Member of the International Society for Soilless Culture, College of Agriculture, University of Osaka Prefecture
Soilless culture is, as readily seen from the term, a technique for crop production using no soil. In this method, crops are grown in the nutrient solution or on a proper medium where crops are planted and nutrient solution is applied. Therefore, soilless culture involves no work using such tools as spades and hoes, and machines such as tractors. It has many more characteristics which differ from soil cultivation. First, it needs a certain amount of cost for preparing growing systems. In addition, chemical fertilizers and water of good quality must be available. In general, no injury is caused by continuous cropping. Such works as intertillage and weeding are eliminated and almost all of management works, including fertilizing and watering, are automatized. As a result, acreage per farmer can be increased. Since this method utilizes a limited supply of water efficiently, it can be adopted in arid areas.
In actual cultivation, soilless culture offers earlier growth and higher yield. At the same time, more equal supply of nutrient solution can be achieved and so more homogeneous crops can be obtained. While the concentration and composition of mineral elements to be applied to crops can be adjusted at will, the buffer capacity of nutrient solution is low and pH and mineral composition of the solution are easily changeable. For this reason, shortage and excess of nutrients in the solution are apt to occur. It takes a long time to grow seedlings in traditional agriculture, but soilless culture can reduce the time substantially as this process can be performed in a concentrated manner. In the case of leaf crops, soilless culture can use cultivation facilities more efficiently and far more yield than soil culture can be secured. For example, hornworts can be harvested up to only five times per year when they are cultivated in traditional soil culture. But when hydroponics is employed and seedlings are raised separately, this plant can be cultivated as many as ten times every year.
Systems of soilless culture
There are various systems of soilless culture as shown in Fig. l. Here, we will take a look at those systems which have been introduced in Japan or whose introduction has been considered.
(1) Water culture
Fig. 1. Various systems of soilless culture
Many systems used in Japan are included in this category. In water culture, no solid medium is used except at the time of sowing, or in the case of tomatoes, cucumbers, Japanese hornworts and Welsh onions only nutrient solution is used after planting although pots filled with gravel or urethan sponges are used to raise seedlings. The roots of the plant are continuously or intermittently immersed in nutrient solution. In this culture method, how well oxygen and nutrient elements can be supplied to the roots of crops is an important subject. According to the system of providing oxygen and nutrients to crops, water culture is classified into several types (Fig. 2).
In this system, nutrient solution is directly applied continuouslyor intermittently to the roots of crops in the form of mist or aerosol. As no solid medium is employed, aeroponics can be considered to be a type of water culture.
(3) Media culture
This is a system in which crops are planted on solid substrate rather than on soil and nutrient solution is applied to the media. Both inorganic and organic media are used. Inorganic media are classified according to their shape into particles, foam, fiber and others. Particle media culture include sand culture, gravel culture, expanded clay culture and Kuntan culture.
- Sand culture: Sand having a diameter less than 3 mm is used as media.Nutrient solution is supplied by the dripping method in general.
- Gravel culture: The roots of plants are growing in a solid substrate,which porous or non-porous, non-collapsing particles (natural gravel,pumice, lava, etc.) have a diameter more than 3 mm. In most cases, nutrientsolution is applied and discharged periodically.
- Expanded clay culture: In this method, artificial clay is used insteadof natural gravel. This method is mainly employed for the cultivationof ornamental plants. Artificial clay is made by heat-treating clayin a rotary furnace at a temperature of about 1,100ƒC and is red-brownporous particles. The size of particles is varied. In many cases, nutrientsolution is supplied from the bottom.
- Kuntan culture: This is a unique system in Japan and Kuntan used asmedia is the carbonized rice hulls. Kuntan is very light in weight andhas a great capacity to retain water; because of this, it can be usedas media effectively. However, this material has its own defects: chemically,it has high pH values and much water soluble potassium, while physically,it is apt to be broken finely. Nutrient solution is applied by the drippingmethod.
- Rockwool culture: In this method. rockwool slabs are employed as media.The rockwool is a fibrous substrate and consists of various types ofrocks, primarily diabas or slag melted at high temperature. This culturemethod was recently introduced to Japan from Europe. The nature of rockwooldiffers somewhat according to the raw materials, but as a whole thepH value of media is fairly high and their cation exchange capacity(CEC) is extremely low.
Rockwool culture has several types. One of them is the method by which rockwoolslabs are wrapped with plastic films and nutrient solution is applied bythe dripping method. Another method is to place slabs on a tray withoutwrapped with films.
Organic solid media include peatmoss, sawdust, bark and rice hulls. These are all vegetable materials and have their respective characteristics. ln Japan, only sawdust and rice hulls have been adopted on an experimental basis.
i) Sawdust culture: Sawdust is put in U-shaped or V-shaped grooves or packed in polyethylene bags and these grooves or bags are employed as media for cultivating crops. The characters of sawdust differ more or less depending on the types of tree and care should be taken since some sawdust contains elements injurious to the growth of crops. In case the tree has been soaked in seawater for a long time, its salt content may have been raised; in such a case, sawdust made from the tree will give salt damage to crops. This method is widely introduced in Canada and other regions where large-scale lumber industry exists.
ii) Rice hull culture: Rice hulls are used as media without making them into Kuntan. They are the material easiest to be obtained for Japanese farmers and can be disposed after use. Phosphorus and potassium all leached out from rice hulls to some extent, but their pH value is stable, showing slightly acid. Rice hulls are porous, too. As they don't absorb water readily at first, they should be matured for some time or mixed with Kuntan, perlite or other substrates having a good water retaining capacity.
History of soilless culture in Japan
It was in 1946 that soilless culture was started in Japan. The United States forces in Japan began this culture in Chofu, Tokyo. The acreage was 20 hectares and said to be the largest scale in the world at that time. The system adopted was gravel culture. In 1960, the Hort. Res. Sta. of the Ministry of Agriculture and Forestry developed a simple method of gravel culture as an original system in Japan. As the Ministry offered financial assistance named "Agricultural structure improvement projects" to farmers who wanted to introduce this method, this method gradually spread across the country for tomato, cucumber, and sweet pepper production. But gravel culture began to pose some problems, including the disposal of roots after harvesting and difficulty to obtain good gravel. To solve these problems of gravel culture, in 1964 the Kurume Branch of the Hort. Res. Sta. newly developed a water culture system using no medium. Named the forced circulation method, this system grow crops only with nutrient solution. In the second half of the 1960s, a variety of water culture plants began to be manufactured and sold by some companies (Fig. 2). These plants were introduced by farmers with the help of the government's funds for agriculture modernization and other financial support programs. By 1973, the acreage of soilless culture reached about 70 ha. Of them, more than 60% was waterculture, 32% gravel culture and the rest included sand culture, aeroponics and Kuntan culture.
In thosedays, some of Agricultural Experiment Stations in each prefecture proposed several types of simple soilless culture system. But what spread among farmers mainly was water culture plants made from plastic or styrofoam developed by company. Later, NFT (the nutrient film technique or nutrient flow technique), a simple water culture method developed in the United Kingdom, was introduced. Rockwool culture was adopted, too, recently. In 1983, the total acreage of soilless culture in Japan was about 300 ha.
Present status and problems
In Japan, soilless culture is more extensively introduced in the regions along the Pacific. These areas have rather mild weather even in winter and many big cities. The suburbs of cities are suitable for intensive agricultural production. Japanese farmers have been faced with a big problem in recent years: continuous cropping injury of vegetables, mainly caused by soil microorganisms. In the fields in and around cities, farmers are prohibited from using chloropicrin, methylbromide and other chemical disinfectants having strong stimuli because these chemicals are harmful to residents. In these areas, farmers have no alternatives but to disinfect the soil in greenhouses with solar heat during summer by keeping the greenhouses completely shut. But the effect is sometimes insufficient. Thus, in an attempt to avoid injuries by continuous cropping, many farmers began soilless culture. At present, soilless culture is adopted to vegetable production in virtually all cases, and its introduction to flowering plants is tried in only a few cases. Vegetables grown by soilless culture include mainly tomatoes, cucumbers, Japanese hornworts, melons, strawberries, Welsh onions, perillas and lettuces. The use of soilless culture for Chinese vegetables and condimental plants is also increasing. A rare application is the production of tea by soilless culture.
Since 1980 when the area of soilless culture reached 283 ha. in Japan, no substantial expansion has been seen. One possible reason for this will be too high prices of culture plants. A plant manufactured by certain company costs farmers 7 to 9 million yen to build a 10-acre facility. And they must pay additional money to construct a greenhouse. Another big burden for farmers is that they have to worry about the occurrence of diseases in the roots of crops. In water culture, nutrient solution is circulated in the entire field. Thus, once root rots occur caused by pathogens such as phytophthora, pythium, fusarium spp., it spreads over all the fields in a short period of time, and in an extreme case no harvest will be obtained. Regrattably, no decisive measures against diseases have not been found yet.
There are some problems regarding cultivation techniques, too. The minerals concentration, composition and pH of nutrient solution are apt to change during cultivation. Growers usually control their nutrient solution by a pH meter and EC (Electric Conductivity) meter, but this method does not indicate the changes in the composition of the solution. Most farmers change half or all of the nutrient solution with new solution once every month or every other month to keep the composition uniform, but this is an economically disadvantageous practice.
When gravel culture was introduced in Japan, the Hort. Res. Sta. made a standard nutrient solution: 8mM of KNO3, 4mM of Ca(NO3)2, 4H2O, 3/4 mH of NH4H2PO4 and 2mM of MgS04, 7H2O. This composition has been used not only for gravel culture but for water culture and for any crop. But recently, efforts have been made to create nutrient solution having a composition and concentration suited for each crop and cultivation period.
Actual methods of cultivation
Leaf vegetables: In the case of Japanese honeworts and Welsh onions whose cultivated acreage is the largest, seeds are sown and germinated on urethane sponge. The urethane sponge has been added cuts in advance so that cubes of 2 to 2.5 cm each side may be obtained by just pulling it. Japanese honeworts are transplanted to the culture bed when their cotyledons fully expanded, while Welsh onions are removed to the bed when they grow to 3 to 4 cm On the water surface of the culture bed a styrofoam board with small holes is floated, and urethane cubes are pressed in these holes. Crops are grown in the bed until harvesting. Larger vegetables such as lettuces and spinach are sown on the media of sand, perlite or other materials and then are grown by water culture. They are planted one by one in each hole of styrofoam board and fixed by urethane cubes. In some cases, lettuces and spinach are directly sown on urethane sponge.
Fruit vegetables: These vegetables are sown and germinated on such a suitable medium as sand, Kuntan or perlite. Then they are either grown by water culture without using any medium or raised in a pot with bores around it filled with gravel (gravel pot). After that, the seedlings grown by water culture are transplanted as they are, and those raised in gravel pots are also transplanted with pots, to the culture bed. Strings hung from the above are used instead of props to attract plants.
Water culture has been the major method of soilless culture in Japan, and various plants for this culture have been sold in the market. This may be rare in other countries. But it appears that recently the interest of Japanese farmers has been shifting to those methods using solid media, such as rockwool culture. In general, media culture uses very simple facilities and involves lower cost. In addition, there is a smaller possibility of diseases and this method can be used in the open field, too, when some modification is added. In the near future, vegetables and flowering plants may be grown by this method on the rooftop of buildings. On the other hand, a new system, factory-style vegetable production (called "vegetable factory"), has been put to practical use among some farmers. In this system, the techniques of soilless culture and artificial lights are used to produce vegetables continuously regardless of weather conditions. The role of soilless culture techniques is going to become more and more important.
Courtesy of the Hydroponic Society of America. Used by permission.