356 HOW THE NEEDS OF SOILS MAY BE DETERMINED 



the growing period, and especially because of gravel or coarse 

 sand subsoils (Fig. 42). 



(c) Physical Condition of the Soil. — Since there is a close 

 relationship between the physical condition of a soil and its 

 fertility, attention should also be given tliis factor in diagnosing 

 infertility or the cause of low yields. A striking difference in 

 growth of plants due to physical improvement of the soil is shown 

 in Figure 225. Cultivation improved the poor growth as shown 

 by Figure 226. 



(d) Inoculation. — In determining the cause of failinres or 

 unsatisfactory crops in growing any legume, especially for the 

 first time, it is unportant to determine whether or not the soil is 

 properly inoculated. This can best be done by digging up several 

 plants with a spade and examining the roots for nodules. If none 

 are found and the soil is not acid, the lack of proper organisms 

 must necessarily be the limiting factor, provided, of course, other 

 conditions are favorable. If the soil is acid, liming is the first aid, 

 then inoculation (see index). 



(e) Harmful Agents in the Soil. — ^Not only should injury from 

 diseases like rust, barley stripe and blight, and from insects like 

 leaf aphids, boll-weevil and beetles be considered, but also the 

 harmful agents which may infest the soil. These may be diseases, 

 insect pests, certain poisonous substances, etc. (Chapter XIV). 



(/) Organic Matter. — Organic matter (see index) in soils affects 

 practically all the factors which determine fertility. It is important, 

 therefore, to note the amount of organic matter in any soil under 

 examination, especially upland soils. The color of the soil usually 

 indicates the amount present. 



CHEMICAL AJSTALYSES AND THEIR VALUE 



Total vs. Available Plant-food Elements. — Chemical analysis 

 which shows the total quantity of the important plant-food ele- 

 ments contained in a soil is not always an index to its productive- 

 ness, because it is not so much the total supply as it is the amoimt 

 that is available, or which can easily become available to meet 

 the needs of the crop, that determines the productivity of the 

 soil. When this fact became established, chemists tried to dis- 

 cover some method to measure the availability of the elements, 

 or ascertain to what extent the crop can secure its requirements 

 from any particular soil. Attempts, therefore, have been made 

 to imitate the action of the plant roots in securing the elements, 



