SHINZO NITTA
SHINZO NITTA is Assistant Professor of Landscape Engineering at Kyoto University. See also Unasylva, Volume 9, Number 2.
Japan produces an original technique for conservation
THE ESTABLISHMENT of vegetative cover is much practiced in Japan in order to prevent soil erosion on denuded slopes (Figure 1).
The most widely accepted method of establishing such a cover has so far been sodding with Zoysia grasses. But recently it has been felt that this method could not on its own meet the increasing volume of work resulting especially from the use of heavy earth-moving equipment, because the operations require too much labor to be economic. Moreover, the supply of Zoysia sod is insufficient. The need to devise a new efficient method of establishing vegetative cover has therefore been keenly felt in connection with erosion control, public works, or landscaping activities.
One such new method is hydro-seeding, which has been developed in the United States, but this is not generally suitable under Japan's conditions of climate and topography which include heavy rainfall and steep hills. For instance, highway or building locations in Japan are frequently sited on 1:1 (45 degrees) or steeper slopes. No grass seed and fertilizer can be applied on such slopes by an ordinary hydraulic spraying method without being washed away. Beside this, mulching operations on these slopes are rather difficult.
Research has been conducted to find more effective ways of depositing seed and creating proper germinating beds on steep slopes and, after a series of experiments, a new and practical method has been developed using a machine that can spray a mixture of water, seed, soil, fertilizer and other soil conditioners at a high speed.
The machine
It has been found that concrete guns can be applied successfully to the spraying of the muddy mixture (Figure 2). A concrete gun is equipment used to spray concrete mortar by the pressure of compressed air.
There are two types. One sends dry materials through a hose, as far as a nozzle, where water is added just before release. The other type sends wet materials directly to a nozzle from a chamber in which all the materials, including water, are mixed. Both types are useful, but the latter, the wet type, has the advantage of enabling the use of a wider variety of soil material which may be found near a construction site and which may contain a lot of water and/or fine gravel.
The True Gun-All, made in the United States, is one of the more promising machines of the wet type. It has a special mechanism by which the muddy mixture is sent from the tandem chambers into a hose, alternately injected With compressed air. The machine measures about 2 feet 8 inches wide by 8 feet 4 inches long and is 6 feet high (79 × 252 × 182 centimeters), and weighs about 2,640 pounds (1,200 kilograms). Combined with a 75 h.p. air-compressor, it can spray a maximum load of 190 cubic feet (6.37 cubic meters) per hour, which, if spread at a depth of 3/8 inch (95 millimeters), will cover an area over 5,000 square feet (465 square meters). In other words it is theoretically possible to spray 30,000 square feet (2,790 square meters) in six hours - that is, in a working day. In practice, however, the area that can be dealt with per day may more conservatively be estimated at about 10,000 square feet (930 square meters), because it is rare to find an extensive continuous slope just right for spraying, even although the hose may be stretched horizontally as far as 1,000 feet (300 meters), and vertically 300 feet (100 meters). The working pressure on the mixed materials is 76 pounds per square inch (6.27 kilograms per square centimeter) inside each chamber, and 43 to 64 pounds per square inch (3 to 4.5 kilograms per square centimeter) at the nozzle.
Grass seed
The seed of Japanese lawngrass, Zoysia japonica, is not ordinarily available in commercial quantities. Even if available, it does not germinate well unless pretreated. Consequently, some other species of grass had to be chosen that can be seeded and grow fast on an infertile slope subject to drought. As a result of experiments and other research studies, the following three species have been found most successful in the southern part of Japan:
Weeping lovegrass (Eragrotis curvula)
Creeping red fescue (Festuca rubra)
Kentucky 31 fescue (a variety of Festuca arundinacea)
These are perennial grasses. Weeping lovegrass becomes dormant in winder while the two fescues are evergreen. Appropriate seeding rates for these grasses when used alone are about 30 pounds per acre (33 kilograms per hectare) for Eragrostis and 150 pounds per acre (180 kilograms per hectare) for the fescues. The best time for seeding varies with the species of grass and the locality. Generally, speaking, in the southern warm humid region of Japan, the seeding of Eragrostis is done during a period between a last frost and early summer. For the fescues, it is either early spring (March-April) or autumn.
Fertilizers
Manuring is an absolute necessity when seeding is done on inert subsoils. Starter fertilizer for the grass seed should be liberally contained in the blowing mixture.
There should be approximately equal proportions of nitrogen, phosphoric acid and potassium, on the basis of about 80 pounds of nitrogen per acre (90 kilograms per hectare). It is desirable that at least a half of the nitrogen be in organic form. To secure a permanent vegetative cover, however, additional feeding of the grass, especially with nitrogenous fertilizer, will be required year by year even after the initial establishment.
Lime
The grasses mentioned above tolerate a soil reaction between pH 5.5 and pH 7.0 If the soil is more acid than pH 5.5, an adequate application of lime is needed.
Soil
Clayey soil, which contains 80 percent or less of sand with less than 30 percent clay, is suitable. Gravel should be sifted through a 3/8 inch (1 centimeter) sieve; larger sizes choke up the nozzle of the machine. The amount of soil to be used is calculated against the depth of coverage required. The standard depth is considered to be about 3/8 inch (1 centimeter) because small seeds like Eragrostis do not germinate well in a greater thickness of soil.
Water
The quantity of water to be added depends on the dryness of the soil. Roughly speaking, the volume of water should be 30 percent of the volume of soil used.
Seed, fertilizer, soil, lime and water are thrown into tandem chambers which are operated alternately and are stirred for more than 1 minute, preferably for 2 minutes. Insufficient mixing results in failure to obtain a uniform distribution of seed and other materials. The mixed mud should be blown evenly onto a slope, to the standard depth of 3/8 inch. It is not easy in practice to check whether the required depth has been attained all over a steep slope. Thus, prior to spraying, it is better to mark the slope with ropes into small divisions of equal area for each of which a certain number of chambers of mixture will be used. By so doing, and by regulating the quantity in a chamber and also by spraying as steadily and continuously as possible, a uniform depth may be obtained.
The distance and angle between nozzle and slope must be adjusted to the ground conditions. Obviously, if the nozzle is held at too close an angle to the surface, the ground will be broken up by the pressure. The standard nozzle for the True Gun-All is 1 foot (30 centimeters) long. It is better substituted by a hard vinyl pipe of 13 feet (4 meters) length. Using this long nozzle, the mud mixture can be sprayed onto slopes less than 50 feet (15 meters) in height, without climbing up the slopes. For slopes of greater height, a mobile crane with a gondola can be effective.
The spraying of the seed and its bed is the first of two successive operations. The second operation is to conserve moisture for germination and to protect against erosion. Mulching with straw is undoubtedly good for protection, but it is ordinarily too difficult on steep slopes. At an early stage of the experiments in Japan, a coarse matting made of rice straw was used as a covering material. The matting is called mushiro in Japanese and chiefly used as a wrapping material. The mats were spread over the sprayed slopes and secured with straw ropes and bamboo pins. The grass was expected to grow through the matting and the results were not at all bad (Figure 3). Covering with mushiro, however, has the disadvantage that the matting is liable to be blown off by strong winds; it depends largely on manual labor which cannot keep up with the spraying operation by the machines; and the matting itself is also relatively expensive.
Experiments were made to replace the straw matting with some other material and at present asphalt emulsion seems to be one of the most promising materials. Asphalt emulsion can be sprayed with the same machine, the concrete gun, over the seedbed. It lies as a thin film on the seedbed, preventing loss of moisture and possible erosion (Figure 4). Asphalt emulsions of the cationic type are considered better than the anionic type, because the film forms more rapidly, but care must be taken in their use, because they may occlude the chambers or hoses. They also need to be nontoxic to plants. Approximately 1,000 gallons of asphalt emulsion per acre (11,0.00 liters per hectare), diluted with water to a consistency of 30 percent, is required for spraying.
FIGURE 4. - A slope just after being sprayed with asphalt emulsion
The grass seed begins to germinate within two days after the seeding operation but takes about a week to appear above the ground, breaking through the asphalt crust. In the growing season, the grasses will cover the entire surface in one mouth. The method is now beginning to be widely applied in Japan.
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