EFFECTS OF LIMING AND MYCORRHIZAL COLONIZATION ON SOIL PHOSPHATE-DEPLETION AND PHOSPHATE-UPTAKE BY MAIZE (ZEA-MAYS L) AND SOYBEAN (GLYCINE-MAX L) GROWN IN 2 TROPICAL ACID SOILS

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phospha te (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate in actions in rhizosphere soil (A I-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil p H was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with diffe rent amounts of CaCO3. In both soils, liming increased shoot dry weigh t, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root d ry weight in some cases, but much more markedly increased shoot dry we ight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal a nd non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depen ded in some cases on the soil pH. At all pH levels, the extent of P de pletion in the rhizosphere soil was greater in mycorrhizal than in non -mycorrhizal plants. Despite these quantitative differences in exploit ation of soil P, mycorrhizal roots used the same inorganic P sources a s non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the effi cient P uptake from soil solution by the roots determines the effectiv eness of the use of the different sail P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.