We were unable to secure exact data regarding the amount of fuel burned annually in these countries, and of ashes used as fertilizer, but a cord of dry oak wood weighs about 3500 pounds, and the weight of fuel used in the home and in manufactures must exceed that of two cords per household.  Japan has an average of 5.563 people per family.  If we allow but 1300 pounds of fuel per capita, Japan’s consumption would be 31,200,000 tons.  In view of the fact that a very large share of the fuel used in these countries is either agricultural plant stems, with an average ash content of 5 per cent, or the twigs and even leaves of trees, as in the case of pine bough fuel, 4.5 per cent of ash may be taken as a fair estimate.  On this basis, and with a content of phosphorus equal to .5 per cent, and of potassium equal to 5 per cent, the fuel ash for Japan would amount to 1,404,000 tons annually, carrying 7020 tons of phosphorus and 70,200 tons of potassium, together with more than 400,000 tons of limestone, which is returned annually to less than 21,321 square miles of cultivated land.

In China, with her more than four hundred millions of people, a similar rate of fuel consumption would make the phosphorus and potassium returned to her fields more than eight times the amounts computed for Japan.  On the basis of these statements Japan’s annual saving of phosphorus from the waste of her fuel would be equivalent to more than 46,800 tons of rock phosphate having a purity of 75 per cent, or in the neighborhood of seven pounds per acre.  If this amount, even with the potash and limestone added, appears like a trifling addition of fertility it is important for Americans to remember that even if this is so, these people have felt compelled to make the saving.

In the matter of returning soluble potassium to the cultivated fields Japan would be applying with her ashes the equivalent of no less than 156,600 tons of pure potassium sulphate, equal to 23 pounds per acre; while the lime carbonate so applied annually would be some 62 pounds per acre.

In addition to the forest lands, which have long been made to contribute plant food to the cultivated fields through fuel ashes, there are large areas which contribute green manure and compost material.  These are chiefly hill lands, aggregating some twenty per cent of the cultivated fields, which bear mostly herbaceous growth.  Some 2,552,741 acres of these lands may be cut over three times each season, yielding, in 1903, an average of 7980 pounds per acre.  The first cutting of this hill herbage is mainly used on the rice fields as green manure, it being tramped into the mud between the rows after the manner seen in Fig. 114.