i6i4 The Cornell Reading-Courses 



where the exploitation of the soil by continuous cropping is yet going 

 on. There are instances of profitable continuous cropping with com, 

 potatoes, or grass where barnyard manure is available and green manure 

 crops are grown in order to maintain the soil fertility; but the largest 

 part of our farm lands must have a succession, or rotation, of crops, cover- 

 ing a period of two years or more, if their cultivation is to be maintained 

 at a profit. 



TEN REASONS FOR ROTATING FARM CROPS 



1. Farm crops differ in their requirements of available plant food. 

 If a crop be grown continuously on the same land, the nitrogen or phos- 

 phorus or other plant-food elements mainly required by that crop may 

 be used faster than other kinds and the balance between the available 

 plant foods be thus disturbed. Crops produce less in yield and we say 

 the soil is getting poor. A succession of crops that require different 

 quantities of nitrogen, potash, phosphorus, lime, and sulfur will main- 

 tain a better balance between the quantities of these plant-food elements 

 in the soil. If we wish to apply manures and fertilizers to help in main- 

 taining this balance, the rotation of crops affords better opportunities for 

 doing it profitably. 



2. Farm crops vary in their habits of root growth. Grasses and small 

 grains are rather shallow-rooted plants and feed comparatively near the 

 surface of the soil. Corn, potatoes, and root crops send their roots 

 somewhat deeper and therefore have a larger feeding ground in the soil. 

 Red clover and alfalfa are deep-rooted plants and are able to utilize stores 

 of mineral plant food, such as potash, phosphorus, and lime, which are 

 below the reach of the shallower-rooted crops. When a clover or alfalfa 

 sod is plowed under for the next crop, the roots and crowns decay and give 

 up some of the plant food that they obtained from below to the new and 

 perhaps shallower-rooted crop that succeeds in the rotation. 



3. Humus is a very important constituent of soils. The continuous 

 tillage incident to growing successive crops of corn, potatoes, or beans, 

 without returning liberal applications of barnyard manure to the soil, 

 tends to hasten the decay of humus and reduce the amount of it present 

 in the soil. Continuous grass culture on a field may increase the amount 

 of humus-forming material on the surface, but gives little opportunity for 

 mixing into the soil the accumulating leaves, stems, rootstocks, or top 

 dressings of manure. The rotation of crops where sods, stubble, and barnyard 

 manure are plowed under, tends to keep up the supply of humus in the soil 

 and thus to conserve the moistiire supply and liberate plant food when 

 crops need it. 



4. If nitrogen-gathering crops are grown in rotation with others and 

 the entire plants o'* their residues of root and stem are plowed under to 



