Buying Unmixed Materials.--Acid phosphate, animal bone, and tankage can be bought of any fertilizer agent, but when one pays cash, he does well to get quotations from various leading manufacturers. The names of dealers in nitrate of soda can be secured from the New York agency which keeps its address before the public in agricultural papers. This is likewise true in the case of the syndicate controlling all the potash. When the addresses of leading distributors of all needed materials have been secured, quotations should be obtained on a cash basis. The best terms are obtained by groups of men combining their orders. CHAPTER XX MIXTURES FOR CROPS Composition of Plant not a Guide.--It has been pointed out that a chemical analysis of a soil is not a dependable guide in the selection of a fertilizer. Years ago the theory was advanced that the analysis of the crops desired should be a guide, but it has proved nearly worthless. This theory does not take into account the soil's supply of plant-food. Moreover, a certain crop may demand a large supply of an element at a time of the year when the soil's supply is inactive. The need of nitrogen for grass in the early spring, before nitrification in the soil is active, is an illustration. Let the causes be what they may, the fertilizer formulas that call for plant-food in a fertilizer in the same proportions that it is found in plants are disappointing in their results. The analysis of the plant is not a dependable index. The Multiplication of Formulas.--Fertilizer manufacturers have made all possible combinations of fertilizer materials, using them in various quantities. Each manufacturer has given a mixture a brand of his own, and confusion reigns. There is no formula for a soil or crop that will remain absolutely the best, even for one particular field. It represents one's judgment of the present need, and is employed subject to change, just as is the prescription of a physician. It is usually only an approach toward the most profitable amount and kind of plant-food that may be supplied. The one important consideration is that no manufacturer can know the need nearly so well as the intelligent farmer who knows the history of his field and constantly tests its ability. [Illustration: On the productive farm of Dr. W. I. Chamberlain, in Northern Ohio.] A Few Combinations are Safest.--It is the best judgment of scientists to-day that greater results would be obtained from the use of commercial fertilizers if the number of formulas could be reduced to ten, or even a less number. The satisfactory fertilizers fall into three classes: