the asparagus strains in the container must satisfy the above numerical range. However, it is preferable that the immersion rate of the roots of all asparagus strains in the container satisfies the above numerical range.

The position of the liquid level 8a can be set by adjusting the depth of the cultivation container 2, the fixed position of the seedling or the root stock 5, the position of the nutrient solution discharge port 9 for discharging the nutrient solution from the cultivation container, and the like. When a plurality of cultivation containers 2 and 2 are used in one system as shown in FIG. 2, the depth of the cultivation container 2, the fixed position of the seedling or the root stock 5, the position of the nutrient solution discharge port 9, etc. are set for each container. By changing to, it is also possible to hydroponically cultivate seedlings or root stocks having different cultivation years in one system at the same time.

The seedlings or root stock 5 may be fixed by a known means applied when fixing crops in hydroponics, and although not shown, for example, a styrofoam with holes or a resin net is desired. This may be done by fixing the seedlings to a height and placing the seedlings or root stocks 5 straight on them. When the seedlings or root stocks 5 have highly developed roots, as shown in FIG. 2B, each seedling or root stock 5 is laid down and arranged in the cultivation container 2. It can also be fixed with. At this time, it is preferable to set up a partition plate such as a resin net between each seedling or root stock 5 to ensure a smooth flow of the nutrient solution 8.

By the way, in the present invention, as shown in FIG. 2, hydroponics may be carried out while circulating the nutrient solution in the system using the pump 3, or, although not shown, the nutrient solution is not shown. Hydroponics may be carried out while replacing the nutrient solution in the container at an appropriate timing without circulating the nutrients, but for reasons such as the distribution of nutrients evenly and the labor of exchanging the nutrient solution can be saved. It is preferable to carry out hydroponics while circulating, and in particular, it is preferable to carry out submerged hydroponics. When hydroponics is carried out by circulating the nutrient solution, in principle, it is necessary to maintain the position of the liquid level throughout the cultivation period, in other words, the distance between the bottom of the container and the liquid level (hereinafter referred to as "the nutrient solution". It is necessary to keep the water depth (sometimes called "water depth") almost constant from the start to the end of hydroponics.

However, when hydroponics is continued for many years by the method of the present invention, the root immersion rate gradually changes due to root development while the water depth of the nutrient solution is maintained, and the nutrient water content The number of strains that are insufficiently absorbed may increase. Therefore, in such a case, it is preferable to reset the position of the liquid level in the middle of cultivation. Of course, the liquid level at the time of resetting may also be set at a position where the root immersion rate is within the above-mentioned numerical range. As a guideline for the timing of resetting, it is preferable that the number of strains having a root immersion rate in the above numerical range is less than 80% among all the strains existing in the container. The resetting may be performed by adjusting the water depth of the nutrient solution, the fixed position of the asparagus being cultivated, the position of the nutrient solution discharge port 9, and the like.

As described above, in the present invention, if the asparagus seedlings or root stocks 5 and the nutrient solution 8 are prepared and the liquid level 8a of the nutrient solution in the cultivation container 2 is set at a desired position, hydroponics can be performed. Although it is possible, it is preferable to set the following conditions or use the following equipment.

The temperature during hydroponics is appropriately determined according to the variety, growth stage, etc., but from the viewpoint of achieving a better germination rate, at least the temperature should be maintained at 20 ° C or higher when the young shoots germinate. It is preferable that the temperature is kept at 20 ° C. or higher throughout the cultivation period. The upper limit of the temperature is preferably, for example, 40 ° C. or lower, particularly 30 ° C. or lower, throughout the cultivation period.

As the light condition, it is sufficient that the same amount of light as the amount of light required by asparagus at each growth stage can be secured in cultivation in soil, and a light irradiation means such as a fluorescent lamp or an LED lamp may be used. , The light source may be only natural light.

However, when it is desired to harvest so-called white asparagus, it is necessary to prevent the young shoots from being exposed to light by a known method. As a light-shielding means, for example, a light-shielding box is installed in a portion where young shoots are likely to germinate, or if the hydroponic cultivation of the present invention is carried out in the house, the entire house is covered with a light-shielding sheet. There is.

When the amount of dissolved oxygen was changed to 2-8 mg / L by bubbling oxygen or nitrogen or both in the nutrient solution, there was no change in germination rate and number of germination (data not shown). Regarding the oxygen conditions in the liquid, there are no conditions for each stage in the present invention, and oxygen may or may not be supplied by a known method.

With reference to FIG. 2B, in the present invention, it is preferable to cover the seedlings or root stocks 5 installed in the cultivation container 2 with the light-shielding film 10. This is to prevent moss and algae from growing in the cultivation container 2. If moss and algae grow and flow through the system together with the nutrient solution, it can cause pump failure. It is preferable to make a notch in the light-shielding film so as not to hinder the elongation of young shoots.

Further, in the present invention, it is preferable to use a strainer to prevent moss, algae, a part of torn roots and the like from reaching the pump. A known strainer may be used. The strainer may be provided at an arbitrary location, for example, near the nutrient solution outlet of the tank 4.

Furthermore, it is preferable to use activated carbon from the viewpoint of maximally suppressing the influence of allelopathic substances. More specifically, in the present invention, asparagus is cultivated with a nutrient solution, so that asparagus is less affected by allelopathic substances than when cultivated in soil, but the influence of allelopathic substances is more reliably eliminated. Therefore, it is preferable to provide a means for removing the allelopathic substance. And, as described in Non-Patent Document 1, since the allelopathic substance released by asparagus is well adsorbed on the activated carbon, it is preferable to use the activated carbon as the allelopathic substance removing means. The activated carbon may be provided at an arbitrary place, and may be used by providing an adsorber filled with activated carbon at the outlet of the strainer or the outlet of the pump described above.

In the present invention, it is preferable to carry out hydroponics while measuring the EC value (electrical conductivity, electrical conductivity) and pH of the nutrient solution at any time. This is because these values are indicators for confirming changes in the composition of the nutrient solution and changes in the CO ₂ concentration, and the changes in these values can be used to know the appropriate timing for replacing the nutrient solution. ..

Even when hydroponics is carried out while circulating the nutrient solution in the system, it may be necessary to replace the nutrient solution, although the frequency is less than that of the non-circulation type. For example, the same strain of asparagus may be cultivated over many cycles and the young shoots may be harvested repeatedly. In this case, the nutrient solution in the tank 4 is generally replaced, but from the viewpoint of minimizing the stress given to asparagus by the nutrient solution replacement, two tanks are installed in parallel in the system in advance. First, fill one tank with nutrient solution and perform hydroponics, and when it is time to replace the nutrient solution, fill the other tank with nutrient solution and switch the flow path, and then in the first tank. It is preferable to adopt the method of exchanging the nutrient solution.

In the present invention, asparagus is cultivated in a nutrient solution, germinated, the young shoots are grown to a desired length, and the young shoots are harvested, while appropriately selecting the above-mentioned conditions and equipment as necessary. .. There is no particular limitation on the time of harvest, and in detail, in general, it is often picked when the length of the young shoots reaches 27 to 28 cm, but it may be picked at a shorter stage, or the commercial value. You may pick it after it has been stretched for a long time. The young shoots picked at a short stage may be distributed on the market as so-called mini asparagus.

If the same strain is to be hydroponically cultivated by the method of the present invention in the next year or later, it is preferable to leave at least one young shoot at the time of harvesting and grow such young shoot. This is to allow the roots to store sufficient nutrients and prepare for cultivation in the following year. In the present invention, rock wool that hinders root development is not used, and the influence of allelopathy is also suppressed, so that cultivation can be continued for such a long period of time. Of course, the description in this paragraph does not preclude the application of the present invention to short-term cultivation of asparagus (for example, cultivation period of 2 months or less).

Although the present invention has been described with reference to FIG. 2, the present invention does not necessarily have to be carried out by the system of FIG. 2, and is appropriately applied to the system without departing from the spirit of the present invention. Design changes may be made. For example, the number of cultivation containers, pumps, and tanks may be changed.

Hereinafter, the present invention will be further described with reference to Examples and Comparative Examples. The technical scope of the present invention is not limited thereto.

<Examples 1 and 2, Comparative Examples 1 and 2>
A hydroponic cultivation system as shown in Fig. 2 (A) is installed in a plastic greenhouse using two waterway-shaped cultivation containers (width 60 cm x length 320 cm x height 14 cm), a pump, and a 200-liter tank. It was. The temperature was set to 25 ° C. As a light source, a fluorescent lamp for plant cultivation (Biolux HG, manufactured by NEC) was used.

When the nutrient solution (Farm Ace No. 1 and No. 2 were mixed according to the manufacturer's recommended prescription, OAT Agrio Co., Ltd.) was refluxed into this hydroponic cultivation system at a flow rate of 10 liters / min, the depth of the nutrient solution was 11 cm. became.

In one cultivation container, as Example 1, welcome seedlings (1st grade, longest roots 10 to 15 cm, 12 plants) were fixed at a position where the root immersion rate was 80% and planted. Further, in the same cultivation container, as Example 2, welcome seedlings (1st grade, longest root 10 to 15 cm, 12 plants) were fixed at a position where the root immersion rate was 50% and planted.

Further, as Comparative Example 1, welcome seedlings (1st grade, longest roots 10 to 15 cm, 10 plants) were fixed in the other cultivation container at a position where the root immersion rate was 100% or more, and planted. The bulbs were thoroughly immersed in the nutrient solution. Further, in the same cultivation container, as Comparative Example 2, welcome seedlings (1st grade, longest roots 10 to 15 cm, 12 plants) were fixed at a position where the root immersion rate was 20% and planted.

The fixed state of the seedlings in each cultivation container is schematically shown in FIG.

Hydroponics was carried out for 10 days while maintaining the water depth of the nutrient solution, the flow rate of the nutrient solution and the air temperature, and then the germination rate was investigated. The results are shown in Table 1.

After investigating the germination rate, hydroponics was continued and it was confirmed whether or not flowers would grow. As a result, it was confirmed that the strains with flowers and the strains without flowers were almost evenly present in all the test plots, and that the unevenness of males and females did not affect the experimental results.

In Table 1, the "number of planted plants" means the number of asparagus plants grown in this example. "Number of germinated strains" means the number of strains of asparagus in which young shoots have germinated. "Total germination number" means the number of germinated young shoots. "Germination rate (%)" means the ratio of the number of germinated plants to the number of planted plants.

<Example 3>
In the greenhouse, install a hydroponic cultivation system as shown in Fig. 2 (A) using two waterway-shaped cultivation containers (width 22 cm x length 130 cm x height 11 cm), a pump, and a 200 L tank. It was. The temperature was set to 20 ° C. or higher. The light source was only natural light.

When tap water was refluxed into this hydroponic cultivation system at a flow rate of 6 liters / min, the depth of the nutrient solution was 6 cm.

A welcome root strain (4th grade, longest root 25-35 cm, 61 strain) was fixed in one of the cultivation containers α and planted. The root strain contained almost evenly male and female. The root stocks were fixed side by side so as to be laid down as schematically shown in FIG. 2 (B). In all strains, the root immersion rate was greater than 20% and less than 80%.

In the same manner, the welcome root strain (4th grade, longest root 25-35 cm, 62 strains) was fixed in the other cultivation container β and planted. The root strain contained almost evenly male and female. In all strains, the root immersion rate was greater than 20% and less than 80%.

After covering the upper surfaces of both cultivation containers α and β with a light-shielding film with slits (trade name: Rakuhagi Multi, manufactured by Mitsubishi Plastics Agridream Co., Ltd.), the water depth of the nutrient solution, the flow rate of the nutrient solution, and the air temperature for 30 days. Hydroponics was carried out while maintaining the above, and then the germination rate was investigated. The results are shown in Table 2.

<Example 4>
As shown in FIG. 2A, using two hydroponic cultivation containers (width 22 cm × length 130 cm × height 6.5 cm), a pump, and a 200 L tank in the same greenhouse as in Example 3. A water-based cultivation system was set up. The temperature was set to 20 ° C. or higher. The light source was only natural light.

When tap water was refluxed into this hydroponic cultivation system at a flow rate of 6 liters / min, the depth of the nutrient solution was 6 cm.

Welcome root strains (4th grade, longest roots 25-35 cm, 47 strains) were fixed in one of the cultivation containers α and planted. The root stocks were fixed side by side so as to be laid down as shown in FIG. The root strain contained almost evenly male and female. In all strains, the root immersion rate was greater than 80% and less than 100%.

In the same manner, the welcome root strain (4th grade, longest root 25-35 cm, 50 strains) was fixed in the other cultivation container β and planted. The root strain contained almost evenly male and female. In all strains, the root immersion rate was greater than 80% and less than 100%.

After covering the upper surfaces of both cultivation containers α and β with a light-shielding film with slits (trade name: Rakuhagi Multi, manufactured by Mitsubishi Plastics Agridream Co., Ltd.), the water depth of the nutrient solution, the flow rate of the nutrient solution, and the air temperature for 30 days. Hydroponics was carried out while maintaining the above, and then the germination rate was investigated. The results are shown in Table 2.

1; System 2; Cultivation container 3; Pump 4; Tank 5; Seedling to root stock (root stock in this case) 6; Longest root 7; Bulb 8; Nutrient solution 8a; Liquid level 9; Nutrient solution outlet 10; Light-shielding film

Claims (4)

A method for hydroponic cultivation of asparagus, which comprises setting the position of the liquid level of the nutrient solution so that the immersion rate of the roots of asparagus is greater than 20% and 80% or less . The method for hydroponic cultivation of asparagus according to claim 1, wherein asparagus is cultivated using the nutrient solution and young shoots are germinated. The method for hydroponic cultivation of asparagus according to claim 1 or 2 , wherein the temperature is maintained at 20 ° C. or higher at least when the young shoots germinate. The method for hydroponic cultivation of asparagus according to any one of claims 1 to 3 , wherein the root immersion rate is the root immersion rate of all asparagus strains.