OUTLINE AND TABLE OF CONTENTS XX111 



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Relation of soil-productiveness to crop-rotations 503 



a. Principles underlying crop-rotation (356). 



1. Nutrients removed from the soil by different crops 



(357). 



2. Root systems of different crops (358). 



3. Some crops or crop treatments prepare food for 



other crops (359). 



4. Crops differ in their effect upon soil structure (360). 



5. Certain crops check certain weeds (361). 



6. Plant diseases and insects checked by removal of 



hosts (362). 



7. Loss of plant food from unused soil (363). 



8. Accumulation of toxic substances (364). 



