24 FIELD OPEKATIONS OF THE BUKEAU OF SOILS, 1903. 



more porous substrata to lower levels and local drainage basins. In 

 this way large areas of valuable land at lower levels are damaged, being- 

 converted into alkali flats, bogs, or marshes. The effect of this upon 

 the composition and distribution of the alkali salts of the soil and upon 

 the valuable organic and mineral plant foods is important. Not only 

 does a water-logged soil lead to the formation of alkali salts of the 

 most dangerous character and their concentration within the zone of 

 root activity, but the water leaches from the soil the valuable mineral 

 elements, retards proper aeration, produces harmful changes in the 

 physical structure of the soil, and arrests the development of the nitri- 

 fying and other bacteria so important to the growth of plants and the 

 fertility of the soil. 



Small bodies of land, somewhat damaged by seepage water, occur 

 along the lower valley level throughout the area. The most extensive 

 area, and the one subjected to the greatest injury, occurs in the Gila 

 fine sandy loam and the Maricopa silt loam, about 1 mile north of 

 Thatcher. Here is an area of once valuable land, covering hundreds 

 of acres, extending westward along the river for several miles, where 

 the water stands upon the surface or is found but a few feet below. 

 The accumulation of this seepage water results from excessive irriga- 

 tion of surrounding lands, coupled with insufficient natural drainage. 



Underground drainage in irrigated regions has been too long a neg- 

 lected subject. It is now, however, becoming of equal and in some 

 respects greater importance than the drainage of lands in humid 

 regions. Even the digging of a few open ditches through the water- 

 logged tracts would reduce the water content of the land to a great 

 degree. Upon the more badly damaged areas a system of tile drain- 

 age would prove of great value. 



ALKALI IN SOILS. 



The alkali of the Gila Valley is confined for the most part to the 

 Maricopa silt loam. The areas in which the salts prevail to a danger- 

 ous extent occur only in a half dozen or so places, usually following 

 the boundaries of the Maricopa silt loam and covering from one-half 

 square mile to 3 square miles each. 



The position of these areas, and the average salt content to the depth 

 of 6 feet, may be ascertained by reference to Plate I. The proportion 

 of alkaline soil as compared with the total area surveyed is consider- 

 able. The damage thus far done is, however, in many cases slight, 

 the alkali and seepage conditions upon the whole being very much 

 better than in many of the irrigated districts of the West. The condi- 

 tions, however, demand careful attention to prevent an extension of 

 the areas affected by seepage water and the further deterioration of 

 lands already more or less alkaline. Over a considerable proportion 



