180 A. F. GUSTAFSON 



than did unhealed soils. Heating at 150°C. still furtlier increased the solubility of both 

 organic and mineral matter. In most comparisons, heating increased the amount of water- 

 soluble phosphorus, yet a few gave slightly less. Pot experiments with oats showed that 

 heating in a vacuum at 95 to 98''C. increased the growth. 



Wilson (106) secured slightly increased growth of plants in soil heated at 95°C., but 

 retardation at higher temperatures, the effect varying with the kind of soil and nature of 

 crop grown. 



Buddin (8) found the nitrate content, immediately after drying in a thin layer in labora- 

 tory for 24 hours, unafifected, but reducing the moisture content further during 46 hours 

 did increase the nitrate content slightly. When the dried soils were remoistened and in- 

 cubated for 40 days, there was a marked increase in nitrates; untreated moist soil 36 parts 

 per million, soil spread in gallery 46, and in glass-house 53.5 parts per million. 



King (48) relates that the residents of northern China build flues ("Kangs") of sun-dried 

 bricks made of "soil or subsoil mixed with short straw or chaff." After two to four years' 

 use, these flues become defective, so that they must be replaced. When removed the bricks 

 are finely pulverized and used as fertilizer, being planted in hills with the seed. The soil 

 while used as a flue has become thoroughly air-dry and on the inside of tlie flue, undoubtedly, 

 much of it has at times been freed of uncombined water. During this long-continued drjdng, 

 the plant nutrients have been rendered more available and this fact is made use of by the 

 Chinese farmer. King (47) suggests "absorption of the products of combustion" by the 

 "brick" as an additional factor in giving tliem value as fertilizer. 



Kellej' and McGeorgc (40) revievs- briefly the history- of burning soils. There are, in 

 Hawaii, large areas of heavy soil which do not, when first plowed, produce satisfactory crops. 

 It requires several months of cultivation before crops thrive. It has been noticed that on 

 small spots where brush has been burned cotton grows exceptionally well. It is suggested 

 that this effect may be due to heating the soil rather than to the soluble oxides of phosphorus, 

 potassium, calcium and magnesium in the ash, since fertilizers do not produce such beneficial 

 results. 



They report results of analyses of the 1 : 5 water-extract of a brown ferruginous clay soil 

 and its subsoil, and a similar type which had been plowed and was growing pineapples. 

 Determinations were made on fresh, air-dried and oven-dried samples, extracted respectively 

 for 1 hour, 24 hours and 7 days. Phosphoric acid (P2O.O was ahvaj^s highest in the oven-dried 

 soil, manganese oxide (Mnj04) always higher in air-dried than in fresh soil (not determined in 

 oven-dried soil). Lime (CaO) was highest in the oven-dried soil in three of nine comparisons 

 only. Magnesia (MgO) varied, in some samples higher in the air-dried. Sulfuric acid 

 (SOs) was highest in the fresh soil of tener than in either of the others, and potash was highest 

 in the air-dried soil (with one exception) in the two surface soils, while in the subsoil the 

 fresh soil held first place. 



When all the comparable data are considered we see that in three cases the fresh soil was 

 highest in total soluble solids, in three cases the air-dried and in the other three cases the 

 oven-dried soil stood first. So no conclusion as to the effect of heating on total soluble salts 

 can be drawn from these figures. 



In general, extracting for 24 hours or for 7 days gave but slightly higher results than ex- 

 tracting for 1 hour, except in the case of phosphorus which increased in solubility with longer 

 extraction. In two out of eight trials, heating at 100°C. increased the nitrate content; in 

 four soils it was decreased while in the other two there was no change. As the soil was 

 raised to higher temperatures, 150 to 200 and 250 nitrates decreased rapidly until almost 

 none was recovered at the highest temperature. 



These investigators think both chemical and physical factors enter into an explanation of 

 the effect of drying on the soluble constituents of soils, but that "the most important set of 

 factors affecting the solubility of inorganic soil constituents are physical in nature. Also 

 that the physical factors act through the effect of changes in soil moisture on the physical 

 properties of the soil." "The conditions conducive to the formation of a colloidal state and 



