256 Bureau of Farmers' Institutes. 



How Can We Determine the Amount of Available Plant 

 Food Present in the Soil ? — The method given above is es- 

 sentially the only practicable and reliable means we have at pres- 

 ent for ascertaining whether or not there is in a soil enough avail- 

 able plant food of any particular kind to grow a crop satisfac- 

 torily. As already pointed out this information cannot be gained 

 with certainty by a chemical analysis of soil. Farmers cannot 

 be too strongly impressed with the necessity of carrying out 

 some such experiments as those indicated above. Such experi- 

 ments can be carried on with very little extra work. This line 

 of work, properly conducted, will not only furnish the farmer 

 information that is of the greatest practical and economical value 

 to him, but it will exert a marked educational influence upon 

 him, making him personally familiar with plant-food materials, 

 causing him to take a deeper interest in the growing of crops, 

 and leading him to practice better methods. 



Amount of Plant Food Required by the Kind of Crop 

 Grown. — It is a well-known fact that different crops need different 

 quantities of nitrogen, potash and phosphoric acid compounds. 

 If we know with a fair degree of accuracy how many pounds of 

 nitrogen, potash and phosphoric acid a crop of any kind will 

 remove from the soil, then we have fairly definite knowledge of 

 the amounts of different forms of plant food to apply to the soil 

 to insure a crop. If we could not depend upon the soil to fur- 

 nish any plant food, then we should use, at least, the amounts 

 of fertilizing materials removed by one crop. 



In the following table, we give the number of pounds of nitro- 

 gen, phosphoric acid and potash used by different kinds of crops 

 grown on one acre of land. The yields have been made variable 

 within fairly wide limits so as to cover most cases found in actual 

 experience. The calculations are based upon the most reliable 

 analyses available. 



In studying this table, we must keep in mind that the figures 

 do not in every case represent the amount of plant food removed 

 from the soil. Thus, with clovers, beans, peas and other legu- 

 minous crops, a portion of the nitrogen is obtained from the 

 air and hence we need to apply less nitrogen in the form of 

 fertilizer than appears to be called for by the table. In the case 

 of fruits, like apples, pears, plums, etc., it will be found sa ! 'e 

 often to apply larger quantities than the table calls for, becnuse 

 the figures in the table do not indicate the demands made by 

 the tree in increasing growth.: 



