THE NECESSITY OF ZINC FOR PINUS RADIATA

Abstract

In Western Australia a number of softwood plantations have been established on poor soils. Of the many species of Pinus planted, only P. radiata and P. pinaster are practicable on many of the areas. Even in these hardy species numerous disorders arise, fundamentally owing to soil poverty. Soil analyses (5) show that phosphate is extraordinarily low in the majority of cases. At the request of the Forests Department of Western Australia, the nutrition of Pinus radiata in culture solution was studied. Field experiments (4, 5) had shown that zinc deficiency, and probably phosphate deficiency, were the most common causes of disease. Zinc spraying, and fertilizing with superphosphate, have been resorted to on a large scale. A number of puzzling features related to ''rosetting'' (as the zinc deficiency disease is called) require an explanation, such as the very slow response to even large applications of a zinc salt to the soil, and the favorable results often obtained with superphosphate. There is reason to believe that the soil contains sufficient zinc for the needs of the plant, but that it is in some way rendered unavailable. The unavailability seems to be accentuated by clearing and burning of the native forest or scrub. It is conceivable that burning, followed by heavy rains, will result in almost complete loss of zinc from the surface layers (3) of a deep sandy soil such as the one in question. The problem is further complicated by the mycorrhizal habit of the pine. One would expect the associated fungal mycelium to be a more capable assimilator of zinc than the roots of non-mycorrhizal pines. And in fact analyses of fructifications of the mycorrhizal fungus indicate a much higher concentration of zinc than in healthy leaves of Pinus radiata. This suggests the possibility that in a zinc deficient soil, the fungus exerts a priority over the limited amount of zinc absorbed by the mycorrhizae. Field trials (5) with diseased trees, using a large number of metals (including zinc, copper, manganese, boron, molybdenum, cobalt, iron, and magnesium) applied by injection, or by spraying the foliage with dilute solutions, showed that zinc alone remedied the rosetting.'' This indicated that the disease is due to a deficiency of zinc, a conclusion which is supported by analyses in which the needles of resetted,'' cured, and healthy untreated plants, exhibited a fairly definite increasing gradation of zinc content. It is not yet known, however, whether rosetting ' ' will subsequently recur in trees which have been cured by spraying, and the spray-