Fish Emulsions: These fertilizers are produced by soaking trash fish, offal and scraps in water to extract all the solubles. This extract is then condensed until it contains less than SO per cent water. Surprisingly, the condensed product does not have an offensively fishy odor. The method of extracting insures that all elements are present in soluble form and are readily available to plants. Like dried blood, fish emulsions (they contain considerable blood) provide every element needed for growth. In my experience they are ideal for shade-loving plants like tuberous begonias, gloxinias, African violets, and so on. Fish emulsions have a nitrogen content of about 5 per cent, but they should not be judged solely on nitrogen.
Sewerage Sludge: Perhaps this is the most widely used of all organic fertilizers for lawns. Activated sludge is a black, flocculated organic material produced by treating solids in sewerage and allowing them to settle out in special beds. If the nitrogen content is more than 5 per cent and the analysis shows any amount of potash, the chances are that the sludge has been doctored with additional chemical nitrogen and potash. Activated sludge is a good conditioner for other fertilizers that tend to cake in the bag, hence it is used to a far greater extent than most gardeners realize.
Tankage: This is made up of packing house wastes steamed to extract the animal fats. The remaining tankage contains between 6 and 10 per cent nitrogen.
Bone Meal: This is possibly the most overrated of all fertilizer materials. Beloved by tradition-bound British gardeners, it has been the universal remedy recommended whenever the "authority" was stumped and had to say something. I suspect the reason bone meal is so frequently recommended is that since it does nothing to the plant of any importance, it does no harm. In fairly acid soils, for example those with pH readings of from 5.8 to 6.2, phosphorus becomes available if bone meal is used liberally. In soils of higher or lower pH readings, phosphorus locks up in insoluble forms that cannot be used by plants. Where phosphorus is needed, superphosphate will supply it at a fraction of the cost of bone meal, and in much more available
form. If bone meal can be had practically for nothing it has some slight value, largely for its small content of nitrogen.
Rock Phosphate: In raw form, just as it is dug from the ground and pulverized, rock phosphate is a fairly good source of slowly available phosphorus. The studious gardener who consults foreign texts should not be deceived, however, by results reported in Europe. Phosphate rock used abroad comes from Africa and is of a different type than American rock. The African product provides much more available phosphorus. Finely ground American phosphate rock, in acid soils, becomes slowly available after the second year. In alkaline soils, it is practically worthless.