6 
It has been known for a long time that burning improves the struc- 
ture of clays by causing a coalescence of the smaller particles into 
larger granules, thus effectively improving the drainage. The in- 
creased size of the pores and air spaces within the soil permits of better 
aeration and encourages deeper root development. By means of heat 
the hydrous compounds become dehydrated, plasticity and adhe- 
siveness are overcome, the movement of soil moisture facilitated, and 
a more congenial environment for root development is produced. 
If sufficiently great heat be employed, the clay may be baked into 
hard lumps, which yield to cultural and weathering influences with 
difficulty, and, therefore, injury may result. In any event the 
dehydrated silicates and oxids return very slowly to their former 
state and the crumb structure induced by the heat persists for years- 
The physical effects of heat on clays are so pronounced that the 
admixing of a few tons per acre of the burnt with the natural soil 
was formerly employed in Europe as a means of ameliorating heavy 
clay lands. 
Regarding the chemical effects of burning it is also well known that 
clay soils undergo chemical changes. In general the solubility of 
aluminum and potassium in acids is greatly increased up to a certain 
temperature, beyond which a decrease sets in. It is generally held 
that under the influence of high temperatures, especially with the aid 
of oxidizing conditions, a wasteful destruction of soil organic matter 
and consequent loss of nitrogen takes place. 
In addition to the above-named physical and chemical effects the 
killing of weed seeds, parasitic fungi, disease-producing organisms, 
and insects are generally looked upon as being among the beneficial 
effects of soil burning. 
While the old system of burning the soil has gradually fallen out of 
use, the closely related partial sterilization by means of heat and vola- 
tile antiseptics is of great interest at the present time. In greenhouse 
work steam sterilization finds extensive application and has been the 
subject of interesting investigations during the past few years. 
Likewise the action of dry heat in its relation to partial sterilization 
and in comparison with the effects of volatile antiseptics on subse- 
quent biological activities has received considerable study. The 
old idea of considering the subject in a restricted physical and limited 
chemical sense is, therefore, giving way to a broader view of the 
question. The more specific chemical effects involved, including cer- 
tain physico-chemical effects dealt with more in detail in this paper, 
and the biological results are now being studied. 
It has been found that moderate temperatures bring about an 
increase in the solubility not only of the mineral constituents of soils 
but also in the organic matter. Furthermore, a number of investi- 
gators have found that steam sterilization, particularly when under 
