Fertilizers are measured by NPK: nitrogen, phosphorus, and potassium. This is a means of rating synthetic fertilizers for their uses. Nitrogen is connected to the foliage of the plant – how big and bushy the plant and leaves will grow.Phosphorus relates to the productivity of the plant – its root system, flowers, and fruit. And Potassium helps the plant to function well. Each of the three are represented by a number. The higher the number, the higher the concentration of that nutrient.
Worm castings have an NPK rating of 1-0-0 because the slow release value of the nutrients doesn’t count for an NPK rating. So while worm castings don’t look good on paper as compared to synthetic fertilizers, rest assured that they boost soil nutrients in biological ways that synthetic fertilizers simply cannot.
Scientifically speaking, worm castings, “Castings contain a highly active biological mixture of bacteria, enzymes, remnants of plant matter and animal manure, as well as earthworm cocoons (while damp). The castings are rich in water-soluble plant nutrients, and contain more than 50% more humus than what is normally found in topsoil."