228 SOILS AND FERTILIZERS 



where limestone is injurious to soil or crop, and it is 

 frequently most helpful. 



269. Marl. Underlying beds of peat, deposits of 

 marl are occasionally found. Marl is a mixture of 

 disintegrated limestone and clay, and contains variable 

 amounts of calcium carbonate, phosphoric acid, and 

 potash. When peat and marl are found together, they 

 may be used jointly with manure as described in Sec- 

 tion 182. Many sandy lands in the vicinity of peat 

 and marl deposits would be greatly improved, both 

 physically and chemically, by these materials. 



270. Physical Action of Lime. - -The addition of lime 

 fertilizers to sandy soils improves their general physi- 

 cal condition. Heavy clays lose their plasticity when 

 limed and the fine clay particles are cemented together 

 and act as sand, which improves the mechanical con- 

 dition of the soil. The physical action of lime in soils 

 is well illustrated in the case of ' loess soils,' which are 

 composed of clay and limestone. The lime cements 

 together the clay particles to form compound grains, 

 making the soil more permeable and more easily tilled. 

 The better physical condition which follows the appli- 

 cation of lime fertilizers is frequently sufficient to war- 

 rant their use. 



271. Application of Lime Fertilizers. - - Lime is gener- 

 ally used as a top dressing on grass lands at the rate of 

 200 to 500 pounds per acre. Excessive applications are 



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