In estimating plant-food, chemists usually catalogue only the three elements (or combinations of them) that are likely to be much depleted by the growing of crops, - nitrogen, phosphorus, potassium. (These determinations were made by the solution-in-hydrochloric-acid method, sp. gr. 1.115. Other analytical methods in use would give higher readings, particularly in phosphorus and potash, as stronger acids are used to make the soil solutions.)

Plant-food in surface soils, with calculations to pounds in an acre (Roberts)

Plant Food in the Soil 15Plant Food in the Soil 16