II/E-10
A SURVEY OF THE ALGAL PASTURE SOILS OF MILKFISH PONDS IN TAIWAN

by

YUN-AN TANG¹ and SHING-HSIANG CHEN
Tainan Fish Culture Station of the Taiwan Fisheries Research Institute
Tainan, Taiwan

A SURVEY OF THE ALGAL PASTURE SOILS OF MILKFISH PONDS IN TAIWAN

Abstract

The total area of milkfish ponds in Taiwan at present is about 18,100 ha. The average yield of pasture algae mainly consisting of filamentous blue-greens and diatoms during the fish rearing season is estimated to be approximately 25,000 kg/ha which can maintain an average milkfish production of about 2,000 kg/ha.

The optimum elevation of the algal pasture soil is from the mean sea-level to an altitude of 0.45 m. Elevations of 0.6 m or more are also suitable for algal pasture provided that the water for these ponds can be supplied by pumping. However, pond bottoms below the mean sea-level (0.2 m or more) are unsuited because of seepage which may destroy algal beds.

The algal pasture soils of milkfish ponds consist of alluvial tidal flats of recent origin. Silty loam and loam soils are the most favourable for growing pasture algae.

The pH values of algal pasture soils generally range from 8.0 to 9.5 which is considered to be optimum for the growth of these algae. This alkaline reaction is predominantly due to the presence of sodium chloride and sodium sulphate.

The organic matter content of the soils ranges from 0.27 to 5.79, and it has been observed that higher organic matter contents contribute to better growth of pasture algae.

The C/N ratio of organic matter in the algal beds is relatively wide, with a range of 21.1 to 25.1, the total nitrogen content ranges from 0.07 to 0.24 percent, the NH₃-N and NO₃-N contents are mostly above 25 and 15 ppm respectively and the available phosphorus and potassium contents are generally above 45 ppm and 400 ppm respectively.

The available Ca and Mg contents range from 700 to 1,200 ppm and from 300 to 600 ppm respectively and chlorine content generally from 3,000 to 6,000 ppm.

ETUDE DES SOLS A PATURAGE ALGAL DANS LES ETANGS DE CHANOS A TAIWAN

Résumé

La superficie totale des étangs consacrés à l'élevage des chanos à Taïwan représente actuellement environ 18 100 ha. La production moyenne des algues de pâturage, consistant principalement en algues bleues filamenteuses et en diatomées, au cours de la période de croissance du poisson est estimée à environ 25 000 kg/ha, quantité suffisante pour assurer une production moyenne de chanos d'environ 2 000 kg/ha.

L'élévation optimum des sols convenant aux algues de pâturage est 0,45 m au-dessus du niveau moyen de la mer. Les élévations de 0,6 m ou davantage conviennent également à condition que l'alimentation en eau puisse être assurée par pompage. Toutefois, lorsque le fond des étangs se trouve au-dessous du niveau moyen de la mer (0,2 m ou davantage), ces emplacements ne conviennent pas à cause des infiltrations qui risquent de détruire les lits d'algues.

Les sols à pâturage algal des étangs à chanos sont des plages alluviales de formation récente. Les sols argileux et argilo-limoneux sont ceux qui conviennent le mieux à la croissance des algues de pâturage.

La valeur du pH des sols à pâturage algal se situe en général entre 8,0 et 9,5, ce qui paraît être l'optimum pour la croissance de ces algues. Cette réaction alcaline est essentiellement due à la présence de chlorure de sodium et de sulfate de sodium.

La teneur en matière organique des sols varie entre 0,27 et 5,79, et il a été constaté qu'une teneur élevée en matière organique contribue à favoriser la croissance des algues de pâturage.

Le rapport C/N de la matière organique des lits d'algues peut aller de 21.1 à 25.1, la teneur total en azote se situe entre 0,07 et 0,24 pour cent, la teneur en NH₃ - N et NO₃ - N est en général au-dessus de 25 et 15 ppm respectivement, et la teneur en phosphore et en potassium utilisables dépasse généralement 45 ppm et 40 ppm respectivement.

La teneur en Ca et en Mg utilisables va de 700 à 1 200 ppm et de 300 à 600 ppm respectivement et la teneur en chlore oscille généralement entre 3 000 et 6 000 ppm.

ESTUDIO DE LOS SUELOS DE PASTOS DE ALGAS EN LOS ESTANQUES DE SABALOTE EN TAIWAN

Extracto

La superficie total de los estanques dedicados en Taiwan a la cría del sabalote (Chanos chanos) es actualmente de unas 18.100 Ha. El rendimiento medio de las algas de pasto constituídas principalmente por algas filamentosas verde-azules y diatomeas durante la temporada de cría del pez se estima que es aproximadamente de 25.000 Kg/Ha, lo que puede mantener una producción media de sabalote de unos 2.000 Kg/Ha.

La elevación óptima del suelo de pasto de algas se encuentra entre el nivel medio del mar y una altitud de 0,45 m. También son apropiadas para los pastos de algas las elevaciones de 0,6 m o más, a condición de que el agua para tales estanques pueda suministrarse mediante bombas. Sin embargo, los fondos de estanques situados por debajo del nivel del mar (0,2 m o más) son inadecuados, a causa de las filtraciones que pueden destruir los lechos de algas.

Los suelos de pastos de algas en los estanques de sabalote están constituídos por tierras aluviales de origen reciente, bañadas por la marea. Los suelos francos y fran colinos son los más favorables para la producción de algas para pasto.

Los valores del pH en los suelos de algas de pasto, varía generalmente entre 8,0 y 9,5 lo que se considera óptimo para el desarrollo de estas algas. Esta reacción alcalina se debe principalmente a la presencia de cloruro de sodio y de sulfato sódico.

El contenido de materia orgánica de los suelos varía de 0,27 a 5,79, y se ha observado que cuanto mayor es el contenido de materia orgánica tanto mejor se desarrollan las algas de pasto.

La relación C/N de la materia orgánica en los lechos de algas es relativamente am plia, oscilando entre 21:1 y 25:1; el contenido de nitrógeno total varía de 0,07 a 0,24 por ciento; los de NH₃-N y NO₃-N casi siempre son superiores a 25 y 15 ppm, respectivamente, y los contenidos de fósforo y potasio disponibles normalmente son superiores a 45 ppm, respectivamente.

El contenido de Ca y Mg disponibles oscila entre 700 y 1.200 ppm y entre 300 y 600 ppm, respectivamente, y el de cloro generalmente entre 3.000 y 6.000 ppm.

¹ Present address: FAO/UNDP/TA Inland Fisheries Biologist, Philippine Fisheries Commission, Manila, Philippines

1 THE ROLE OF BOTTOM SOILS IN PRODUCTIVITY OF MILKFISH PONDS

The culture of milkfish, Chanos chanos (Forskal) in brackish-water ponds is intrinsically similar to cattle and sheep husbandry in which good growth of pasture is the foundation for efficient production. Pasture algae - mainly the filamentous blue-greens and the diatoms - supply the most desirable and economical food for milkfish during the growing season, and unless good algal pastures are maintained on the pond bottoms during a large part of the fish rearing season, the yields of milkfish will be low and the profits much reduced. Moreover, a luxuriant growth of these pasture algae can crowd out undesirable groups of algae, and possibly other harmful organisms, from the ponds. As the producion of milkfish from brackish-water ponds is directly proportional to the growth of the algal pasture, the management of milkfish ponds lays strong emphasis on techniques for regulating the environmental conditions to favour the growth of these groups of algae, and on a constant supply of the necessary plant nutrients for their growth by fertilization and management of the bottom soils.

In fresh-water ponds soils are mainly important as a nutrient storehouse, but in milkfish ponds they also act as a “bed” for the establishment of the algal pasture and the fertility of the bottom soils of milkfish ponds governs the algal pasture production.

2 AREA AND DISTRIBUTION

The total area of milkfish ponds in Taiwan at the end of 1964 amounted to 18,100 ha (including 1,600 ha which are under construction). These ponds are distributed on the tidal lands along the rivers and creeks. The west coast of the Chai-Nan Plain is the centre of this industry. Here more than 80 percent of the total area of this type of pond is distributed, and the rest are scattered southward of the coast.

3 CLIMATE

The Chai-Nan Plain is the boundary between the tropic and temperate zones as the Tropic of Cancer crosses this plain. The western inland edge of this plain stretches about 120 kilometers along the coast. The annual water temperature of the milkfish ponds in this region ranges from 8° to 33°C. The temperature during the fish rearing season from the early part of April to the middle of November is from 25° to 33°C. Both the pasture algae and milkfish prefer a relatively high water temperature, above 23° to 25°C. Milkfish die from cold when the water temperature falls to about 9° to 11°C. The annual rainfall in this region is about 2,159 mm and the annual evaporation is about 1,454 mm (1951–1960). Wet and dry seasons in this region are very pronounced; it is wet from May to September and dry during the rest of the year. The pasture algae and milkfish grow well from the latter part of May to the end of August when the pond water salinity ranges from 10 to 30 ppt due to the dilution by rain. The salinity is about 50 ppt or more before the rainy season, and less than 5 ppt at the end of the rainy season which often destroys the growth of pasture algae. The water flowing out of the ponds due to heavy rain causes the loss of plant nutrients and the deterioration of soil fertility. This is especially true during the latter part of the rainy season when the salinity of the pond water is low and when the topmost layer of bottom soil is not well covered with algal pasture.

4 ELEVATION OF POND BOTTOM

The elevation of the bottoms of milkfish ponds in Taiwan ranges from -0.5 to + 1.2 m from the datum plane of the mean sea level. Since the mean water levels of neap and spring tides in this region are respectively 0.69 m and 0.85 m, the optimum range for the pond bottom elevation is approximately at ±0.0 to +0.45 m. For ponds with bottoms at +0.6 m or more, supplemental filling of the ponds with water by pumping is necessary. However, the ponds located on the lower tidal lands, more than 0.2 m below the mean sea-level, are difficult to manage because the seepage water causes changes in the physical conditions of the pond bottom making the topmost layer of the soil unfavourable for the growth of algal pasture.

5 KINDS OF ALGAE AND THEIR YIELD

The algal pasture in milkfish ponds consist of two main groups of microscopic algae, the filamentous blue-green Cyanophyceae and the diatoms, Bacillariophyceae. The major genera of Cyanophyceae are Oscillatoria, Lyngbya, Phormidium, Spirulina, Microcoleus and possibly other genera of the order Oscillatoriales; the major genera of Bacillariophyceae are Navicula, Pleurosigma, Amphiprora, Mastogloia, Stauroneis, Amphora, Nitzschia and various unidentified genera of the order Centrales.

It is estimated that the total amount of pasture algae grazed by the milkfish during the fish rearing season from April to October approximates 25,000 kg per ha.

6 TILLAGE AND SOIL MANAGEMENT

Milkfish ponds in Taiwan are filled with water from April to October to a depth ranging from 25 to 50 cm adjusted in accordance with the growing conditions of the algal pasture and with the amount of the fish stocked in the ponds. The pasture algae generally thrive better in relatively shallow water, whereas the fish need a greater volume of water. During the fallow period, from November to March, alternate drying and wetting of the bottom soils with sea water is practised to increase the supply of salt and other algal nutrients in the soil, and to ameliorate the soil conditions. Detailed accounts of this practice are given by Chen (1952) and Tang and Chen (1957).

The most common fertilizer applied in the milkfish ponds is rice bran. Compost manures are also used, but the area of ponds where this kind of fertilizer is used is very limited. Rice bran is also used as supplementary feed during the rainy days of the latter part of the fish rearing season. Actually a large portion of the rice bran applied in this case is accumulated on the pond bottoms and acts as fertilizer for algal growth. Chemical fertilizers such as ammonium sulphate, urea and phosphates, have recently been used but only on a small scale. According to general practice, the N-P-K requirement of the soils to maintain an average level of algal pasture production during a fish rearing season is about 60 kg/ha nitrogen, 45 kg/ha P₂O₅, and 25 kg/ha K₂O. Grasses and sedges grown on the pond dykes, and rice straws after they had been used for thatching the wind-breaks of the wintering ponds, are scattered on the pond bottoms at the end of the fish rearing season every year. This is also one of the important means for the maintenance of soil fertility.

7 GEOLOGY

The milkfish ponds of Taiwan are distributed on the recent alluvial deposits on the tidal flats. The parent materials of these deposits are derived from calcarious slates and crystalline schists that have been transported from the central mountain ranges of the island. The surface run-off and streams running down from the mountains are always so heavily loaded with these particles that they are muddy. When the creeks and streams reach the esturine areas, the flow slows down and the muddy particles are deposited causing these areas to become gradually higher. The formation and expansion of this alluvial tidal flat results in constant flooding. Because the alluvium is pushed westward to the sea, the older ponds gradually become farther from the sea and it becomes more difficult for them to obtain a supply of sea water. Consequently, these ponds are gradually converted into farm land by desalinizing with fresh water. This process of land reclamation is practised in many of the milkfish pond areas along the west coast of Taiwan.

8 SOIL TESTS

Fertility tests for the soil samples were made for the surface soil only, as it was considered that the subsoil of the pond bottom has less effect upon the growth of pasture algae because algae are rootless and also because the bottom soils of milkfish ponds have never been ploughed in accordance with the traditional method of cultivation. In sampling the surface soil, the method that is customarily used by agronomists for sampling pasture lands was employed. In this method, dead and living vegetation of the surface were excluded in the samples.

8.1 Soil texture

Mechanical analyses of the soil samples were made by the pipette method. Six textural classes (Atterberg scale) were found from the algal pasture soils, i.e. sand, loamy sand, sandy loam, loam, silty loam and silt. The area of each textural class of the algal pasture soils in Taiwan was estimated (Table I). The average compositions of sand, silt and clay of these textural classes are graphically shown in Fig. 1. Of these textural classes, silty loam is the most favorable one for the growth of the pasture algae. The majority of the most productive ponds are distributed in regions with this type of soil. Loam soil is also suitable for these algae because of its balanced composition of sand, silt and clay. Algal pastures also grow well on silt and sandy loam soils provided that the soil fertility of these textural classes is well managed and maintained. The soils of sand and loamy sand texture do not grow good quantities of pasture algae because of their light texture and low content of clay and fine silt, which constitute the most active portions of the bottom soils for the growth of algal pasture by formation of soil colloids.

Table I

Textural classes of the algal pasture soils of milkfish ponds

Experiments on the growth of pasture algae on the different types of soil represented by the bottom soils of milkfish ponds in Taiwan were also conducted in a greenhouse under controlled environmental conditions. The results of these experiments are summarized in Table II. It can be seen from this table that the quantities of algae produced from the respective classes of soil texture coincide with the results observed from the field.

Table II

The growth of pasture algae in relation to the textural classes of the bottom soils of milkfish ponds ¹
(The quantities of algae produced from the soils of respective classes of texture are indicated by the loss of weight on ignition from the algal pasture samples)

¹ This trial was made with a series of 42 glass bowls with a capacity ofπ 45 × 30 cm each: each class of soil texture was tested with 7 duplicates of 3 repeats in a greenhouse in identical environments.

Fig. I - Graphic representation of the mechanical analysis of the representative classes of algal soil textures

8.2 Soil reaction

The pH values determined by the glass electrode method, ranged from 7.5 to 9.7 for these soil samples. The pH fluctuates greatly with the season, being highest at the end of the dry season, and lowest at the end of the rainy season (Table III). This alkaline reaction is mainly due to the presence of sodium chloride and sodium sulphate. But there were exceptional cases where an acid reaction, with a pH range from 5.5 to 6.8 was found in some of the soil samples from the ponds where the pond bottom is below the mean sea level. This acid reaction is probably due to the decomposition of organic matter under anaerobic conditions. Generally speaking, the alkaline soils rich in sodium are favourable for the growth of these algae.

Table III

The pH values of the algal pasture soils of milkfish ponds

8.3 Soil organic matter

8.3.1 Organic matter in the surface soil

The organic matter content of the soil samples was determined by Graham's wet combustion colorimetric method with a photoelectric colorimeter. The results of this analysis are given in Table IV. The organic matter in the surface soil in relation to the productivity of milkfish ponds based on this survey is shown in Table V. Since the growth of these groups of algae, particularly the blue-greens, requires much organic matter in the soil, pond bottoms which are rich in organic matter are indicative of high productivity.

Table IV

Organic matter in the algal pasture soils of milkfish ponds

Table V

Organic matter content of the algal pasture soils in relation to the productivity of milkfish ponds in different regions

¹ Based on the data obtained from an investigation under the project “The Economic Statistics of Fish Culture Industry” in 1964

² Ponds are under construction

Table VI

Chemical composition of organic matter contained in the algal beds of the bottom soils of milkfish ponds in different regions

8.3.2. Organic matter in the algal bed

The top-most layer of the pond bottom is termed “algal bed” as it comprises a large portion of organic soil colloids which are considered to be the most active portions of the soils for production of pasture algae. In sampling the algal bed for chemical analysis of the organic matter, the top-soil to a depth of 10 cm and the mat of the algae pasture which had been destroyed by desiccation were included.

The organic matter content of the algal beds was measured by the loss of weight on ignition, and by the organic carbon and total nitrogen content. The determination of organic carbon was made by the modified Schollenberger's method and the total nitrogen by the usual Kjeldahl method.

The chemical composition of the organic matter in the algal beds of the bottom soils in different regions indicated in Table V are shown in Table VI. These tables show that the most productive ponds are usually those having a higher content of organic matter in the algal bed. In other words, the better algal beds are usually established on those bottom soils rich in organic matter. The higher C/N ratio indicated in the organic matter of the algal beds is due mainly to the fact that the decomposition of algal gel, extracted from the residues of the pasture algae by desiccation, is still in process.

8.4 Nitrogen

Total nitrogen, ammonia nitrogen and nitrate nitrogen contents of the surface soils were examined by the usual chemical methods. Total nitrogen is richer in the bottom soils where the soil texture is relatively heavier, and where good management has retained the soil fertility for a long period of time.

The NH₃-N content in the surface soil of the pond bottoms generally range from 25 to 45 ppm. The differences were too small to indicate any direct relationship between the amount of NH₃-N and productivity. The range of NO₃-N content in the soils of the pond bottom was even smaller than the NH₃-N, generally between 15 to 30 ppm.

The total nitrogen in the soils is, of course, directly proportional to the amount of organic matter. Soils of higher total nitrogen content undoubtedly produce higher yields of pasture algae. This fact has also been proved both by culture of these algae in greenhouse under controlled conditions, and by fertilization field trials.

Table VII

Total nitrogen in the algal pasture soils of milkfish ponds

8.5 Available phosphorus

The content of available phosphorus in the samples was determined by Bray's method and the results obtained are indicated in Table VIII. Generally, the algal pasture soils are rich in available phosphorus and contain an average of 42 ppm for the whole sample. As available phosphorus promotes the growth of pasture algae, the application of either organic or inorganic phosphate to the bottom soils, alone or in combination with nitrogen fertilizers, improved the growth of pasture algae both in the greenhouse and in field tests. Therefore, the application of phosphate to the algal pasture soil is deemed very necessary.

Table VIII

The available phosphorus in the algal pasture soils of milkfish ponds

8.6 Available potassium

Available potassium in the soil samples was determined by Peech's cobalt nitrate turbidimetric method. The range of available potassium content of the algal pasture soil is from 326 to 709 ppm. The wide range of available potassium content is due mainly to the accumulation of potassium chloride from sea water by periodical evaporation. It is doubtful whether the algal pasture soil needs any application of potash since the results from experiments conducted in both greenhouse and field indicate that the algal pasture soil shows no response to potash from both organic and inorganic sources.

Table IX

Available potassium in the algal pasture soils of milkfish ponds

8.7 Available calcium and magnesium

The available Ca and Mg were determined by Peech's soap solution and titan yellow colorimetric methods respectively (Table X). As there is no soluble carbonate in the algal pasture soil, the Ca and Mg may not change the alkaline reaction of the soil samples.

Table X

Available Ca and Mg in the algal pasture soils of milkfish ponds

8.8 Chloride content

The chloride content was determined by use of the usual chemical methods and the results are indicated in Table XI. The high chloride content of the algal pasture soil is of course derived from sodium and magnesium. As the pasture algae prefer to grow in water of a certain range of salinity, an increase of chlorides in the soils, by evaporation of sea water during the fallow season, may be necessary.

Table XI

Chloride content of the algal pasture soils of milkfish ponds

9 REFERENCES

Chen, Tung-Pai, 1952 Milkfish culture in Taiwan. Fish.Ser.Chin.-Amer.Comm.rur.Reconstr., (1):17 p.

Tang, Yun-An and T.P. Chen, 1957 The use of chemical fertilizers in the milkfish ponds of Taiwan. Fish.Ser.Chin.-Amer.Comm.rur.Reconstr., (3):19 p.