by
B. HEPHER
Fish Culture Research Station
Dor, Israel
Abstract
Three groups of limiting factors affecting the fertilizer dose - chemical, biological and economic - are discussed.
The chemical equilibrium in pond water is relatively stable. The added mineral nutrients upset this stability and are, therefore, withdrawn from the water. The higher the concentration of the added nutrient in the water, the higher the rate of its withdrawal. This limits the dose of fertilizers, as larger amounts of fertilizer do not result in higher concentration of the added nutrient.
Various factors may limit the growth of fish or their food chain biologically. Higher phytoplankton production does not result in higher fish yields in the presence of any of the limiting factors. In such cases fertilizer dose may be reduced.
The economic limit of fertilization is reached when the value of the increased amount of fish yield due to fertilization is equal to the value of the fertilizer added to obtain it.
CERTAINS FACTEURS LIMITATIFS INFLUANT SUR LA DOSE D'ENGRAIS APPLICABLES AUX ETANGS DE PISCICULTURE, NOTAMMENT AU MOYEN-ORIENT
Résumé
Trois groupes de facteurs limitatifs influant sur la dose d'engrais sont examinés: les facteurs chimique, les facteurs biologiques et les facteurs économiques.
L'équilibre chimique de l'eau est relativement stable en étang. Cet équilibre étant rompu par l'apport d'éléments nutritifs exogènes, ces éléments tendent donc à être éliminés de l'eau. Plus la concentration des éléments ajoutés est élevée dans l'eau, plus le rythme d'élimination est rapide. Ce phénomène limite la dose utile d'engrais puisqu'un apport plus important n'entraîne pas une teneur plus élevée en éléments nutritifs.
Du point de vue biologique, divers facteurs peuvent limiter la croissance des poissons ou leur chaîne alimentaire. Une production plus forte de phytoplancton n'entraîne pas une productivité plus élevée du poisson en présence de l'un quelconque de ces facteurs limitatifs. Dans de tels cas, la dose d'engrais peut être réduite.
La limite économique de fertilisation est atteinte lorsque la valeur de la production supplémentaire de poisson due à l'utilisation d'engrais devient égale à la valeur des engrais utilisés pour obtenir ce surcroît de production.
ALGUNOS FACTORES LIMITANTES QUE INFLUYEN EN LA DOSIS DE FERTILIZANTES ADICIONADOS A LOS ESTANQUES PISCICOLAS EN LO QUE SE REFIERE ESPECIALMENTE AL MEDIO ORIENTE
Extracto
Se examinan tres grupos de factores limitantes que influyen en las dosis de fertilizantes: químicos, biológicos y económicos.
El equilibrio químico en las aguas de los estanques es relativamente estable. Los minerales nutrientes añadidos alteran esta estabilidad y, por consiguiente, se eliminan del agua. Guanto mayor es la concetración del nutriente adicionado al agua, tanto más elevada es la tasa de su eliminación. Esto limita la dosis de los fertilizantes, ya que al aumentar las cantidades de éstos no da por resultado una mayor concentración del nutriente añadido.
Varios factores pueden limitar biológicamente el crecimiento de los peces o su cadena alimentaria. Una mayor producción de fitoplancton no da por resultado mayores rendimientos de peces en presencia de cualquiera de los factores limitantes. En tales casos, debe reducirse la dosis de fertilizante.
El límite económico de la fertilización se alcanza cuando el valor del aumento de rendimiento de peces, debido a la fertilización, iguala al valor del fertilizante añadido para obtenerla.
Experiments carried out in various parts of the world since the beginning of the century have proved that fertilization of fish ponds can increase fish yields considerably. Thus fertilization is considered one of the more important means of intensifying fish culture. No wonder, then that in the intensive fish culture in Israel, fertilization has become a common practice in commercial fish farms. Research and empirical experience have made it possible to establish some standards in fish pond fertilization in Israel, yet a great deal still remains to be determined. A particular problem in this regard relates to dose and frequency of fertilization. Some of the factors affecting fertilization dosage under Near Eastern conditions are discussed below.
Three main groups of factors limit the fertilizer dose - chemical, biological and economic.
The water in fish ponds is a complicated chemical system of equilibria in which the bottom soil takes an active part. These equilibria govern the cycles of the elements in the pond and their concentration in the water. There is a close interaction between these chemical equilibria and various physical, biological, and edaphic factors such as temperature, photosynthesis, bottom soil composition, etc. These factors affect, and are at the same time affected by changes in the chemical equilibria and the cycle of events in the pond.
Some of the factors affecting the equilibria such as temperature, oxygen tension, etc. may vary to some extent, but these variations usually remain within fixed limits which are determined by local conditions of climate, geology, and soil composition. As a result, there is a certain stability in the chemical equilibria in the water and the extent of variation in the concentration of the elements in the water of the pond is limited.
The adding of mineral nutrients in fertilization of the pond upsets the chemical equilibria. The equilibria, however, tend to re-establish themselves and the added elements are withdrawn from the water by various chemical and physico-chemical reactions.
In Near Eastern conditions, the phosphorous, which is the most important fertilizer added to the pond, is absorbed to bottom soil colloids (Hepher, 1958) or precipitated as calcium phosphate (Matida, 1956; Hepher, 1958). In the temperate region similar reactions bring about the same result. In this case the reacting elements are usually iron and aluminium, precipitating the phosphate as iron and aluminium phosphates, or adsorbing it on the ferrio hydroxide gel of the bottom soil (Ohle, 1937; Einsele, 1938).
The reactions causing the disappearance of nitrogen from the water have been studied to a lesser extent. It seems that in alkaline water some of the nitrogen is converted to gaseous ammonia which escapes from the water into the air. Another part of the added nitrogen is probably absorbed by cation exchange reactions to soil colloids (Mandal, 1962), and then leached away.
All these reactions cause a rapid disappearance of the added nutrients from the water. Moreover, the rate of disappearance is dependent on the concentration of the nutrient in the water. The higher the concentration, the quicker the disappearance of the nutrient. This, of course, limits the amount of fertilizer that can be added to the pond. Past a certain point the rate of disappearance is so high that actually no further increase in the concentration of the nutrient in the water can be achieved. This was clearly shown in the experiments of Rashkes (Sarig) (1949) and Reich (1950), where even after adding large doses of phosphate fertilizer, the recovery was seldom over 0.5 mg/l P - only a fraction of the phosphorus added. It has already been shown theoretically (Hepher, 1958) that under Israeli conditions, the fraction of the phosphate added above this level is precipitated as calcium phosphate. Similar results have been observed when large amounts of nitrogen were added to the water. The concentration seldom exceeded 2.0 mg/l N.
Fig. 1. The average daily growth rate of fish in ponds receiving different fertilization treatments (for actual amounts see text).
It is clear then, that chemical factors limit the dose of fertilizer, as there is no point in adding nutrients which will be lost immediately.
No doubt, part of the nutrients added as fertilizer is taken up by the phytoplankton and serves the initial purpose of increasing the pond's productivity. The phytoplankton is the first and basic link in the food chain of the fish. Increasing the phytoplankton productivity activates the whole food chain, with the resulting increase in fish yields. The food chain and the growth of the fish are affected, however, by a number of factors, of which phytoplankton productivity is only one. Factors such as temperature, pH, etc. may affect any link in the food chain or the fish itself. If any one of these factors becomes a limiting factor, increasing phytoplankton production would not result in increased fish yield. In such cases the dose of fertilizer may be smaller.
An example of such a case may be seen in the results of a fertilization experiment given in Fig. 1. In this experiment, which began in August 1963 and continued through 27 October, 11 ponds were fertilized every two weeks with 60 kg/ha superphosphate and 60 kg/ha ammonium sulphate (‘Standard fertilization’), and nine ponds were fertilized with only 30 kg/ha superphosphate and 30 kg/ha of ammonium sulphate. In summer there was a noticeable difference in the growth of the fish in the two treatments, the former being higher. As temperature dropped in autumn this difference disappeared.
Limiting factors other than nutrient elements added by fertilization may affect phytoplankton production itself. Such a case is described by Hepher (1962) where light became a limiting factor. Fertilization of fish ponds with large amounts of fertilizers increased production in the upper layers of the water where favourable light conditions exist, but decreased production in the lower layers, where shading by the plankton caused a decrease in light penetration. The effect of a large dose of fertilizer may thus cause a lowering of production per unit area.
The effect of fertilizers on fish yield, like the effect of any other growth factor, is governed by the law of diminishing returns. In our case, this means that each successive portion of fertilizer causes a smaller return in yield, until the optimum point is reached where there is no further increase of yield with the increase in fertilizer.
From an economic point of view the optimum dose of fertilizer is reached before this levelling off of the yield curve occurs. It is reached when the price of the ‘last’ portion of fertilizer equals the value of the increase in fish yield received in return. The following may illustrate this point.
Experiments at Dor have shown that fertilization with a ‘standard’ dose of fertilizers increased the fish yield by 88 percent in ponds where fish were fed (Hepher, 1962a). Considering the average yield of 2,100 kg/ha in Israeli commercial ponds, this gain is equal to about 980 kg/ha.
Another experiment (Hepher and Chervinski, 1965) showed that the yield in ponds fertilized with half the ‘standard’ dose (30 kg/ha of each of the fertilizers mentioned every two weeks, or 425 kg/ha per year) was about 6.1 percent lower than in ponds receiving the full ‘standard’ dose. (And not half the gain, or 44 percent as could be expected if a linear relationship had existed between fertilizer and yield). This decrease is equal to about 128 kg/ha of fish, the value of which, in Israel, is three times the value of the fertilizers added to achieve it. One can see that in this case fertilization is still economical. By further experiments (which unfortunately were not carried out in this case) the economic limit of the fertilization dose can be found more accurately.
It may be concluded that these three groups of factors, which may differ between localities, determine the dose of fertilizers that should be added to fish ponds. Only by studying the effects of these factors on fertilization and on fish yield can the optimum fertilizer dose be found.
Einsele, W., 1938 Uber chemische und kolloidchemische Vorgänge in Eisen-phosphatsystemen unter limnochemischen und limnogeologischen Gesichtspunkten. Arch.Hydrobiol., 33:361–87
Hepher, B., 1958 On the dynamics of phosphorus added to fish ponds in Israel. Limnol.Oceanogr., 3(1):84–100
Hepher, B., 1962 Primary production in fish ponds and its application to fertilization experiments. Limnol.Oceanogr., 7(2):131–6
Hepher, B., 1962a Ten years of research in fish pond fertilization in Israel. 1. The effect of fertilization on fish yields. Bamidgeh, 14(2):29–38
Hepher, B. and J. Chervinski, 1965 Studies on carp nutrition - the influence of protein-rich diets on growth. Bamidgeh, 17(2):31–46
Mandal, L.N., 1962 Nitrogenous fertilizers for brackish water fish ponds, - ammonium or nitrate form? Indian J.Fish.(A), 9(1):123–4
Matida, Y., 1956 Study of farm fish culture. 3. Fates of fertilized elements and the relationship between the efficiency of fertilizer and biochemical environment in the pond. Bull.Freshw.Fish.Res.Lab., Tokyo, 6(1):27–39
Ohle, W., 1937 Kolloidgele als Nahrstoffregulatoren der Gewässer. Naturwissenschaften, 25:471–4
Rashkes (Sarig), S., 1949 (On the nitrogen and phosphorus regime in the river Asi and the experimental ponds at Nir David). Alon Lemegadley Dagim, (26/27):1–12; (28):1–7 (In Hebrew)
Reich, K., 1950 (On a small scale experiment for the determination of a fertilization method). Alon Lemegadley Dagim, (32):1–9 (In Hebrew)
