A lysimeter study of vadose zone porosity and water movement in gypsum amended soils.

摘要

One phase of this study involved the collection of undisturbed soil columns by using a free-standing portable hydraulically-powered device (Hydraulic Ram) to compare gypsum effects on insertion rate, penetration resistance and force required to insert PVC pipes into the soil. Four soil columns per treatment/soil were collected to measure and compare gypsum application regime effects on the variables previously mentioned. Lower penetration resistance and pressures were needed to insert soil columns on gypsum treated soils compared with control; however, response varied with soil drainage class. Celina long-term gypsum treated soils took 30 and 44% less time to completely insert cores than for short-term treatment and control, respectively with an average total insertion rate of ∼24 cm/min. For Brookston loam soils, insertion for both treated soils took 20% less time than for control and an average total insertion rate of 19 cm/min. In our study, core collection time ranged from 30 to 45 minutes and an average of eight cylinder insertions per day was accomplished.;The second phase of this study involved determining the effect of gypsum application regime on alfalfa growth in a controlled environment lysimeter study. Three of the four soil columns of each treatment/soil collected from fields with historical gypsum application were used for the alfalfa greenhouse lysimeter study and at the same time for a water balance study. The fourth soil column of each treatment/soil was used to chemically and physically characterize the soils under the different gypsum regimes. During the alfalfa greenhouse lysimeter study, six alfalfa harvests were performed and alfalfa water use, evapotranspiration, stored water and drainage were recorded. To be able to determine the effect of gypsum on ion redistribution throughout the soil profiles, soil, water and alfalfa tissue analyses were performed. After the greenhouse alfalfa and water balance studies, the three soil columns/treatment/soil were cut in half to facilitate alfalfa root length and biomass measurement, for further chemical and physical analysis of the soils and for a micromorphological study of thin sections.;In the alfalfa greenhouse study, enhanced growth of alfalfa was observed under both gypsum treatments/soils compared with controls. However, no statistical difference (P0.05) was found for average dry weight and cumulative dry weight of alfalfa. Higher yield production was measured under Brookston soil (1.5X) compared with the Celina series. However, for both soils, higher root biomass was found in the first 10 cm of soils for alfalfa plants growing under both gypsum treatments. No significant effect of gypsum was observed for the remaining depth increments.;For the hydrology study, statistical differences (P0.05) were found for drainage (D) and evapotranspiration (ET) variables. Greater drainage (∼60%) was obtained under Brookston long-term and Celina short-term soils. Lower evapotranspiration was found on the treatments previously mentioned. Results from the water balance study demonstrated gypsum application regime had a positive effect by increasing exchangeable Ca and Ca:Mg ratio which influenced clay flocculation, improved soil structure and increased Ksat.;Micromorphological results showed total porosity (phi), pore size distribution (PSD), pore shape (PS) and aggregate size (AS) varied with treatment/soil/soil sample orientation. Also, there was no consistent response for gypsum application regime. Both Brookston gypsum treated soils had higher phi in the whole soil profile compared with control. However, in Celina gypsum treated soils, higher phi was found at some depths and soil faces. Also, gypsum treatments had a positive effect on micropores and mesopores percentage, whereas, higher macropores percentage was found on untreated soils. Pore size distribution and aggregate size varied with respect to treatment, soil depth and soil face. In general, gypsum treated soils had larger aggregates than control in the whole soil profile. Aggregates 100 microm predominated in Brookston control soils and 200 microm in Celina control soils; however there was no predominant size for gypsum treated soils. Based on these micromorphological results, lower drainage on the long-term Celina treatment cannot be attributed to soil structure. Restricted drainage on long-term Celina treatment was attributed to secondary calcium carbonate precipitation and deposition in pores. (Abstract shortened by UMI.)