CHAPTER XXV. 

 CROP ROTATION. 



FROM time immemorial it has been considered a self-evident fact 

 that where crop rotation is practised there is a bigger and better 

 yield. The farmers of ancient Rome understood that crops fol- 

 lowing beans, peas and vetches were usually better than those 

 following wheat or barley, but it was not until the last quarter 

 of the nineteenth century that it was learned that the leguminous 

 plants, with the aid of associated bacteria, have the power of feeding 

 on the free nitrogen of the air, whereas the non-leguminous plants 

 have not this power and require a supply of combined nitrogen. 

 Today the best farmers practise some system of crop rotation. 

 They have learned from experience that where crop rotation is 

 practised the crops are bigger and better than under the single 

 crop system. This is usually interpreted as indicating that crop 

 rotation has increased the fertility of the soil. Many , farmers 

 plant legumes for a number of years on run-down soil, remove the 

 entire crop and feel confident that their soil is becoming richer in 

 plant-food. Let us examine some of the results obtained in care- 

 fully planned experiments to see if this conclusion is warranted by 

 the experimental evidence. 



Essential Elements. Plants are composed of ten elements, each 

 one of which is absolutely essential to growth and seed formation. 

 Only two carbon and oxygen are secured from the air by all 

 plants; only one hydrogen from the water; the other seven are 

 secured by all plants from the soil. One class of plants the 

 legumes may, under appropriate conditions, obtain their nitrogen 

 from the air. Six elements phosphorus, potassium, magnesium, 

 calcium, iron and sulphur are obtained from the soil by the growing 

 plant. 



Element Added by Legumes. The great majority of agricultural 

 soils contain large quantities of all these elements, with the excep- 

 tion of nitrogen, phosphorus and potassium. These are used by 

 the growing plant in larger quantities than are any of the other 

 elements which are obtained directly from the soil. In the great 

 majority of soils nitrogen, phosphorus, or potassium is the limiting 

 element in crop production. Therefore,, our problem resolves 

 itself into the question : Can crop rotation maintain these elements 

 in the soil in quantities sufficient for maximum yields? Phosphorus 



