210 NEBRASKA STATE HORTICULTURAL SOCIETY. 



almost any rock. These by their death and decay formed a very thin 

 film of vegetable matter on the rock and stronger growth took place which 

 in turn died, decayed, and gave way to a still stronger growth and higher 

 form of vegetation, and so on till grass, shrubs and even trees, were able 

 to exist. This decayed vegetable matter in the soil is called humus. And 

 this humus helps to disintegrate the rock by holding moisture and by sup- 

 plying acids which increase the solvent powers of the water on the rock. 

 When a soil contains much of this humus it is called a vegetable mold. 

 Rich garden soils are good representatives of this class. A soil contain- 

 ing vegetable matter that has only partly decomposed under water is 

 called peaty soil. Such soils are found in swamps and bogs, and are gen- 

 erally sour and need to be aerated and limed before using. 



While organic matter or humus is by no means indispensable to plant 

 life, and though it is a debatable question whether plants derive any nour- 

 ishment from it direct, it is of great importance in enabling the soil to 

 hold water, in making the soil more friable and easily worked, and in sup- 

 plying the carbonic acid which feeds the plant and acts on the soil, dis- 

 solving and making available other foods such as soda, potash and mag- 

 nesium, which are held in an insoluble state in the soil. Humus also gives 

 the dark color to the soil which enables it to absorb the heat rays of the 

 sun more readily and thus warm up more quickly in the spring. 



True clay is composed of silicate of alumina, but the term clay in 

 agriculture is employed rather loosely, being given to soils that contain 

 a large percentage of impalpable rock dust with very little of the true clay 

 present. Some authorities give the following percentage composition of 

 the various soils: 



Clay or impalpable matter Sand 



Heavy clay contains 75 to 90 per cent 10 to 25 per cent 



Clay loam 60 to 75 per cent 25 to 40 per cent 



Loam 40 to 60 per cent 40 to 60 per cent 



Sandy loam 25 to 40 per cent 60 to 75 per cent 



Light sandy loam 10 to 25 per cent 75 to 90 per cent 



Sand to 10 per cent 90 to 100 per cent 



These percentages are by weight of the dry soil. We hear the term 

 light and heavy soil applied to soils. These terms have no reference to 

 the weight of the soil but are used in reference to the mechanical condi- 

 tion. A light soil is one that contains considerable sand, falls apart and 

 works easily. A heavy soil is one that is stiff and tenacious, with more 

 clay than sand. It is a fact that a heavy soil actually weighs less than a 

 light one. 



A tenacious or adhesive clay soil can be greatly improved by the ad- 

 dition of sand, lime or vegetable matter, which tend to separate the par- 

 ticles of clay. It is this adhesiveness of clay which causes heavy soil to 

 crack when drying. Clay expauds very much more than light soils when 

 wet, and shrinks upon drying, and owing to the adhesiveness of the par- 

 ticles of which it is composed the shrinkage causes the cracks to appear. 

 These cracks are naturally injurious to the roots of the plants, breaking 



