Nitrogen Carriers: I. Soil Effects 1

Replicated field plots on moderately acid sandy loam soils were sampled after 3 to 7 years' annually repeated applications of eight nitrogen sources at rates of 40 to 300 lb N/acre. Soil pH, exchangeable K, Ca and Mg, and lime requirement to pH 6.5 were measured. Acidifying effects to a depth of 15 inches were of the order: (NH₄)₂SO₄ > NH₄Cl > NH₄NO₃ ≊ NH₃ ≊ urea > ureaform. Residual basicity from Ca(NO₃)₂ and NaNO₃ was approximately equal to the acidifying potential of climate and management. Observed declines in exchangeable bases were ⅓ to ½ the calculated equivalent acidity of acidifying carriers; increases in lime requirement were 1.5 to 6 times greater than calculated. Major increases in lime requirement occurred below the pH range 5.0 to 5.5 and were ascribed to retention of protons in the form of ionic complexes with Al and Fe. Uniquely greater residual acidity from (NH₄)₂SO₄ may have been due to greater exchange affinity and base displacement by hydroxy-Al-SO₄ complexes as compared with complexes with monovalent anions (Cl⁻, NO₃⁻). It is proposed that a fundamental objective of liming practice is to maintain soil pH well above the range where pathways of proton transfer are dominated by mobile sesquioxide buffer systems.