XX.—­Farm manures, concluded 183
        Green-crop manures: 
          Functions 183
          Benefits 185
        Character of best plants for green-crop manuring 185
        The time for green-manure crops 186
        Leguminous green-manure crops 186
        Non-leguminous green-manure plants 191

XXI.—­Commercial fertilizers                                192
The raw materials                                     192
Sources of nitrogen                                   193
Sources of phosphoric acid                            195
Sources of potash                                     199
Sources of lime                                       200

 XXII.—­Commercial fertilizers, continued 202
        Mixed fertilizers: 
          What they are
          Many brands
          Safeguard for the farmer
          Low grade materials
          Inflating the guarantee 202
        Valuation 205
        Low grade mixtures 207
        Buy on the plant food basis 209

XXIII.—­Commercial fertilizers, concluded 211
        Home mixing of fertilizers 211
          Kind and amount to buy 212
          The crop 213
          The soil 215
          The system of farming 215
        Testing the soil 215

XXIV.—­Rotation of crops                                     219
Systems of cropping                                   219
The one crop system                                 221
Rotation of crops                                   224
Benefits derived from rotation of crops               230
The typical rotation                                  231
Conditions which modify the rotation                  232
General rules                                         233
Length of rotation                                    233

  XXV.—­Farm drainage 235
        How surplus water affects fertility 235
        Indications of a need of drainage 235
        Drains: 
          Surface drains
          Open ditch drains
          Covered drains or under drains 236
        Influence of covered drains on fertility 237
        Location of drains: 
          Grade
          Tile drains 238