Experiments carried out during the past ten years at Wareham in England fully confirm the opinion expressed long ago by Professor Elias Melin, Upsala, Sweden, that the growth of trees and other plants on poor soils of the raw humus type is greatly influenced by the root-fungus association. By fostering the appropriate combination it has been possible to carry out successful afforestation of heathland so poor that ordinary cultural methods prove inadequate for the least exacting tree species. Satisfying the mineral requirements of the trees by direct application of fertilizers is not in itself sufficient treatment to ensure continued healthy growth; biological factors also play an essential role in promoting soil fertility. The experiments have shown that failure of the trees to establish a satisfactory biological equilibrium with the necessary fungi is due in this case, not to the absence of these fungi in the soil, but to their inactivation by toxic products of biological origin. The factors inhibiting the activity of the fungi can be removed by the application of comparatively small amounts of organic composts which produce dramatic and lasting effects on the growth of roots and shoots. The special composts used are prepared from organic materials such as straw, hop waste and sawdust. The mechanism by which they stimulate growth is still obscure. All of them contain small amounts of directly available plant foods such as phosphates and potash, but careful investigation both in laboratory pot cultures and in the field, has shown that these can account for only a relatively temporary effect on growth. It is suggested that the composts act mainly by modifying the course of humus decomposition, thus bringing about drastic changes in the biological activities of the organic substrate of the soil. This demonstration of the profound influence of biological factors on the nutrition of trees challenges the attention of foresters and has important practical applications. By making use of suitable composts, it will be possible to carry out the successful afforestation of land formerly regarded as wholly unproductive. For further information see "Problems of Tree Nutrition"[5]. From the two foregoing examples it is seen that in the case of banana blight, fungi had to be suppressed by bacteriae, but that for pine trees on poor English soils fungi had to be activated for proper tree nutrition. 4) Inorganic Tree Nutrients Other information also from England concerns the use of so-called "minerals" which I prefer to call "essential inorganic nutrients," and name by the element or the compound in which the element is contained. "Minerals", strictly speaking, refers to compounds formed by nature as rocks, ores, brines, salt deposits, etc.