In regard to inorganic nutrients, more attention has probably been devoted to citrus trees than to any other tree species, largely because the soils of Florida and California require additions thereof. It would be unfair to say that such main fruit crops as apples, cherries, peaches, plums have been neglected; we merely possess more information on the nutrients of citrus trees than on other tree crops, as far as the micro essential nutrients are concerned. Most orchards and groves are fertilized only with nitrogen, phosphorus and potash, and limed when necessary. Nitrogen can stimulate size of fruit at the expense of quality. A paper by P. W. Rohrbaugh[16], Plant Physiologist of the California Fruit Growers Exchange, Ontario, California, deals with eleven mineral nutrient deficiencies and their causes, viz: calcium, magnesium, potash, phosphorus, sulphur, nitrogen, iron, boron, zinc, manganese, copper, and this might well be used as a guide for nut trees. 6) Miscellaneous A few oddities may also be mentioned for anyone inclined to experiment: From Holland it is reported that an avenue of large handsome shade trees close to a century old, all died in one year, except where a junk dealer had stacked a pile of old metals. The trees had exhausted the inorganic nutrients within reach of their roots in the soil, but the junkpile had replenished them sufficiently, so that those within reach of it kept alive to this day, twenty years later. A rock mulch is reported to have improved the growth of lime and lemon trees considerably[17], and it would seem that similar experiments should be made on young nut trees, just before bearing age in a comparative test with a check planting. Stones can be selected for the nutrients they contain, and a geologist can easily point out those containing the greatest number of elements. No one could go wrong in placing a few rocks of limestone or dolomite near the base of a tree, and let rain and sunshine, heat and frost attend to the fertilizing in a slow but perpetual manner. Maple sugar contains manganese[18], showing this as a distinct quality over cane sugar. Manganese and other essential nutrients are known to facilitate the production of proteins[19], and the question of better quality nut production may well be examined from the viewpoint of the indirect effect from activities of soil microbiology by manganese, copper, cobalt and zinc. Some of these elements have also been classed as inorganic plant hormones[20]. "Chlorosis," the yellowing of leaves, may not only be a deficiency symptom of manganese, but also one of iron, copper and magnesium. Lack of manganese can cause a decrease in photosynthesis[21], so much so that in manganese deficient leaves the CO2 assimilation may be reduced to half of normal. Herein, too, may lie the cause of low yields, smaller roots and lowered resistance of those roots to invading detrimental organism.