390 SOIL PROTOZOA IN SOUTH AFRICA. 



tozoa in soil by direct examination. In the case of soils in South 

 Africa, it seems from the above-mentioned experiments that the 

 use of finely-divided soils will reveal more Protozoa than when the 

 soil is examined without previous pulverisation. 



Finely tilled soils are stated generally to yield heavier crops 

 than those less well cultivated- It is possible that the finer pul- 

 verisation of the soil may release more protozoal cysts, which, in 

 the presence of the moisture necessary for their development as 

 trophic forms, will produce Protozoa that may react on organisms 

 inimical either directly or indirectly to plant growth. There may 

 thus be a protozoal factor in the production of heavier crops after 

 fine tillage. 



A minimum moisture content in the soil seems necessary for 

 the growth of Protozoa therein. 



It was found that when a genus of Protozoa such as Amoeba 

 Actinophrys, Pararnoecium, Euglena or Cyclidium appeared irs 

 one culture in the trophic condition, several other cultures from) 

 different localities contained the same organisms simultaneously. 

 This seems to indicate that these Protozoa in water culture react to 

 some external stimulus, as yet not fully determined, and assume the 

 trophic condition. 



General Remarks. 



The following general remarks may, for convenience, be col- 

 lected and set forth here. 



On the whole, relatively few bacteria have yet been en- 

 countered, though in some cases bacteria were seen ingested by 

 Amoebae, Mastigamoeba, Bodo and Colpoda. This may be 

 due to the fact that the nitrogen content of most South African 

 soils is relatively somewhat high, so that the environment for bac- 

 teria may not be quite so favourable as in other parts. Dryness 

 may also be a factor. 



The nitrogen content of South African soils is a difficult sub- 

 ject to a protozoologist. According to Mr. A. Stead (1920) of the 

 Grootfontein School of Agriculture, "the nitrogen factor would 

 not appear to be deficient in our average soil ; on the other hand 

 it is doubtless often excessive." Again, "the average soil of the 

 Union would seem to be more deficient in phosphatic food than in 

 any other." The relatively large amount of sunshine causes partial 

 sterilisation of the soil and prevents denitrifying bacteria from 

 thriving. The relative lack of rains implies that the nitrates are 

 not much lost by washing away (leaching). An important factor 

 of the soil in South Africa for plant food is the balance between 

 nitrogen and phosphates. 



According to Waksman (working in the United States of 

 America) soil Protozoa do not have any appreciable influence upon 

 ammonification by bacteria. As far as the present evidence goes, 

 Waksman's (1916) remark also seems to apply to South Africa. 



The natural medium for moistening soil is water, more especi- 

 ally rain water, and so water cultures of soil should reveal a content 



