Again, a point of great importance in a universal manure is the proportion in which the necessary plant-foods are present. If it be asked, Are the nitrogen, phosphoric acid, and potash in farmyard manure present in the proportion in which crops require these constituents? the answer must be in the negative. Heiden[173] has very strikingly illustrated this point, in so far as the relations between the two ash ingredients are concerned, by some computations as to the amount which would be removed from the soil in the course of different rotations.[174] In the case of five different rotations it was found that the ratio between the potash and phosphoric acid removed was as follows:[175] (1) 2.96 to 1; (2) 2.76 to 1; (3) 2.95 to 1; (4) 4.13 to 1; (5) 3.78 to 1. This would give a mean of 3.32 to 1. This is not the ratio in which these ingredients are generally present in farmyard manure. Farmyard manure may be said to be much richer in the mineral constituents of plants than in nitrogen. Professor Heiden found that in the case of a farm at Waldau, the crops in the course of ten years removed from a _morgen_ (.631 of an acre) the following quantities:-- lb. Nitrogen 329 Potash 263 Phosphoric acid 121 In order to supply these amounts the following quantities of manure would require to be supplied:-- 1. For the nitrogen, 26 or 27 tons (manure containing .606 per cent nitrogen). 2. For the potash, 20 to 25 tons (manure containing .672 per cent potash). 3. For the phosphoric acid, 13 to 19 tons (manure containing .315 per cent phosphoric acid). From the above it will be seen that farmyard manure contains too little nitrogen in proportion to its ash ingredients.