Organic fertilizers depend on soil organisms to break them down to release nutrients. Most are effective only when the soil is moist and warm enough for microorganisms to be active. Nutrient release by microbial activity, in general, occurs over a fairly long time period. One potential drawback is that the organic fertilizer may not release enough of their nutrients when the plant needs them for growth. Blood meal is a rich source of nitrogen, so rich in fact that it may burn plants if used in excess.
Fish emulsion is high in nitrogen and is a source of several trace elements.
A strong odor is associated with most brands of fish emulsion fertilizer, but the smell dissipates in a day or two. Manure is a complete fertilizer, but low in the amount of nutrients it supplies. Compared to synthetic fertilizers, organic fertilizers contain relatively low concentrations of actual nutrients, but they increase the organic content and consequently the water holding capacity of the soil. They improve the physical structure of the soil, which allows more air to get to plant roots. Where organic sources are used, bacterial and fungi activity increases in the soil.