What is agrobiology? (that question is answered); The indivisible kingdom of plants (in which the agrobiology is chiefly interested); Thinking it out (the method of approach is decided on); The first agrobiologists at work (all kinds of plants are scientifically interrogated); Setting up the scale of soil fertility (the zero and the Ioo-point are fixed); Graduating the scale of soil fertility (the points between o and Ioo are filled in); The fertility index (much simpler than one might think); Using the scale of soil fertility (a very important and practical matter); Sources of frustation (some interfering circumstances are identified); The agrobiology evaluation of water (the why, what, when and how of soil moisture); The stand of plants (wherein mass action enters the picture); The quantity of plant life (agrobiologists have a yardstick for that too); More about the formula 318/N (it indicates some wide possibilities); Looking at the limits (Nature is generous but men are mostly inept); Public soil science in the United States (what it is- and isn't); Review of the basal axions (the important principles are recapitulated); Agrobiologic equilibria or end- sates (nature is always arithmetical); Mathematical details (mainly for serious students).