Calcium-Magnesium Nutrition with Special Reference to Serpentine Soils

Abstract

tions of V-lamis andl Jenny (16) and V'lamis (15) indi-cate(l that the principal reason for the infertility of these soils is their low Ca/Mog ratio or low Ca satura-tion. Apparently crop plants are unable to absorb sufficient Ca from these soils for proper growth. V'arious factors affecting plant growth on serpentine soils and the dominant role of Ca availabilitv in these habitats were discussed recently bv the senior author (1S) . Although agricultural plants grow poorly on ser-pentine soils, a considerable number of native species thrive in this medium. Obviously the latter must differ physiologically from crop plants, and perhaps some phase of their Ca-'Mg nutrition is the most likely point of difference. In orcler to investigate this possibility, the Ca-MIg nutrition of several native serpentine species is compared with that of some com-mon agricultural plants in this paper. AIATERIALS AND AIETHODS Two natural soils were used for the pot culture experiments. They are characterized as follows: No. 30. A shallow, primary, reddish-brown soil overl-ing serpentine rock in Lake County, California, classified in the Heneeke soil series. The pH is 6.8 as read on the saturation paste with a glass electrode; the cation exchange capacity is 16.1 meq/100 gm; Ca saturation is 14 %; Ca/MIg ratio in the cation ex-change complex is 0.16. An x-ray diffraction pattern indicated that the 2 ,u colloidal fraction is predomi-nantlv kaolinitic. No. 85. A shallow, primary, gray soil overTing serpentine rock in Chelan County, Washington-The pH is 6.4; the cation exchange capacity 24.4 meq/100 gin; Ca saturation is 32 %; and the Ca/_Mg ratio in the cation exchange complex is 0.48. After screening through 1/4-inch mesh to remove rock, 15 kgm lots of the above soils were reconsti-tuted with respect to exchangeable cations by leach-ing in inverted bell jars with appropriate mixtures of CaCl2 and MIgCl2 (17). In most cases 36 liters of leaching solution wiith a total salt concentration of 0.25 molar were used. For the very low Ca soils, the solution contained MIgCl2 alone. After reconstitution with the chloride mixture, the soil lots were leached with water until free of chloride to the AgNO3 test, then dried, screened again, and mixed. Samples were analyzed for exchangeable cations by ammonium ace-tate extraction and determination of the bases in the extract. Although no K was included in the leaching solutions, some residual K persisted in the soil after treatment. The cation status of the soil lots after reconstitution is given in table I. A slightly different but similarly prepared series was used for the experi-ment with tomato on Soil 30. The range of Ca satu-ration extends from below that of any naturally oc-curring soil up to over 80 %, which is higher than that present in most fertile agricultural soils. The pot culture methods used followed those of Jenny et al (7). Six-inch clay pots, coated on the