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Soil properties and their interactions on organic farms in Nebraska

Citation

Wortmann, Charles (2022), Soil properties and their interactions on organic farms in Nebraska, Dryad, Dataset, https://doi.org/10.5061/dryad.15dv41nzp

Abstract

Sustainable soil productivity is a function of soil chemical, physical, and biological main and interaction effects that vary with inherent soil properties, topography, crop, location, and management. These effects were investigated using soil samples collected at geo-referenced points from 119 fields on 15 organically certified farms across Nebraska. Observations are reported for 46 variables. Most fields had adequate soil conditions for high productivity. The irrigated versus rainfed effects differed for several soil properties in western compared with eastern Nebraska. On average, about 80% of the soil was in water-stable aggregates (WSA) of >0.05 mm diameter with more aggregation and arbuscular mycorrhizal fungi (AMF) with pasture compared with cropland. Variation in soil microbial biomass (SMB) was more affected by variation in soil and climate properties than by management. Increased SOM for cropland in eastern Nebraska was positively related to SMB but not to AMF or the biomass ratios of saprophytic fungi (Fs) to bacteria and of actinomycetes to other bacteria. After accounting for the effect of SOM, SMB was most positively related to crop growth at the time of sampling, and availability of K and S. Mehlich-3 P was positively correlated with SMB but P and Zn availability were negatively correlated to AMF. Increased soil pH was associated with greater biomass of Fs and eukaryotes but not of bacteria. The SMB was generally less for pastures in western Nebraska compared with other field types. In conclusion, the soil properties of these organic farms were generally suited to high productivity and SMB had a strong, positive relationship to SOM.

Methods

Soil samples for the 0 to 7.5 and 7.5 to 15 cm depths were collected in May and June 2013 at geo-referenced points from 119 fields on 15 certified organic farms across different topographic forms of Nebraska. Nine farms were in the rolling hills of eastern Nebraska with loess and glacial till soils, five were on the plains with primarily loess soil, and one farm was in the Platte River valley. Observations were made on the current crop, rotation sequence, irrigation, and use of manure or other organic material. Soil samples were collected in three sets

One set of soil samples was dried, ground, and analyzed for SOM, pH, buffer pH, electrical conductivity, Mehlich-3 P, SO4-S, NO3-N, DTPA-extracted Zn, and 1-N ammonium acetate exchangeable Ca, Mg, K, Na, and CEC (Ward Laboratories, Inc., Kearney NE). Organic C and N were analyzed using a Flash 2000 Organic Elemental Analyzer.

Another set of samples was analyzed for particulate organic matter, particle size distribution (Kettler et al., 2001), and water-stable soil aggregation (WSA). These were air-dried, sieved to remove and discard aggregates of >8 mm diameter, and plant materials were removed. Determination of cPOM and fPOM was by weight-loss on ignition (450oC) according to Cambardella et al. (2001). Dry weights (g kg-1) of WSA>2, WSA0.25-2, and WSA<0.25 were determined (Quincke et al., 2007).

Soil samples for fatty acid methyl ester concentration (FAME) analysis were kept in a cooler during transport to the lab and then stored frozen at -20 oC. Frozen soil was slightly thawed in the refrigerator, sieved to 4 mm diameter, and extracted for FAME. The FAME biomarkers used to represent taxonomic and/or functional groups were: C18:2c9,12 for saprophytic fungi (Fs); C16:1c11 for arbuscular mycorrhizal fungi (AMF); iC14:0, iC15:0, aC15:0, C15:0, iC16:0, iC17:0, aC17:0, cyC17:0, C17:0, cyC19:0(9,10), cyC19:0(11,12), 10MeC18:0 and 10MeC19:0 for total bacteria. The cyC17:0 and cyC19:0 represented bacteria that form cyclopropyl rings. The 10-methyl fatty acids (B10Me) were indicative of some actinomycetes. Eukaryotic biomarkers, exclusive of Fs and AMF, were represented by C20:4 (5,8,11,14). C20:2, C20:3, and C20:5. The total of all FAME biomarkers was used to represent SMB concentration.

Usage Notes

There are a few missing values (marked N/A) due to the values being not applicable or unavailable due to missing sample or problems in lab analysis but the dataset is sufficient for statistical analyses.