20 BULLETIN 699^ U. S. DEPARTMENT OF AGEICULTUBE. 



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, is 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, 

 Mdiether natural or artificial, is seldom uniform on a field.^ If a soil 

 has good underdrainage, but the topography of the field is not 

 regidar, the accumulation of silt and the greater moisture content of 

 the low ground Avill probably render these depressions more produc- 

 tive than the high ground. On the other hand, if the low gi'ound 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 sufficiently 

 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 readilj^^ 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 

 strengths is of little value as an index to its fertility or to its content 



»Hall, A. D., and Russell, E. J. Chem, News, 102, 180 (1910). 



