Effect of Humus Amendments on Growth and Nutrient Uptake of Monterey Pine Seedlings in Biocide‐Treated Soils

To investigate the possibility of a counting hysteresis existing in probe 1, two equal size (3-foot cube) constanttemperature baths (1 and 2) at 24.5° and 46.0° C., respectively, were used. When the probe reached thermal equilibrium in bath 1 (or 2) it was removed and immediately inserted into bath 2 (or 1) where 1-minute counts were recorded until thermal equilibrium was reached. The results of the above procedure show the counting hysteresis of this probe (figure 2). A counting hysteresis is not present in probe 2 below 60° C. (140° R). The shape of the curve (probe 1) figure 1 may not be the same for other neutron probes and a thermal characteristic curve should be determined for each probe. Probe 1 used in this study gave incorrect counts when its temperature was above 32° C. The effect on neutron counts in going from a high to a low temperature with probe 1 in either air or soil may be seen in figure 2. Consequently, above 32° C. a temperature correction may not be feasible. —JAMES M. DAVIDSON, DONALD R. NIELSEN, and EUGENE R. FERRIER, Laboratory Technician II, Assistant Professor, and Laboratory Technician IV, respectively, University of California, Davis. EFFECT OF HUMUS AMENDMENTS ON GROWTH AND NUTRIENT UPTAKE OF MONTEREY PINE SEEDLINGS IN BIOCIDE-TREATED SOILS THE USE OF CHEMICALS for weed, insect, and disease control in forest tree nurseries is becoming standard procedure. Although these materials aid in reducing seedling costs, their addition to the soil involves certain hazards. Investigators in other fields (1, 2, 3, 4, 5, 6, 8, 12) have found that many of the commonly used biocides are toxic to plants, accumulate in the soil, and are highly persistent. The effects of these compounds in forest nursery soils assumes even more importance because changes in size, vigor, or chemical constitution of the tree seedlings may cause reduced survival or growth in forest plantations which represent large investments over a long period of time. The purpose of this greenhouse study was to evaluate the changes in growth and nutrient uptake of Monterey pine (Pinus radiata) seedlings caused by the addition of certain biocides to the soil; and also to test the ability of different types of humus to counteract the toxic effects of the chemicals. Previous research relating to nursery practices (9, 10) indicated that humus material might be useful for this purpose. Two types of humus, an arthropod fine mull and a matted mycelial mor (7, 11), which preliminary trials showed were particularly beneficial were selected. The properties of these two humus types and the control soil, Plainfield sand, are described in table 1. The biocide and humus additions were mixed thoroughly with the soil which was placed in K-gallon glazed crocks. Treatments were duplicated except for the control which was replicated four times. Twenty-five Monterey pine seeds were sown per crock. The rates of biocides added were heavier than those normally used in practice in order to obtain definite toxic symptoms and beneficial effects. Such concentrations might occur in soils through uneven applications, accumulation from successive treatments, or movement through the soil. The seedlings were harvested after a 6-months' growing period. Dry weight of tops and roots was determined and the tops were analyzed for nutrient content. Growth data and chemical composition of seedlings for selected treatments are presented in table 2. Growth was retarded by all of the biocides at the rates used. Calomel, thiosan, and chlordane caused the greatest reductions in growth. Calomel, allyl alcohol, and thiosan caused noticeable reductions in survival. Both humus types partially or wholly offset the toxic effects of the chemicals.