IR spectroscopy of soil fertilised with farm manure reveals that soil quality can be improved significantly compared to "traditional" modern inorganic farming methods and allow poor quality land to be farmed.

Jingdong Mao of the Department of Chemistry and Biochemistry, Old Dominion University, in Norfolk, Virginia and colleagues Dan Olk of the USDA-ARS, National Soil Tilth Laboratory, in Ames, Xiaowen Fang and Klaus Schmidt-Rohr of the Department of Chemistry, at Iowa State University, Ames, Iowa, and Zhongqi He of the USDA-ARS, New England Plant, Soil, and Water Laboratory, in Orono, Maine, explain how spectroscopy can reveal the benefits of manure-based soil improvers compared to inorganic fertilisers.

The researchers observed that the yearly application of cattle manure to farmland in eastern Nebraska over a five-year period led to significant improvements in the nitrogen and phosphorus availability of the soil for crop growth. This lead to increased yields of corn, Zea mays L, even in less productive parts of the field when compared with land treated with the same amount of nitrogen obtained from inorganic fertiliser.

The benefits of using animal manure for improving soil organic matter (SOM) quantity, nutrient availability, soil aggregation, and other soil functions have been documented repeatedly. Manure application can apparently improve crop yields even in otherwise poor quality agricultural land well beyond what is achievable using "traditional" inorganic fertiliser. Moreover, release rates of soil nutrients from manure can be slow, which builds soil quality gradually and is less prone to nutrient loss during rainy periods.

In order to ascertain what properties of the soil were affected by the bovine manure-based fertiliser, as opposed to the conventional inorganic fertiliser, the team extracted two humic fractions for analyses. The first extract was the mobile humic acid (MHA) and the second the calcium humate (CaHA) portion of the field soil.

They used solid-state ¹³C NMR and Fourier transform-infrared (FTIR) spectroscopy to identify quantitatively the functional groups present. NMR techniques employed included quantitative direct polarization, cross polarization/total suppression of sidebands, ¹H-¹³C heteronuclear correlation NMR, ¹³C chemical shift anisotropy filtering, CH-signal selection, and other spectral-editing techniques, the researchers explain. The FTIR spectra were consistent with the NMR data.

The researchers found that the CaHA extracts displayed high aromaticity and spectra typical of Mollisol-type soils rich in oxidized charcoal. In contrast, the researchers explain, the MHA extracts had a lower aromaticity and greater concentrations of lignin aromatic ethers.

Moreover, their analyses revealed a rather surprising result in that the chemical nature of each humic fraction "did not differ significantly" between a field left unfertilized as control and a field treated with inorganic nitrogen fertilizer.

Further detailed analysis of the data showed that the CaHA fraction from the manure treatment had much lower levels of aromatic rings than the inorganic but was enriched in non-polar alkyl compounds. The likely identity of the latter, the team suggests, are fatty acids. Manure CaHA also had an enhanced peptide component, which is consistent with an improvement in the soil's nitrogen supply.

"The key issue in this study was whether amendment of animal manuring for 5 years affected the chemical nature of SOM in a manner that could account for the observed improved availability of soil N and P to a corn crop," the researchers say. They point out that NMR was much more effective in revealing quantitative differences between manured, non-manured, and inorganic fertilised soils.

"An effect of a 5-year manure application on the chemical nature of the MHA and CaHA fractions was observed solely by NMR, namely the more aliphatic and peptide-rich nature and less aromatic nature with manure addition for CaHA," they explain, "An effect of a 5-year manure application on the chemical nature of the MHA and CaHA fractions was observed solely by NMR, namely the more aliphatic and peptide-rich nature and less aromatic nature with manure addition for CaHA."

Related links:

• Geoderma, 2008, in press
• Mao Page

Article by David Bradley