David E. Kissel

Effects of drying and rewetting on carbon and nitrogen mineralization in soils and incorporated residues

An understanding of nitrogen mineralization from residues and soil organic matter is important to understand the quantity of N available from the soil for crop production. The objective of this study was to determine effects of repeated wetting and drying of soils on rates of N mineralization. The study compared mineralization rates in three kaolinitic, low organic matter soils, utilizing cotton leaves or compost as residues. One set of treatments was subjected to repeated drying and rewetting, whereas the other was kept at constant moisture content. Mineralized N was measured by leaching with 0.01 M CaCl2 periodically, for 185 days. Rates of C mineralization were measured in the treatment containers by periodic measurement of CO2 respiration rates. In constant moisture conditions, soils with cotton leaf residue mineralized between 25% and 40% of N applied as residue, whereas soils with compost mineralized between 3.8% and 9.3%. In fluctuating moisture conditions, soils with cotton leaf residue mineralized between −1.3% and 6.9%, whereas soils with compost mineralized from 1.6% to 3.3%. Moisture effect was not significant in soils without residue, with soils mineralizing between 16 and 47 mg N kg–1. Carbon mineralization rates were not significantly affected by moisture. Both residue and soil type affected rates of C mineralization.

Salt Concentration and Measurement of Soil pH

The measured value of soil pH depends in part on the laboratory procedures used, such as the soil–solution ratio and soil solution electrolyte composition. One of the most significant factors affecting the measured value of soil pH is the electrolyte concentration of the soil solution. Since electrolyte concentration of agricultural soils can vary greatly during the year and between years, the date of sampling can result in highly variable pH values for samples with the same percentage of base saturation when soil pH is measured in deionized water. For example, we found a different relationship between extractable calcium (Ca) and pH (1:1 in deionized water) for about 18,000 soil samples from the same geographic area taken during winter of 2 years, differing in winter rainfall. On average, samples taken during the wetter year had higher pH for a given value of extractable Ca, consistent with a reduced ionic strength (more leaching) in the wet year. In a comparison of pH in water with pH in 0.01 M calcium chloride (CaCl2) for 1,186 soil samples received from clients, the median difference in pH was 0.67. It is notable that 20% of the samples had a difference of >0.8 and 10% had a difference of >0.9 pH units. Some samples with differences larger than the median may not receive a lime recommendation when needed because of the erroneously high pH reading in water caused by low ionic strength. The stability of pH readings in 0.01 M CaCl2 essentially eliminates this problem.

Field Scale Mapping of Surface Soil Clay Concentration

The surface soil clay concentration is a useful soil property to map soils, interpret soil properties, and guide irrigation, fertilizer, and agricultural chemical applications. The objective of this study was to determine whether surface soil clay concentrations could be predicted from remotely sensed imagery of bare surface soil or from soil electrical conductivity for a 115 ha field located in Crisp County, Georgia. The soil clay concentrations were determined for soil samples taken at 28 field locations. Three different data sources–an aerial color photograph image, two infrared bands from an ATLAS data set, and the electrical conductivity of the surface soil layer were used in the research. Principal components analysis was applied to the color photograph image, whereas the ratio of two infrared bands was applied to the ATLAS data set. Filtering was applied to both resulting images. The distribution of soil electrical conductivity was derived from the measured soil electrical conductivity data by spatial analysis. Statistical relationships between soil clay concentrations and the principal component 3, the ratio of two ATLAS infrared bands, and the soil electrical conductivity were analyzed, and three linear equations were derived with r2 values 0.83, 0.52, and 0.78, respectively. The distribution of the soil clay concentrations was derived based on these three equations. Six levels of soil clay concentrations were classified in these three methods, and the advantages and disadvantages were discussed. The predicted and measured soil clay concentrations, based on additional soil samples from 30 field locations, were compared using linear regression (r2=0.76, 0.45, and 0.77 for the three methods). The overall accuracy for these methods were 84%, 66%, and 76%, respectively. The principal components method had the highest accuracy in our research, while the result for the depressional areas is the best from the ratio method.

Water soluble phosphorus released by poultry litter: effect of extraction and time after application

The normally alkaline pH of poultry litter limits the solubility of P forms, especially the inorganic ones. Poultry litter acidification after field applications could result in increased P solubilization, so the use of water-soluble P (WSP) concentrations measured at the original litter pH might lead to an underestimation of the risk of P contamination of runoff water. In the laboratory, we studied the influence of pH (original and target pH 6 and 7) and shaking time (0.5, 1.0, 2.0, and 4.0 h) on the amounts of Total Dissolved P (TDP) and Molybdate Reactive P (MRP) extracted from two broiler and one breeder litters. Additionally we measured pH, MRP, and TDP evolution in the thatch and top 1 cm of soil during 115 days after application of broiler litter to a Bermudagrass pasture. Acidification of litter suspensions increased TDP by 34 to 72% and MRP by 24 to 69%. In the field, broiler litter pH decreased from 8.1 to 6.7 within 30 days after the application. The following evidence suggests that the WSP measured at the original litter pH might have been considerably less than that released in the field: (a) Based on adsorption isotherm data, the 97 μg P g–1 of soil applied as MRP would have been insufficient to result in an increased concentration of 16 μg P g–1 of soil as MRP; (b) The total increase in Dissolved Unreactive P (DUP) observed in soil (38 μg cm–2) was twice the amount measured in the litter at the original pH; and (c) The increase in MRP measured in soil 59 days after litter application could be linked to additional amounts of DUP not accounted for in the analysis at the original pH. These results highlight the importance of measuring WSP under conditions similar to those encountered by the litter after application.

Predicting N mineralized in a Georgia Coastal Plain field

The N mineralized from soil organic matter provides an important portion of N available for crop production. The objective of this study was to determine the amount of spatial variability in N mineralization potential in a field and to evaluate three different methods that might be used to estimate this variability. The three methods tested included predicting the N mineralized from surface soil properties as well as from a biological and a chemical procedure. Three soils varying in N mineralization potential were selected for the study from a field in the Georgia Coastal Plain. The N mineralized from these soils was determined by an N balance of unfertilized and cropped plots. The amount of N mineralized could not be reliably predicted from surface soil organic C, although surface soil clay concentration was positively correlated with the N mineralized. The N mineralized that was predicted using mineralization parameters determined by aerobic incubation, adjusted daily for soil water content and temperature, was approximately 50% of the field measurements of N mineralized. The values of NH4-N extracted with hot 2 M KCl were related significantly to N mineralized in the field (r2= 0.60) and also to the zero order rate constant of mineralization, k0 (r2= 0.77), determined from the N mineralized in the aerobic laboratory incubation.

Nitrogen concentration in cottonseed as an indicator of N availability

Research on corn and winter wheat has shown that a critical N concentration in the grain exists above which a yield response to N fertilizer is unlikely. This indicator can be used for post-harvest evaluation of N sufficiency and for mapping N availability in the field, which may be helpful for making future N fertilizer decisions. The purpose of this study was to determine if a critical N concentration in the seed exists for cotton. The study was conducted in the Georgia Coastal Plain during 1998, 1999, and 2001, using a different variety of cotton in each year. In 1998, 12 N fertilizer rates ranging from 38 to 203 kg ha–1 were applied to Delta Pineland 90 at three locations within one field that differed in soil organic matter and clay concentration, and in 1999 and 2001, 6 N fertilizer rates ranging from 22 to 179 kg ha–1 were applied to Stoneville 474 and Delta Pineland 458 in a different field. At all locations, the N concentration in the cottonseed increased linearly with increasing N fertilizer rates. Maximum yields were obtained at less than maximum seed N concentration. Lower seed N concentrations indicated some degree of N deficiency. Based on these results, it appears that a critical N concentration of 35 g kg–1 exists for cottonseeds, above which no yield response to N fertilizer is likely. Information on the spatial distribution of cottonseed N concentrations could therefore help to evaluate the adequacy of N fertilization for cotton, thereby providing a basis for adjustment of N fertilization rates in future crops.

Comparison of Conductimetric and Colorimetric Methods with Distillation–Titration Method of Analyzing Ammonium Nitrogen in Total Kjeldahl Digests

The distillation–titration method (DTM) is a standard procedure used by most laboratories to measure ammonium-nitrogen (NH4-N) in the total Kjeldahl N (TKN) digests of various kinds of agricultural and environmental samples. These samples may have TKN contents ranging from less than 100 ppb to as high as percentage levels. However, the DTM procedure generally leads to a very low throughput because it is labor intensive and time-consuming. At the current practical quantitation limit (PQL) of 300 ppb established at the Feed and Environmental (FEW) Laboratory, University of Georgia, the DTM procedure is less applicable to low TKN surface water samples. In this study, we therefore compared the performance of diffusion conductivity method (DCM) and colorimetric method (CM) with DTM in measuring NH4-N in the TKN digests of 29 different samples representing surface waters, lagoons, manures, poultry litters, and environmental wastes. Acceptable accuracy and precision were achieved for various QC samples by all three methods. For widely different sample matrices and TKN contents, the NH4-N in the TKN digests measured by DCM and CM both agreed well with that measured by DTM. However, the linear working range of CM is limited within 0.2 to 5.0 ppm, whereas DCM is linear at a wider range of 0.01 to 2000 ppm. With DCM, the PQL of TKN is at 13 ppb, much less than the 300 ppb in DTM and 520 ppb in CM. Both DCM and CM require increasing the pH of the working TKN digest to a highly alkaline range. To meet such pH requirement, the minimum dilution need for DCM is twofold, where as that CM is fourfold. Because of greater mandatory dilution requirement coupled with a greater PQL, CM may often fail to measure NH4-N in the working TKN digest of some low TKN surface water samples. On the other hand, with some environmental waste samples containing TKN at percentage level, CM would require multistep dilution of the digests prior to measurement, thus allowing dilution-related error as well as requiring additional labor. In contrast, DCM can measure both low TKN surface waters and high TKN environmental wastes without any major limitations. Moreover, DCM may work well without any adjustment of sample background in the calibration standards. Thus DCM appears to be an attractive alternative to the labor-intensive and time-consuming DTM for measuring NH4-N in the TKN digests of various kinds of agricultural and environmental samples in the analytical services laboratories.

Near-Infrared Reflectance Spectroscopy for the Analysis of Water and Total Nitrogen Contents in Poultry Litter

Near-Infrared Reflectance Spectroscopy (NIRS) offers advantages over gravimetric water content and dry combustion nitrogen determinations that could be significant for routine laboratory operations. Water content in ground and blended poultry litter samples was successfully estimated by NIRS, but total nitrogen predictions differed significantly from measured ones. The failure to predict total nitrogen could be related to the unspecific nature of the relation and to the quality of the data used for calibration. Additionally, a model was established that permits the estimation of water content in poultry litter in its original state from that measured after grinding and blending.

Assessing the Market for Poultry Litter in Georgia: Are Subsidies Needed to Protect Water Quality?

Concerns about nutrient loads into our waters have focused attention on poultry litter applications. Like many states with a large poultry industry, Georgia recently designed a subsidy program to facilitate the transportation of poultry litter out of vulnerable watersheds. This paper uses a transportation model to examine the necessity of a poultry litter subsidy to achieve water protection goals in Georgia. We also demonstrate the relationship between diesel and synthetic fertilizer prices and the value of poultry litter. Results suggest that a well functioning market would be able to remove excess litter from vulnerable watersheds in the absence of a subsidy.

Composition of Aqueous Extracts of Broiler Litter Treated with Aluminum Sulfate, Ferrous Sulfate, Ferric Chloride and Gypsum

More knowledge on the composition of aqueous extracts of broiler litter amended for Water Soluble P (WSP) reduction would help to understand how amendments work. We measured pH, concentrations of Ca, Mg, Fe, Al, Cu, Mn, Zn, Molybdate Reactive P (MRP), and Dissolved Unreactive P (DUP) in water extracts of broiler litter treated with aluminum sulfate (ALS), ferrous sulfate (FES), ferric chloride (FEC), and gypsum (GYP) at 0, 5, 15, and 25% w/w. In order to study the effects of acidification, the same properties were measured in aqueous extracts of broiler litter suspensions that were titrated to end-points 3, 4, or 6 with 0.5N HCl. Concentrations of MRP, DUP, Ca and Mg, were 61%, 53%, 3.8 times, and 2.6 times greater in extracts from suspensions acidified to pH 6 than at the original pH of 8.9. ALS, FES, and FEC reduced pH, and showed similar effects on WSP concentrations, which were greater than with GYP. The magnitude of the reductions in WSP by ALS, FES, and FEC is uncertain because the actual amount of WSP immobilized cannot be determined. This is because of two opposite effects: 1) Through adsorption, soluble aluminum and iron remove phosphates from solution, and 2) Through acidification, iron and aluminum compounds release phosphates to solution.