344 JotritNAL OF THE DEPARTMENT OF AdRlCULTtTRE. 



is necessary to pievenl soil acidily. to [jidiiiotc h;ic1cii;il jictioii niid 

 to prevent certain diseases. 



Tlie K^ures tor total lime are ot less si/^niHcance, hut i1 niiisl lie 

 remembered that soils containing ,a good quantity of total lime 

 usually, though not always, contain adequate amounts of calcium 

 carbonaie. It is obvions that if the total lime be low the percentage 

 of calcium carbonate cannot be high. 



To illnstrate the importance of the percentage of carbonate of 

 lime in contradistinction to total lime it may be mentioned that some 

 South African soils have been found to contain what would usually be 

 regarded as high percentage of lime l)ut have been found to contain 

 little or no ca)l)onate of lime. Three samples of soil analysed a few 

 years ago contained 1.48 per cent., 0.79 ])e]' cent., and 2.87 per cent, 

 of total lime but were actually acid. 



(fi) Mafjnesia. — Magnesia is one of the essential ])lant foods 

 though, in general, soils contain sufficient magnesia for crop require- 

 inents. The assimilation of magnesia appears to be connected with 

 that of phosphoric oxide. There is a certain amount of evidence to 

 show that when magnesia is in excess over lime in a soil the amount 

 of phosphoric oxide assimilated is greater than when lime is in excess 

 over magnesia. 



It is also considered that a lai gc excess of jnagnesia over lime is 

 detrimental. The whole question is obscure, but it may be accepted 

 that it is ])referal)le that a soil should contain more lime than 

 magnesia. 



(Ji) Potash. — This is one of the essential plant food constituents 

 and is also one of those which it is customary to supply in the form of 

 fertili;5crs. It is therefore necessary to determine whether a soil stands 

 in need of potash manures or not. The question of the supply of 

 potash to the plant by the soil is an extremely difficult one. Many 

 of our soils contain enormous stores ot potash, some of the granite soils 

 of the Transvaal contain about 4 per cent, of total potash, that is. 

 the potash extracted by hydrochloric acid together Avith that remain- 

 ing in tke " insoluble matter." As previously mentioned, Hopkins 

 has shown that plants are able to assimilate potash from the acid 

 extracted " insoluble matter" obtained in soil analysis. It is one of 

 the most difficult problems in the interpretation of soil analysis to 

 determine whether the soil stands in need of potash fertilizers or not. 



There can he no question that the value of potash fertilizers is 

 tremendously overrated. As an aid to intensive cultivation as 

 practised in Europe potash manuring is necessary, but it is the writer's 

 belief, based on an examination of the available data obtained in 

 reliable manurial experiments, that potash manures are superfluous, in 

 fact money wasted, in at least 90 per cent, of cases in this country. 

 Yet according to the methods for the chemical analysis of soils used 

 in South Africa (there are two) about 50 per cent, of our soils are 

 reported as deficient in potash. 



The "available potash," as determined by Dyer's citric acid 

 method, seems to he a much more reliable guide to the potash require- 

 ments of South African soils. 



(i) Phosplioric Oxide. — Phosphoric oxide is one of the most 

 important plant food constituents. In South Africa it is perhaps of 

 the greatest importance economically. In fact the chief manurial 



