Phosphorus Nutrition of Banana as Influenced by Mycorrhizae and Fertilizers

Lin, Mu Lien
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The objectives of this study were to (1) assess field observations of the long-term effects of N and K fertilization on P nutrition of banana, (2) determine the external and internal P requirements as influenced by inoculation with mycorrhizal fungi, (3) test the hypothesis that high N fertilization is associated with low root sugars and low mycorrhizal activity and leads to low P uptake, and (4) examine the agronomic effectiveness of different rock phosphate sources as affected by mycorrhizal infection. Leaf and soil analyses were used to assess the P status of soils and plants from a long-term Williams banana fertilizer experiment that had previously been established in a Mollisol at the Waimanalo Experimental Station, University of Hawaii. The experiment was a continuous function design with 8 blocks, each with 6 N and 8 K levels. Four blocks were fertilized with 224 kg P ha-land four blocks were not fertilized with P. Foliar analyses of control plants fertilized with adequate levels of N and K (assumed from the literature) were used as a guide to determine amounts and frequency of N and K fertilizers. The P status of plants that received intermediate N and K treatments indicated that, although leaf P concentrations declined from the beginning of production, P levels of all blocks were adequate for 8 years. After that, leaf P values were below the assumed critical level (0.18%}. The long-term decline of leaf P concentration was generally associated with a gradual yield decline. It is assumed that the decline of leaf P resulted from a restricted root volume caused by expansion of mats, soil compaction, nematode and weevil infestations, drought, P deficiency and declining mycorrhizal activity. Long-term N fertilization depressed leaf P percentage, whereas K fertilization increased foliar P levels. This suggests that dilution was not the primary reason for depressed banana leaf p. Low P concentrations associated with high N fertilization may result from depressed mycorrhizal activity associated high N fertilization. Ten target P levels, 0.003 to 1.6 mg P L-1 in soil solution, equilibrated with fumigated soils with and without mycorrhizae (Glomus aggregatum), were used to estimate external and internal P requirements of banana. The plants were grown in 100 kg of potted soil (Tropeptic Eutrustox) until the 17th leaf was fully developed. There were substantial increases in vegetative growth of mycorrhizal plants with increasing soil P level in the range from 0.003 mg L-1 to 0.05 mg P L-1 and of non-mycorrhizal plants in the range, 0.003 to 0.1 mg P L-1. High levels of soil solution P depressed plant dry matter production and leaf emergence. Apparent external P requirements of banana for near maximum production were 0.05 (mycorrhizal) and 0.1 mg P L-1 (non-mycorrhizal) in soil solution. This difference suggests that mycorrhizal dependency of banana is not great. The internal P requirement was 0.18% irrespective of mycorrhizal status. If available soil P is high, P accumulates in the conducting tissues of leaves rather than in leaf laminae. Therefore it is concluded that the laminae of No. 3 leaf is not a good tissue for indicating P status over a wide range of P availability. Conducting tissues, for examples, sheaths, petioles, and mid-ribs, should be better indicators of high P availability and luxury consumption. Banana Zn and Mn uptake were enhanced by mycorrhizal inoculation. Three levels of N (target levels of 10, 40, and 80 mg L-1 in solution), and three levels of P (target levels of 0.006, 0.1, and 0.4 mg L-1), with and without mycorrhizal inoculation were used to test the hypothesis, based on field observations, that high levels of N, by reason of growth stimulation, induce low root sugar content and lessen mycorrhizal activity and P uptake. Banana plants were grown in half-barrel (98.4 L) pots with 3 viewing ports for root sampling. The fumigated potting soil (95 kg per pot) was a Tropeptic Eutrustox. Plants were harvested when 17 leaves were fully developed. Leaf punches from No. 3 leaves were sampled during the four month growth. At the low level of P increased N supply decreased P concentration of mycorrhizal banana leaves, but the trend for the effect of N fertilization on leaf P percentage in non-mycorrhizal plants was not obvious. If soil P was adequate, increased N supply resulted in increased leaf P percentage. However, increased N levels tended to increase total P uptake at every soil P level, irrespective of mycorrhizal inoculation. Root sugar content and N supply was negatively correlated, but there was no clear indication that low root sugar was associated with l0i,>1 mycorrhizal activity. The outcome of this study did not support the hypothesis, probably because banana is not highly mycorrhizae dependent. Also, vigorous growth stimulated by N fertilization resulted in increased total P uptake and this seems to have over-shadowed mycorrhizal effects. Field data did not conform with the pot experiment. Some confounding factors, for example a mat system in the field vs. a single plant system in the pot, may have contributed to the discrepancy. Small corms with a diameter of approximately 6 cm were used as planting materials to evaluate the agronomic effectiveness of three rock phosphates (Idaho, Florida, and North Carolina) as mycorrhizal inoculation. The experimental variables also included superphosphate and a no-P control. The soil was fumigated to eliminate native VAM. The amount of superphosphate necessary to establish 0.2 mg P L-1 target P level in solution was based on a P sorption curve. Rock phosphate additions to the potting soils were based on the quantity of material needed to equal the quantity of Padded to the soil as superphosphate. Plants were grown in 9-liter pots for 3 months after germination. Plant dry weight, P percentage in the 3rd leaf, and total P uptake were greater when plants fertilized with insoluble rock phosphates were inoculated with mycorrhizae-producing fungi. Dry weight of plants fertilized with Idaho, Florida, and North Carolina rock phosphates were 48, 53, and 80% respectively of those fertilized with superphosphate.
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