No abstract

The ability of organic acids to affect surface and solution reactions of P and Al in soils containing Al‐oxide surfaces is related to their ability to form stable complexes with Al. This study was conducted to determine if the amount of P and Al released following the addition of an organic acid to a Bh horizon soil could be estimated based on the Al stability constant (logKAl) of the organic acid. The release of Al and inorganic P from spodic horizon material increased in the presence of organic acids that form stable complexes with Al. Overall, the logKAl value was a good indicator of the effect of the organic acid on inorganic P and Al release. Among the 16 organic acids studied, release of Al and inorganic P increased exponentially with increasing stability constants. A threshold value of approximately 4.1, however, was required before substantial amounts of inorganic P were released. This value may reflect the stability of P bond to Al‐oxide surfaces in this soil. Salicylic acid was a notable exception to the observed trend in release of both Al and P; despite a large stability constant, little inorganic P or Al was released. Although significant amounts of soluble organic P were released in all the organic‐acid solutions, the amount of soluble organic P released was not related to the stability constant of the organic acids.

The amount, composition, and persistence of decaying wood in humus layers was examined in yellow birch (Betula alleghaniensis Britton)‐red spruce (Picea rubens Sarg.) stands in the Adirondack Region of New York. The wood is initially attacked by brown rot fungi, but thereafter decomposes very slowly and may persist in the humus layer for a century or more. Separable wood residues weighed as much as 41,800 lb/acre and composed 14 to 30% of the forest floor. These residues were significantly lower in N and P content than the surrounding humus.

Organic anions have been implicated in the release of P to soil solution and may thus increase the availability of P to plants. Oxalate and formate have been identified as the most prevalent low‐molecular‐weight organic anions in the soil solutions in Spodosols under forest vegetation in northern Florida. The purpose of this study was to investigate the effect that these two organic anions have on the kinetics of inorganic‐ and organic‐P release from the A, Bh, and Bt horizons of a representative forested Spodosol. Soil samples were equilibrated for various periods of time with solutions of either oxalate, formate, or water. The extracts were analyzed for pH, inorganic P, organic P, Al, and organic acids. Results showed that oxalate had little effect on P release in the A horizon. This was due to the lack of Al oxides to hold P. Conversely, oxalate greatly increased solution P in the Bh and Bt horizons. In the subsoil horizons, the release of P was rapid, followed the disappearance of oxalate from solution, and was coincident with an increase in pH. These facts suggested that P was released via a ligand‐exchange reaction. A longer term reaction was consistent with the reprecipitation of the P as an amorphous Al phosphate when oxalate was either totally sorbed or degraded. The presence of formate did not increase P release.

The relative value of ammonium and nitrate as N sources for Pinus radiata D. Don seedlings was determined under various conditions. Short‐term experiments varying from 3 weeks to 6 months' duration were conducted utilizing three growth media: solution, soil, and exchange resin beads. In the solution cultures N sources were compared at two levels of acidity (pH 3.6 and pH 5.9), three root‐zone temperatures (15, 20, and 25C), and three N levels (2.8, 11.2, and 22.4 ppm N). In the soil cultures N was added at five levels as NaNO3 or NH4Cl with a nitrification inhibitor. Three levels each of ammonium and nitrate were used in a factorial arrangement in the resin cultures.Radiata pine seedlings grown either in soil, solution culture, or exchange resins exhibited greater growth and N uptake with an ammonium source than with nitrate. This was true in experiments at several levels of acidity, in all three growth media and regardless of the root zone temperature or N level maintained. Increases in root zone temperature in solution cultures increased growth and N uptake and increased the differential response to N source.Eight‐fold differences in N concentration in the solution cultures produced only small differences in growth and N uptake. N levels of approximately 2.0 ppm N in solutions proved satisfactory for normal growth.Picea glauca (Moench) Voss seedlings were tested concurrently in the solution cultures and exhibited the same favorable response to ammonium N source.