26 VERTICAL FARMING 



forms of combined nitrogen that may be added to the soil, and 

 convert it into usable forms. 



Soil changes also materially affect the availability of phos- 

 phate material. In nature it is always combined in slowly 

 soluble compounds. In commercial fertilizers it is usually com- 

 bined with lime. Different relations in the amount of lime to 

 the phosphorus effect the solubility of the phosphorus. In 

 badly drained land the phosphorus is often found combined 

 with iron in little balls of " bog ore " that are very insoluble. 

 Many other examples of combinations of plant foods could be 

 brought out going further to show how the air, water and 

 bacteria assist the changes. In every case the benefits brought 

 about demand deep stirring of the soil, such as is produced by 

 exploding small charges of slow powders in the subsoil and 

 opening a way for the liberators of plant food. No other 

 practical method can bring about the desired results. 



Deficient Plant Food. If the rocks from which a soil is 

 derived are deficient in any needed food, it becomes imperative 

 sooner or later to add some material that will make up the 

 deficiency. The original and best general fertilizing material 

 is manure, as it adds not only certain amounts of plant food, 

 but also large amounts of humus. Forest mould, litter, straw, 

 and other materials of like nature add some of the fertilizing 

 elements and also humus. The number of materials that may 

 be used to add plant food is great. Some materials carry but 

 one needed element, while others carry two or three. There can 

 be no general rule promulgated to guide in choosing fertilizers, 

 as different soils and different crops demand certain chemicals 

 in different forms. 



Soil Amendment or Correction. Some soils well supplied 

 with mineral and organic plant foods have some trouble, such 

 as sourness or an excess of alkali. Materials not classed as 

 foods are used in the correction of such conditions. These 

 materials are generally known as " Soil Amendments." In the 

 correction of black alkali, gypsum is added to change the sodium 

 carbonate to a less harmful compound which can more readily 

 be leached out. 



