232 



system, is very resistant to drought. This accounts for its growth on troi)i- 

 (al and subtropical steppes. The Xegro or brush millet fPennisetum spica- 

 nim) is entirely unsuitable for firm soil, but is excellent for porous soils in 

 dry localities. The other millet varieties behave similarly. 



The Lcijuminoseae, which are suitable for growth as a second crop be- 

 cause of their usually short vegetative period, may, in the tropics and sub- 

 tropics, acquire great importance not only as collectors of nitrogen and as an 

 excellent nutritive substance, but are also valuable on account of their close 

 shading of the soil, preventing it from hardening and as soil loosening, green 

 manuring plants. The plants make good growth in dry soils ; — accordingly 

 heavy soils, in regions with abundant precipitation, are not suitable for them. 

 Busse^ has given more detailed studies of sorghum diseases and their rela- 

 tions to atmospheric conditions. 



Of tuberous plants, the szveet potato requires about the same cultural 

 conditions as our potato. The cassiwas (Manniok) require deep, loose, dry 

 soils, but rich in humus. The moisture-loving Maranta species, furnishing 

 arrowroot, also requires looseness of the soil, on which account virgin soil is 

 found to be less suitable because of its compactness. Even Taro, the tubers 

 of the different Colocasia species, which requires a great deal of moisture, 

 flourishes only when the soil is pervious. The same is true of the Yam, 

 which is derived from different species of the genus Dioscora. In regard to 

 poppy culture and the harvesting of opium, reference should be made to 

 Braun's- work, and in regard to rubber plants and especially the Liana, 

 root and herbaceous rubber plants, to studies by Zimmermann". 



Mf.ans for Overcoming the Disadvantages of Heavy Soils. 



Drainaye. In this we have to take into consideration not only soils 

 rich in clay, but also those sandy ones whose graular structure is so fine 

 that they can become as closely compacted as clay soils. 



Of the practical means used to increase soil aeration, drainage deserves 

 to be named primarily. It facilitates the exchange of air in the soil inter- 

 stices as well as removing stagnant water accumulations after every rain. 

 The drainage pipe acts as an apparatus for sucking up air. When the rain 

 fills the soil, it forces out the air which has a less oxygen content than the 

 atmosphere, but is richer in carbon dioxid. But since the rain is quickly 

 soaked through the drains, air rich in oxygen streams just as quickly from 

 the surface down into the pores increasing, thereby, the processes of oxida- 

 tion in the soil and the activity of the roots and micro-organisms needing 

 oxygen. 



The fear that drainage will impoverish the fields has rarely any founda- 

 tion, since the numerous analyses of drain water show only slight traces of 



1 Busse, Walter, Untersuchungen iiber die Krankheiten der Sorghum-Hirse. Arb. 

 d. Biolog. Abt. f. Land- u. Forstwirtschaft a. Kais. Gesundheitsamte, Vol. IV, Part 4. 

 1904. 



2 Der Pflanzer, 1905, No. 11-12. 



3 Ibid, Nos. 8-10. 



