Effect of ammonium and phosphorus supply on H+ production in gel by two tropical forage grasses

The effect of the supply of ammonium (NH4+) and phosphorus (P) in gel on th e amounts of hydrogen ion (H+) excreted from plant roots was studied with B rachiaria humidicola (a highly acid-soil tolerant tropical grass) and B. br izantha (less acid-soil tolerant) grown in soil in a glasshouse. The H+ pro duction was measured over 24 h in agar gel containing full nutrient solutio n with a range of NH4+-N levels (0, 0.25, 0.5, and 5.0 mM NH4+-N). Highly s oluble P, K2HPO4, or relatively insoluble P, rock P, was supplied at four c oncentrations (0, 11.5, 34.5, or 115 mu M P) in the gel. Increasing NH4+ co ncentration in the gel increased H+ production for both grasses, but there was some inhibition of growth for B. brizantha at the highest N concentrati on. For B. humidicola, but not B. brizantha H+ production was greater with 34.5 mu M K2HPO4 than 1.5 mu M K2HPO4. At 34.5 mu M P for both grasses ther e was no difference in H+ production when P was supplied as rock P or K2HPO 4. With 11.5 mu M P both grasses produced less acid in the gel with the roc k P compared with K2HPO4. The reduced H+ production is probably due to a lo wer availability of P in the rock P compared with K2HPO4. This effect was g reater with B. brizantha than B. hudmidicola, implying that 11.5 mu M rock P was not able to supply sufficient P for the growth of B. brizantha. Brach iaria humidicola was able to dissolve more rock P than B. brizantha or alte rnatively, the growth of B. humidicola was less adversely affected by the l ow P supply from rock P than B. brizantha. Plant-induced acidity does not s eem to occur as a response to a lack of available P, but rather these grass es only produce acid if there are enough nutrients for growth, i.e., both N H4+ and P. if either N or P is limiting, growth is limited as is NH4+ uptak e, so that H+ production is curtailed.