Hydrogen ion production and ammonium uptake by two tropical forage grasses

Abstract

The amounts of hydrogen ion (H⁺) excreted from roots and the relationship of acid production with the uptake of ammonium (NH₄ ⁺) were studied with Brachiaria dictyoneura (a highly acid‐soil tolerant tropical grass) and B. brizantha (less acid‐soil tolerant) grown in soil in a glasshouse. At six dates between 39 and 73 d the H⁺ production in agar gel and NH₄ ⁺ uptake from solution were measured over 24 h for ten replicate plants of each species. The agar gel and solution contained the same nutrients with 5 mM N supplied as NH₄ ⁺‐N. Brachiaria dictyoneura tended to have greater total dry matter (DM) production than B. brizantha mainly as a result of greater root than shoot DM production. Daily H⁺ production increased with increasing DM and was always greater for B. dictyoneura than B. brizantha. Similarly NH₄ ⁺ uptake was always greater for B. dictyoneura than B. brizantha and was linearly related (R²=0.85) to H⁺ production in gel. Hydrogen ion production/ NH₄ ⁺ uptake ratios were 0.3 to 0.5 for B. dictyoneura and 0.1 to 0.3 for B. brizantha. Hence as well as taking up more NH₄ ⁺ B. dictyoneura also produced more H⁺ for every mole of NH₄ ⁺ taken up than B. brizantha. Differences in the pH changes between the gels and solutions and the effects of the inclusion of NO₃ ^‐ in the media are discussed. It is concluded that for pastures B. brizantha would be a better choice than B. dictyoneura in that it would have less of an acidifying effect on the soil without detracting from the herbage yield.