20 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 
Where the check plots and those receiving various fertilizer treat- 
ments are replicated several times throughout the experiment field, 
absolute uniformity in a field is not so essential. This point, how- 
ever, 1S discussed under the heading “ Number and Distribution of 
Plots.” 
TOPOGRAPHY AND DRAINAGE CONDITIONS. 
The soil of an experiment field may be fairly uniform throughout. 
but unless the topography and drainage conditions are regular, cer- 
tain parts of the field will be more productive than others. Drainage, 
whether natural or artificial, is seldom uniform on a field.t Tf a soil 
has good underdrainage, but the topography of the field is not 
regular, the accumulation of silt and the greater moisture content of 
the low ground will probably render these depressions more produc- 
tive than the high ground. On the other hand, 1f the low ground is 
poorly drained, excessive moisture in the spring may keep the ground 
cold, delaying greatly the growth of the crop, preventing aeration of | 
the soil, and hindering the development of root systems sufliciently 
extensive to keep the crop from suffering from droughts encountered 
later in the year. 
Topographic irregularities may also affect crop yields by causing 
certain parts of a field to be shaded more than others. The shaded 
areas may or may not produce greater yields than the unshaded, de- 
pending on the climatic conditions and the crops grown. 
Since it is frequently not practicable to obtain an experiment field 
free from topographic irregularities, the plots should be laid off or 
distributed in such a way that these irregularities are apportioned (as 
nearly as possible) equally among the various treatments. 
PHYSICAL AND CHEMICAL COMPOSITION OF SOILS. 
Fertilizers affect various soil types and soils varying in chemical 
composition quite differently. Very sandy soils, low in organic 
matter, ordinarily respond quickly to the soluble fertilizers, but 
owing to their low absorptive capacity they may often be leached of 
their soluble salts during excessive rains. Clay soils, on the other 
hand, if kept in good tilth, have a great absorptive capacity, but often 
do not respond so readily to fertilizer treatment until they have been 
heavily limed. Soils rich in organic matter usually respond more 
quickly to the relatively insoluble fertilizers than those of low organic 
content, but here again the soil type and its mineral constitutents are 
factors which must be considered. 
The digestion of a soil with hydrochloric or nitric acid of various 
4Hall, A. D., and Russell, E. J. Chem. News, 102, 180 (1910). 
