In fact Nolte was recently able to demonstrate that a potash fertilization is quite pro- 

 fitable in "poor" ponds in the high moor. In four-year experiments in Geaste an increased 

 average growth of 29. /« kilograms per hectare (26.1 pounds per acre) was achieved. One 

 kilogram of K2O per hectare produced 0.23 to 1./V7 kilogram of growth increase. The results, 

 however, varied greatly. Also in regard to the kind of potash fertilizer, great variations 

 were found. In 1930 potassium-magnesium sulphate produced the highest results, kainite by 

 far the smallest, and in 1932 it was reversed. The question as to which kind of artificial 

 fertilizer is to be applied for existing conditions, must be regarded as completely un- 

 solved according to other experiences also. Walter surmises that perhaps the acid constit- 

 uents could act as mobilizers on phosphoric acid and lime so long as there is a sufficient 

 Bupply of these substances on hand. 



The potash salts are most simply mixed with the phosphates and then distributed. In 

 practice as a rule about 2 double hundred weight per hectare of kainite (178 pounds per acre) 

 has been given, which corresponds to Walter's suggestion to give 30 kilograms of pure 

 potash (K2O) per hectare, 26.7 pounds per acre. In potash-poor ponds, in moor and heath 

 ponds it is advisable to experiment with larger doses of about 60 kilograms per hectare 

 (53.'i4 pounds per acre) of K2O. 



In brood ponds, a potash fertilization is also still profitable in some circumstances 

 if the monetary net profit is not increased. In the brood pond, especially in the carp- 

 brood nursery pond every increase of natural nutrition is of the greatest value for the 

 health maintenance of the brood. 



On the question, whether and under what conditions a profitable nitrogen fertili- 

 zation in the pond is to be considered, it is unfortunate that at present it is still not 

 possible to form a fully clear conception. Increased yields can doubtlessly be achieved 

 as well by nitrogen fertilization alone and also by nitrogen doses with phosphate and 

 potash fertilization. Yield increases were also obtained in this way in recent years in 

 Wielenbach, in spite of earlier failures, and similarly even earlier in Sachsenhausen. 

 On account of the relatively high expenditures the net yields mostly do not rise with the 

 gross yields, the fertilization with mineral nitrogen is practically unprofitable. Still 

 it should also not be forgotten that the increase of natural nutrition can indirectly make 

 itself more than profitable by means of a better status of health of the obtained fishes, 

 even when the monetary yield increase with the fishing out seems to be abolished again 

 by the costs of the fertilization. According to the existing pond experiments, also 

 according to experiments started by Naumann and Jaernefeld under completely comparable 

 conditions in half vats, it seems moreover as though even a nitrogen-free fertilization 

 then always suffices for high production, if organic substance is present. The assumption 

 of Walter, that the organic substance is a continuous slow nitrogen purveyor, has not been 

 verified thus far, since equally good results were achieved also with pure practically 

 nitrogen-free cellulose. Prom all this it follows that the "nitrogen problem" in the pond 

 has been and is of great interest and merits further experiments and researches. 



Sodium nitrate, ammonium sulphate and urea in experiments hardly showed any differ- 

 ence in action on the average and when applied individually they caused yield increases of 

 about 50 percent. Jaernefeld believed, of course, that he has had better successes with 

 saltpeter than with ammonium compounds which in agriculture act slower but more persist- 

 ently. In Sachsenhausen it was the reverse. 



Noteworthy are the results which Walter achieved in 1929 and 1930 with only guano 

 fertilization (about 300 kilograms per hectare, 267 pounds per acre), in which the phos- 

 phates contained in guano must naturally have worked strongly cooperative. A yield 

 increase of 100 to 150 percent was obtained. It may be asked, however, whether an organic 

 fertilization would not have given the same success more cheaply. It is to be noted also 

 that in Wielenbach, results with nitrogen without phosphorus remained far behind those 

 with phosphorus without nitrogen. 



Attempts to inoculate the ponds with nitrogen gathering bacteria (Azotobacter) by 

 the use of various preparations, have had no results up to now in Wielenbach. With the 

 strong influence of the environmental conditions upon the metabolic cycle of the nitrogen, 

 this result is almost to be expected. 



170 



