Determination of the total salt content of livestock and poultry manure using gravimetric method

Livestock and poultry manure is often used as organic fertilizer in the farmland surrounding large-scale farms, which is an effective method for recycling manure in agricultural settings. However, the salt content of livestock and poultry manure has not received sufficient attention in terms of the risk of salinization of agricultural land. Before applying livestock and poultry manure to fields, it is crucial to determine its total salt content or water-soluble salt content for salt analysis and management of saline-alkaline land. Unfortunately, there is a lack of standards and methods for determining the total salt content of livestock manure both domestically and internationally. Given the absence of standardized detection methods, the non-uniformity of determination methods, and the loss of potential salt residue, we conducted experimental research by gravimetric method. This study aimed to determine the total salt content in livestock and poultry manure and validate the feasibility, with the goal of providing a more accurate and reliable means of assessing the quality of livestock and poultry manure for agricultural production. Firstly, we conducted a completely randomized trial to optimize the sample-to-water ratio, oscillation parameters, leaching method, and leachate uptake. Subsequently, an orthogonal experimental design was used to evaluate three key parameters, including oscillation frequency, centrifugation time, and leachate uptake. Nine treatment combinations were formed to optimize the conditions for manure leachate extraction. Finally, a spiked recovery test was performed to assess the precision and accuracy of the method. The results indicated that the optimal conditions for determining the total salt or water-soluble salt content in livestock manure, with solid manure as the primary source, were as follows: a ratio of 1:5 for sample-to-water, oscillation at 150 r/min for 3 minutes, centrifugation at 4 000 r/min for 8 minutes, and leachate uptake of 20-50 mL. Moreover, it has been demonstrated that centrifugation can effectively replace the existing air suction filtration method for obtaining leachate. There was a significant correlation observed between the turbidity of the leachate from livestock and poultry manure and the centrifugation parameters, as well as the results of total salt determination. Leachate uptake volume had the greatest influence on the total salt content in the leachate from livestock and poultry manure, oscillation frequency took the second place, and centrifugation time had the weakest effect. According to the gravimetric method, we determined the total salt content of six different livestock manure samples under these optimized parameter conditions. The measured results exhibited relative standard deviations (RSDs) ranging from 3.7% to 13.8% and recovery rates ranging from 89.0% to 106.1%, meeting the requirements of analytical quality control in the laboratory. The established gravimetric method for determining the total salt content in livestock manure offers advantages such as low reagent usage, cost savings, high accuracy and precision, and practicality. It can be applied on a large scale to test the total salt content in livestock manure, thereby improving efficiency. This study provides a scientific foundation for the research and development a technology of salt reduction, and contributes to the sustainable development of integrated crop-livestock agriculture.