The fish are ground up, put in a vat, and enzymes added to cause the mess to deteriorate. The oil is skimmed off, and the remains boiled down. Phosphoric acid is added to take the pH below 4, to stop the enzymes, and then potash added to raise the pH back to 4.5. Fish emulsion can be applied as a liquid fertilizer or as a foliar spray, although it can burn foliage if applied in too high a concentration. Fishbone meal is appearing more often in organic fertilizer mixes as an alternative to bonemeal from slaughterhouses.
Fish emulsion is notably low in calcium, while fishbone meal is quite high.
The issue with phosphorous, however, is that it tends to move slowly through the soil, no farther than an inch each year. Since phosphorous is important for good root growth, it becomes important when planting to make sure a good supply of phosphorous is directly available in the root zone. Place a layer of fertilizer in the bottom of the hole, cover with a shallow layer of dirt and then plant. The roots will grow to seek out the phosphorous. There are a number of soluble fertilizers available in nurseries and by mailorder that promise bigger, better blooms.