The first is the percentage of nitrogen, the second phosphorus and the third potassium.
Some years ago the powers that be in the fertilizer industry decided that these are the only things we need to know about plant foods in order for plants to grow.
Scientists call these the macronutrients, the things that plants need in large doses for good growth.
While this is true, plants also need a lot of other things in small doses for good health.
Chemical fertilizers may or may not supply them in the mineral filler that makes up the other percentage in the bag.
Compost does it as a matter of its makeup.
Nitrogen is used in the manufacture of proteins and is seen in plants by their green, leafy growth.
Most soil should be 5 or 6 percent nitrogen.
As organic matter is decomposed by the actions of earthworms and microorganisms nitrogen gas is released in a form that plants can use easily.
An organic farmer can add nitrogen to the soil by capturing it with leguminous plants such as peas and beans.
Beneficial bacteria fix the nitrogen from the air in nodules on the plants' roots and the tops are later added to the compost bin.
Both the nitrogen from the decomposing root and later added compost add nitrogen to the soil.
There is enough phosphorus in most soils for plants to be healthy, the problem is phosphorus take up is usually hampered by its forming insoluble compounds with iron and aluminum in the soil.
As compost breaks down in high humus soil the organic acids engae the aluminum and iron to release the phosphorus in a form the plants can use..
There is evidence that 99% or higher of the available potassium in the soil is bound up in mineral form that is not available to the plants.
In addition, the soluble form that plants can use is quickly leached from the ground.
However, organic matter has negatively charged particles which prevent potassium departure and maintain its availability for the plants.
As it is taken up by the root systems more is released by microbial activity and an equilibrium is maintained between available and unavailable potassium.
Compost maintains a healthy soil and makes the nitrogen - phosphorus - potassium available for healthy plants.