by
V.P. Lahnovitch
Candidate of Biological Science
A n n o t a t i o n
This report deals with a short history of the development of pond fertilization concepts and practices of their application in various countries The following trends are singled out: the German school, non-nitrogen fertilization theory; the U.S.A. school, treatment with commercial mixed mineral fertilizers containing nitrogen, phosphorus and potassium, and the Soviet school, combined fertilization and integrated investigations of the mechanism of fertilization effect in the ponds.
Treatment with potassium as a mineral pond fertilizer is shown as groundless. The main stages of the complex cycle processes of phosphorus and nitrogen, chief biogens whose limited quantities inhibit the fish pond biological productivity generally, are briefly traced.
As there is no reason to consider calcium a deficient element of the mineral diet of aquatic organisms in carbonate-calcium waters, liming of ponds is treated as an important amelioration measure of preparation for an intensified utilization of ponds with acid water and bottom soils.
This report contains the fundamentals of the primary effect of fertilizers in fish ponds, their influence upon the phytoplankton composition, quantitative development and photosynthesis and states a quantitative relation between the magnitudes of the gross primary output and fish productivity. The nature of fertilizing influence on the natural fish food supply development in ponds, the quantitative relation between the primary output and zooplankton standing crop, as well as a direct dependence of natural fish productivity on the amount of the feed standing crop are treated in the light of the above.
In conclusion, an optimal phosphorus - nitrogen ratio of pond fertilization and efficiency of various fertilizers are considered. Efficiency is evaluated according to fertilizer consumption per unit of fish output increment.
The report is based on “Pond Fertilizing” by G.G. Vinberg and V.P. Lahnovitch.
I.
In practical pond fish culture, fertilization is an effective means of raising fish productivity. As pond fish rearing is being further intensified, ponds are treated with more and more mineral and organic fertilizers. The pond fisheries are concerned with the results of fertilization efficiency investigations because they need to understand the mechanism of the fertilizers' effect upon the fish productivity of ponds.
As to the application of fertilizers and theoretical conceptions, there are two main trends abroad. One of them originated in Germany and the other in the U.S.A., somewhat later.
The theory of non-nitrogen fertilization prevails in Germany and in some other European countries unreservedly.
According to this theory, the ponds treated with superphosphate or other phosphatic compounds do not require any nitrogenous fertilizers. Phosphatic fertilizers are believed to increase the bacterial nitrogen fixation to such an extent that no nitrogenous fertilization is needed.
Thus, the phosphatic fertilizer becomes universal. In pond fish culture in Germany the practicable rate of phosphatic fertilizer application is no more than 25 to 30 kg of P2O5 per ha. Larger dosages of the phosphatic fertilizer result in a very insignificant gain in the fish output and, hence, are not applied. When only one phosphatic fertilizer is used, the natural productivity of fish ponds rises by 50 to 80 percent. Organic fertilizing (manure, compost, etc.) had been long practiced in German fish culture but the patterns of their effect are not investigated systematically as is the case in many other countries.
Fertilizing ponds was started in the U.S.A. much later than in Europe, and the problem was developed independently, disregarding the European experience. Large doses of nitrogenous-phosphatic-potassic fertilizers repeatedly spread on pond waters is the routine practice in the U.S.A.
Unlike the German theoreticians, their U.S.A. colleagues ignore silt sediments and lay a particular emphasis on the direct influence of fertilizers on the phytoplankton development.
In the U.S.A. preference is given to the mineral fertilizers rather than the organic ones, as their biogenous composition is easier to control and they do not bring about undesirable after effects in oxygen conditions. Though liming of ponds is used in the U.S.A. for neutralizing acid waters, it is far less important than in the European practice. In the U.S.A. the ponds are treated with 450 to 1,350 kg/ha of commercial mixed fertilizers containing 10-10-5 percent or 12-12-4 percent of N, P2O5 and K2O, respectively. Fertilization results in a 3 to 4-fold increase in fish productivity. As predatory or semipredatory fish are mainly reared ub the American ponds, the effectiveness of fertilization must be recognized as very high.
Similar methods of fertilization are used in Japan and some other countries. The American method of fertilization used in pond fish culture in Israel has proved especially effective. Superphosphate and ammonium sulphate are used as mineral fertilizers in Israel and chiefly guano as an organic fertilizer. The weekly rate of application of mineral fertilizers into the pond water is such as to maintain the mineral nitrogen content on the level of 1.5 mg/l and that of phosphorus 0.5 mg/l. The total amount of fertilizers applied annually is 1.5 to 2 t/ha. The average fish productivity of the fertilized ponds within three cycles of carp cultivation (1962 data) is 2,040 kg/ha with variations ranging from 1,000 to 4,000 kg/ha.
The maximum annual fish yield of 8,978 kg/ha was achieved in the experimental ponds. That means that nitrogenous phosphatic fertilizers are extremely effective under the subtropical climate conditions of Israel.
Numerous investigations of pond fertilization were carried out in the U.S.S.R. during recent decades. The theoretical substantiation of the ponds requirements in nitrogenous fertilizers, later confirmed by the wide scale practice of pond fish culture in various soil and climatic zones of the U.S.S.R., was very important. At present the further development of theoretical bases for application of mineral fertilizers is conducted along with the wide scale practical treatment of pond waters with crushed superphosphate and Norway saltpeter.
At the same time scientific bases for combined fertilization with mineral salts and organic substances are being worked out. It has been proved that a 4 to 5 fold increase in the natural fish productivity can be attained. Unlike many foreign researchers, the Soviet scientists conduct integrated investigations of the fertilization problems, which are aimed at ascertaining the mechanism of the fertilizer effect, otherwise the creation of a scientifically substantiated theory of pond fertilization is impossible. It is the Soviet authors only who treat the importance of bacteria in the productive processes in the fertilized ponds with due attention. Another task of the Soviet researchers is investigations of plant and various local fertilizers applied to raise the fish productivity.
II.
A pond, as any other water body, is a complex spatially divided system, wherein certain conditions and regularities determine the distribution and conversion of the substances applied in the water, others in the media pertaining to the bottom, etc., with all these phenomena interacting. Fertilizing may result in a rise in fish productivity only after complicated trophic interconnections among aquatic organisms. The final aim of fisheries is to secure the maximum fish output and a high fertilizer efficiency. Therefore, fertilizers should serve to increase the primary product, this primary product should be used as much as possible by food organisms, the latter by fish. The effectiveness of fertilizers, i.e., their influence upon fish productivity, depends on the effectiveness of utilization of the nutrients and food energy at all stages of the productive process. Any imbalance in this mechanism may affect the final efficiency of the fertilizers and even result in a complete failure of the measure.
When mineral fertilizer treatment of ponds was started, it was naturally prompted by the successful experience of fertilizer application in farming, which uses nitrogen, phosphorus and potassium as main nutrients. This is the reason why we deal with these three elements in this case too. It is aquatic vegetation that is affected directly by mineral fertilizers. This also applies to phytoplankton, which is of primary importance to us. Hence, only those fertilizers can cause a higher fish productivity which, under the circumstances, constitute the extra quantities of substances required by phytoplankton. Such requirements are easily determined by biological tests according to the oxygen balance in separate water samples, the biogens under test being added.
As to potassium, there are no theoretical or experimental indications that it inhibits the phytoplankton development in nature, as nitrogen and phosphorus do. And, on the contrary, there is every reason to believe that it does not. A 2 to 6 mg/l content of potassium is most frequent in the ponds fed with surface carbonate waters of low or medium mineralization, the majority of European and North American pond fisheries being located on such waters. It is difficult to expect a potassium deficiency, when the potassium content of phytoplankton substance is low (0.05 to 1.0 percent of dry weight). This content is much higher in chloride waters than that in hydrocarbonate ones. Numerous biological investigations conducted on many ponds in various zones of the European part of the Soviet Union have never revealed a single case of pond plankton being short of potassium. This provides every reason for a conclusion that treatment of ponds with potassic salts as mineral fertilizers is not justified.
As to phosphorus and nitrogen mineral compounds, there is no doubt that they inhibit phytoplankton development. It was observed in many cases that phosphatic or nitrogenous compound treatment is conducive to an intensive phytoplankton development. Consequently, these are the two elements which are deficient in most water bodies of different types. The practical experience with mineral fertilizers in fish culture is a convincing confirmation of this general conception of the water body biological productivity theory.
The regularities of phosphorus and nitrogen cycles in the ponds, studied inadequately so far, are very important for understanding fertilization efficiency. It is well known that pond treatment with a phosphatic fertilizer brings about but a short-term increase in the phosphate content of water. Within several subsequent days this content drops to a certain value typical of the water body and very close to the initial one. Very often at the end of 1 to 2 days only 1 to 2 percent of the initial concentration of the phosphorus introduced with the fertilizer remains in the solution. This reduction is only partially due to the phosphorus consumption by phytoplankton. The larger portion of the fertilizer precipitates and is fixed by the bottom mud of the pond. The mechanism of this process varies under different conditions but a rapid reduction in the dissolved phosphorus concentration after application of fertilizers never fails to take place. Changes in the phosphate content of water are due to changes in the relations between the rate of phosphorus entering the water and that of its utilization and absorption. But these changes give no clue to the rates themselves, which may be low with one and the same phosphate content (low productivity waters) or high, as in the case of water bodies with an intensive substance cycle, a prerequisite for high productivity. The total phosphate content of the pond water at a given moment is very small, as compared with that of aquatic vegetation, animals and particularly the bottom sediments. One ha of the pond bottom mud contains several hundred kg of fixed phosphorus, whereas the phosphorus content of water is generally fractions of a kg, i.e., several hundred times less. The dissolved phosphorus rotates intensely, while the cycle rate of the phosphorus fixed in the bottom mud is much lower.
It has been found by means of the tracer method (p32) that the maximum rate of dissolved phosphorus cycle is with phytoplankton and bacteria. In this case the “rotation cycle” is strikingly short - about five minutes. It is somewhat lower with zooplankton and still lower with higher aquatic vegetation, the phosphorus “rotation cycle” lasting for a few days. The rate is the slowest with the pond bottom soils, where phosphorus is retained longest. However, the quantity of phosphorus available for phytoplankton depends on the conditions of the phosphorus exchange between the water and pond bottom soil, since the bottom soils contain most of it, as was stated above.
The phosphorus exchange between the water and pond bottom soil is determined both by physical, chemical and microbiological patterns inherent in conventional soils (sesquioxide content, medium reaction, calcium content, effect of pH upon microbiological processes) and those specific to submerged soils characterized by the importance of anaerobic processes. To understand the conditions favoring the optimal utilization of phosphatic fertilizers, it is necessary to carry out regular investigations of the regularities of the phosphorus cycle in the ponds.
At present, only general data on the patterns of the nitrogen cycle in the ponds is available to us. This is due mostly to the fact that, when investigating pond fertilization problems, the main attention is attached to the final results of fertilizer application, whereas the study of the general mechanism of fertilizer effects is insufficient.
Nitrogen stocks of the bottom mud are much larger than those of phosphorus. The nitrogen content of the upper 10 cm layer of the bottom soil is up to 2,000 kg/ha, whereas an active biological cycle involves not more than 100 kg of phosphorus per ha even in highly productive ponds. This content is still less in solution, 10 kg/ha.
The nitrogen cycle differs substantially from that of phosphorus, as these two elements play absolutely different parts in living organism metabolism. During the biochemical processes of the energy exchange the phosphoric acid residues keep adding to organic compounds and splitting from them repeatedly, this determining a high lability of this element. When the fermentative autolysis of the dead organism occurs, 70 to 80 percent of phosphorus comes back to solution within a short period of time. The nitrogen fixation in the organic matter of living organisms is much faster, hence only 20 to 30 percent of its total content dissolves during fermentative autolysis. Conversion of the larger portion of nitrogen to the mineral form is brought about by the considerable microbiological decomposition of organic molecules.
Microbiological processes predominate in the nitrogen cycle, while such physical and chemical phenomena as different solubility of nitrogenous compounds, their adsorption, etc. become less important than in the case of the phosphorus cycle.
The highlights of the complex phenomenon of the nitrogen cycle in the water body may be summed up as follows:
Consumption of mineral (ammonium and nitrate) nitrogen by phytoplankton and other plants and conversion of mineral nitrogen to organic.
Consumption of phytoplankton and other plants by plankton and benthic animals, followed by biochemical changes in the assimilated nitrogen containing organic matter.
Various biochemical processes of exchange between plants and living organisms resulting in restoration of some nitrogen into the medium in the form of ammonium ions or some other relatively simple compounds accessible for plants.
Bacterial decomposition of feces, dead plants and animal organisms in water on bottom sediments resulting in a partial nitrogen conversion to the ammonium form (ammonification) and a partial conversion by bacteria which, in their turn, may be used by animal filtrators or detrivorous organisms.
Nitrification, i.e., bacterial oxidation conversion of ammonium nitrogen into nitrate.
Denitrification, which occurs under certain conditions and results in nitrogen losses as well as a decrease in the amount circulating in the biotic cycle. For instance, when the pond is treated with nitrate nitrogen, there follows an increased development of denitrification bacteria (Pseudomonas, Achromobacter, Chromobacterium), though damaging losses of nitrogen do not occur.
Fixation of free nitrogen by bacteria (Azotobacter, Clostridium) and some blue-green algae, i.e. a process the reverse of denitrification.
It is impossible yet to evaluate these processes quantitatively. Not only can mineral compounds of phosphorus and nitrogen applied as commercial fertilizers enrich ponds with biogenous elements which limit productivity, but so also can many organic fertilizers such as manure, compost and higher aquatic vegetation, as well as household and certain industrial sewage. The effect of organic fertilizers in the ponds is accompanied by many undesirable byprocesses, such as intensive oxygen absorption by easily mineralized organic substances. The regularities of the organic fertilizer influence upon pond quantitative productivity has been investigated even less than those of mineral fertilizers. Frequent treatment of the pond water with dissolved mineral fertilizers has proved satisfactory. Many organic fertilizers are presumably also more effective when applied by suitable doses during the vegetation period. Organic and mineral fertilizers are mutually complementary and application of both may prove most effective.
A suitable preparation of ponds ensures a higher effectiveness of fertilizing. Liming of ponds is one of the most popular measures of this kind, particularly in the case of acid bottom soils and waters.
In calcium carbonate waters, calcium prevails over all other cations, therefore, the shortage of calcium as an element of mineral feeding of aquatic organisms is not likely to arise. Thus, there is no reason to consider lime treatment to be a fertilizing measure. It should be considered an important preparatory amelioration measure.
Liming proves most satisfactory in the case of acid waters. An increase in water alkalinity is a specific feature of lime treatment, which is mainly due to the increased contents of bicarbonate ions and accompanying calcium ions. An increase in alkalinity is caused by the formation of easily soluble bicarbonates when the calcium carbonate reacts with carbon dioxide. Consequently, treatment of ponds with CaCO3 is likely to be followed by an alkalinity increase only if surplus carbon dioxide is available or if there is a source of free carbon dioxide supply.
Slaked lime treatment of ponds requires much caution and small doses may be applied only, because when the amount of Ca(OH)2 exceeds that of CO2, the effect is quite the opposite and it may bring about a decrease in alkalinity as a result of the formation and precipitation of sparingly soluble carbonates.
In general, when an increase in water alkalinity is required, lime should be applied along with organic fertilizers and, on the contrary, carbon dioxide accumulation and consequent water acidification as a result of organic matter decomposition may be eliminated by liming. This is one of the fundamentals of the method of joint manure and lime fertilizer treatment of ponds, which was evolved by practical experience.
Lime treatment of the pond bottom proves entirely effective when it serves to eliminate the bottom soil acidity. It results, first in the change of physical and chemical conditions of the exchange between the bottom soil and water of the pond and, secondly, in creation of conditions more favorable for its microflora, the latter being of vital importance for the biogenous element cycle. When the soil is neutralized, the phosphate fixation by sesquioxides is loosened and the release of phosphate ions into the water is facilitated. Microbiological processes that accelerate the nitrogen cycle are also intensified.
III.
The development process from the fertilizer to the final product-fish - is complicated and is subject to numerous intermediate factors in the trophic sequence. The initial effect of fertilizing is an intensified development of phytoplankton. The so-called “water bloom” sets in, the duration of which is entirely dependent upon the fertilizer efficiency.
As a rule in such cases, blue-green algae prevail among phytoplankton, whose importance is treated in different ways in fish culture literature. Some authors consider the growth of blue-green algae undesirable and they search for the means of their removal from fish ponds.
For instance, it is suggested to maintain the mineral nitrogen concentration in water at the level of 5 to 7 mg/l, which involves an extremely high consumption of mineral nitrogenous fertilizers (about 3 t/ha) and still does not achieve the purpose. At the same time many researchers consider the blue-green algae growth to be an indication of high fish productivity of the ponds, which is borne out by fish pond management's practical experience.
The specific composition of phytoplankton is not changed by fertilizing. There are no species in the treated ponds which are not encountered in untreated ones. But the relative abundance of various phytoplankton specimens may be changed greatly, the total phytoplankton standing crop rising considerably.
Byelorussian carp fisheries experience has shown that, as the phytoplankton development is enhanced as a result of application of fertilizers and other means of intensification, the share of the blue-green algae in the phytoplankton specific composition increases. At the same time, it reveals a relation between the phytoplankton standing crop and natural pond fish productivity.
Measurements of the primary plankton output are extremely essential for assessing the primary effect of fertilizing because fish food supplies and, eventually, fish productivity are based on this primary output. The primary plankton output is measured according to the photosynthesis rate by the light and dark bottle method, which is being used more and more in investigations of fertilizer effectiveness.
It was proved by means of this method that the gross primary plankton
output of the fertilized ponds may increase from 0.1 to 7 to 10 mg/l of
oxygen daily and even more than that within the entire vegetation period.
At certain periods of the intensive production the gross primary plankton
output may be as high as 17 to 20 mg/l daily. A direct relation between
the primary output and pond fish productivity is obvious. It has been
figured out that the energy conserved in the fish output makes up about
8 percent of the energy of the gross primary plankton output. Considering
that the theoretically attainable efficiency of energy utilization within
the ideal system: phytoplankton 
food organisms fish, may be about
30 percent of the gross primary plankton output, experimentally the obtained
8 percent may be considered a high efficiency of the primary output
utilization.
Intensification of the processes of primary product formation by means of fertilizing is very important for pond oxygen conditions. The increased phytoplankton photosynthesis activity contributes to saturation of water with oxygen. Conditions ensuring the optimal utilization of the fertilizers applied with a view to raising pond fish productivity coincide with those required for maintaining the normal oxygen content in water. Calculations prove that mineral fertilizer treatment is the most economical way of maintaining oxygen conditions favorable for fish. Favorable oxygen conditions are maintained when not less than one half of the fish output is turned out at the expense of the primary plankton output utilization, i.e., at the expense of natural feed supplies. This should be taken into consideration when planning further intensification of pond fish culture.
Due to fertilization, the primary product in the form of higher aquatic vegetation may grow, which might have intensified the development of water invertebrates as an integral part of the natural fish feed supplies. But, with such development of the productive process, the rate of the substance and energy cycle slackens conspicuously and fertilizing becomes ineffective. Therefore, all macrophytes should be removed from ponds to make fertilization effective.
Investigations of the natural fish feed supplies (zooplankton and zoobenthos) in the treated ponds have shown that fertilizing influences the quantitative zooplankton growth most of all, the specific composition of zooplankton remaining unchanged but the ratios of population specimens varying substantially. The standing crop of zooplankton increases 4 to 5 fold in the treated ponds and in some cases 6 to 8 fold, as compared with the untreated ponds and may average 350 to 400 kg/ha within one vegetation period. As a rule a high zooplankton standing crop in the treated ponds is due to larger species of the zooplankton because Daphnia begin to prevail over rotifers in the aggregate zooplankton standing crop. It is of great practical importance for carp fisheries where bigger and heavier fish need bigger food organisms. The zooplankton standing crop in ponds is in direct proportion to the primary plankton output. On the basis of Byelorussian pond carp fisheries experience, this relation may be expressed by the following equation:
Y = 0.4 + 8.4 X, where
Y - average zooplankton standing crop in grams per m3 within one vegetation period.
X - average gross primary output in grams of oxygen per m3 within the same period.
The zoobenthos specific composition does not change either when the pond is treated. Treatment with mineral fertilizers does not influence the quantitative development of zoobenthos to a great extent, at least during the first year of application.
However, treatment of the same ponds with mineral compounds of phosphorus and nitrogen for several years improves the zoobenthos development and its standing crop is increased.
The benthos standing crop is increased faster when organic fertilizers are used along with certain quantities of household and industrial sewage waters (from sugar refineries, starch plants and dairies) released into the pond water, as well as when carp rearing is combined with duck raising. These findings favor the integrated use of organic and mineral fertilizers in pond fisheries.
Since organic fertilizer treatment is curbed by oxygen conditions and other factors, the zoobenthos development in ponds is poorly stimulated, while mineral fertilizers are used everywhere and on an ever-growing scale. As a result, zooplankton begins to prevail in the fish food supplies over other groups. The investigations carried out in many Byelorussian ponds have proved that, along with the aggregate standing crop increase in food organism populations due to fertilizing, the zooplankton share in its composition rises from 40 to 80 percent, whereas zoobenthos drops from 45 to 20 percent. The fauna of the aquatic vegetation share in fish food supplies is gradually reduced to zero.
A direct quantitative relation between the standing crop of fish food organisms in the ponds and the fish productivity is traced and it may be shown as:
Y = 18 + 2.1 X, where
Y - fish output, kg/ha
X - average food standing crop for the season, kg/ha
IV.
The efficiency of pond fertilizing varies greatly depending on the fertilizer and procedures and conditions of its application. Phosphatic and nitrogenous fertilizers, taken separately, are effective in small doses only but it is not always the case.
Combined application of nitrogenous -- phosphatic mineral fertilizers permits us to substantially increase fertilizer doses, the effectiveness being preserved on the same level.
According to our experience, doses up to 1,000 kg/ha of Norway saltpeter and 700 kg/ha of superphosphate do not result in a lower efficiency of fertilizing, when fertilizers are applied by small portions spread on water during the whole vegetation period, provided the correct phosphorus-nitrogen ratio is selected.
The two-fold increase in the above dosage results in a higher fertilizer consumption per unit of the fish output increment. When pond fertilizing is effective, about 1.5 kg of superphosphate and 1.5 to 2.0 kg of Norway saltpeter produce 1 kg increment in the fish output.
The investigations of the optimal nitrogen/phosphorus ratio for mineral fertilizers applied in the U.S.S.R. fish ponds have just been started. Recent investigations have shown that optimal results are attained with the nigrogen/phosphorus ratio close to 4 to 7 (Baltic area, Beylorussian, north Ukraine and non-black soil zone of the European part of the Russian Federation).
According to the analysis of the efficiency of organic fertilizers used for fishery management purposes, provided conditions are optimal, 30 to 35 kg of manure or 60 to 70 kg of higher aquatic vegetation produce 1 kg of extra fish output. There is every reason to believe that when an integrated application of mineral nitrogenous-phosphorous and organic fertilizers is effected, their efficiency is increased.
