240 MISSOURI AGRICULTURAL REPORT. 



shows that the rock is ground to different degrees of fineness and some 

 soils may be composed largely of the finest particles, while others are 

 made up of the large particles. Clay soils are made up almost entirely of 

 very fine particles, half of them being as small as .00004 inch in diameter. 

 Sandy soils are composed of the largest soil particles. Loams are mix- 

 tures of clay and sand in varying proportions. It must be remembered 

 that all of these classes of soils contain humus. There are, however, 

 some soils that are composed almost entirely of humus and these are 

 classed under the name humus soils. 



Chemically, soils are made up of many compounds composed of var- 

 ious elements united in many ways. A discussion of these would lead 

 us too far into the subject of chemistry for our present purpose, but it may 

 be said that not all of these elements are necessary to plant life. There 

 are about eight of the elements found in soils that seem to be essential to 

 plant life. It follows then that the absence of any one of these would 

 make a soil entirely unproductive. 



But nature has abundantly supplied our soils with all of these ele- 

 ments but three, or possibly four, so that in our discussion we will con- 

 sider only those elements likely to be found absent. They are nitrogen, 

 potassium and phosphorous. One very prominent investigator says that 

 soils may often be deficient in sulphur. Chemical analyses seem to bear 

 him out in this view but until more data is obtained I shall confine myself 

 to a discussion of only the three elements just named. 



WHERE AND HOW FOUND. 



Nitrogen is not strictly a part of the soil since all that is found there- 

 in was originally obtained from the air and in time will return again to 

 the air, but since ordinary plants cannot use it except while it is tempor- 

 arily a part of the soil we will consider it as one of the true elements of 

 the soil. 



There are three sources of soil nitrogen ; These are humus, air in 

 connection with certain bacteria and ammonia brought to the earth in 

 rains. 



Humus contains by far the greater part of the soil nitrogen, but in 

 this form it is entirely unavailable to plants. The himius must be acted 

 upon by several different species of bacteria which change its nitrogen 

 through various forms finally into nitric acid which instantly combines 

 with mineral matter of the soil forming compounds which we call ni- 

 trates. When a nitrate has been found the nitrogen in it may be used as 

 food by plants. A soil without humus lacks potential nitrogen ; a son 

 without bacteria will contain no nitrates ; and a soil containing no nitrates 



