THE SOILS OF THE CANE-GROWING REGIONS 67 



the influence of one decisive feature. This is the law of the minimum 

 first put forward by Liebig, mainly with reference to the chemical composition 

 of the soil. Paraphrased, this law states that the crop yield is determined 

 by the deficiency in one element, and not by a sufhcienc}* or superabundance 

 in others. This law may be extended to other influences such as the physical 

 condition of the soil, the available water supply, and the suitabiUty of the 

 soil as a habitat for beneficial soil organisms ; and it is only when all these 

 conditions are at a maximum that the maximum crops result. Conversely, 

 when no condition necessary for crop production is absent, a soil may be 

 infertile, owing to the presence of undesirable factors. Such factors may be 

 the presence of reducing substances such as ferrous salts, chlorides to which 

 the cane is to some extent resistant, acidity which may be occasioned by the 

 use of overmuch sulphate of ammonia, and alkaUnit}^ particularly that form 

 known as " black alkali," which may be caused by the overlong use of 

 nitrate of soda. 



As now carried out, three schemes are used for determining the chemical 

 analysis of soil. The first, seldom employed except for special purposes, 

 makes a complete analysis, using hydrofluoric acid as the solvent. The 

 second employs strong acid, usually hydrochloric, and the third a weak 

 acid. The two former methods are used to obtain an idea of the potential 

 fertility of the soil over long periods, whereas the third is designed to give 

 information regarding the immediately available plant food. In the com- 

 parison and interpretation of analyses, it is necessary to know the method 

 used : — That known as the U.S. of&cial method uses hydrochloric acid of 

 specific gravity 1-115 (22-96 per cent), 10 grams of soil being extracted 

 with 100 cc. solvent for 10 hoius at 100° C. The German method employs 

 25 per cent, hydrochloric acid, the action being allowed to take place over 

 48 hours at room temperature with frequent shaking. Following on Wiley^ 

 this scheme dissolves only one-fifth to one-sixth the potash obtained by hot 

 digestion, this latter procedure being also followed generally by British 

 chemists. French practice uses nitric acid as the solvent. 



The interpretation of analyses with strong acid as solvent is difficult. 

 Hilgard,* referring to hot hydrochloric acid as solvent, states : — " Generally, 

 phosphoric acid less than 0-05 per cent, indicates deficiency, unless much 

 lime is present. Heavier virgin soils with more than o-i per cent, and a 

 fair amount of lime, are good for 8 to 15 years' continuous cropping ; with less 

 lime 0-2 per cent, is necessary for the same period. Large quantities of 

 organic matter offset low phosphoric acid, which is, on the other hand, 

 rendered inefficient by much ferric oxide. Referring to potash, he fixes the 

 hmits in sandy soils, sandy loams, loams and cla57S as o-i per cent., o-i 

 to 0-3 per cent., 0-3 to 0-45 per cent., and 0-45 to o-8 per cent, respec- 

 tively, and thinks that soils with less than 0-25 per cent, potash are likeh' 

 to benefit by potash manures. As regards lime for sandy soils and clay 

 loams, he adopts o • i per cent, and o - 25 per cent, as the lower admissible 

 limits for normal crop production, and sees no benefit when the lime rises 

 above 2 per cent. The lower limit for nitrogen is usually taken as o-i 

 per cent. 



As regards the available plant food, the method of Dyer, employing 

 I per cent, citric acid as the solvent is very largely used. He considered 

 that when the phosphoric acid or potash fell below o-oi per cent, the need of 

 manuring with these materials was indicated. This standard is to be re- 

 garded as an indication rather than as an absolute figure ; for Demerara 



