Effect Of Soil Variability Research Articles

Autologistic Model of Spatial Pattern of Phytophthora Epidemic in Bell Pepper: Effects of Soil Variables on Disease Presence

The autologistic model is a flexible model for predicting presence or absence of disease in an agricultural field, based on soil variables, while taking spatial correlation into account. In the autologistic model, the log odds of disease in a particular quadrat are modeled as a linear combination of disease presence or absence in neighboring quadrats and the soil variables. Neighboring quadrats can be defined as adjacent quadrats within a row, quadrats in adjacent rows, quadrats two rows away, and so forth. The regression coefficients give estimates of the increase in odds of disease if neighbors within a row or in adjacent rows show disease symptoms; thus, we obtain information about the degree of spread in different directions. The coefficients for the soil variables give estimates of the increase in odds of disease as soil water content or pathogen population density increase. In this problem, the soil variables may also be highly correlated over quadrats, and disease incidence in within-row neighbors may be highly correlated with disease incidence in adjacent-row neighbors. This collinearity makes estimation and interpretation of the parameters of the autologistic model more difficult. We discuss fitting the autologistic model and tools for evaluating the aptness of the model.

SOLUTE TRANSPORT THROUGH THE VADOSE ZONE: A FIELD STUDY AND STOCHASTIC ANALYSES

Solute concentration and water content profiles were measured within an area of 3 × 6 m2 to a depth 1.7 m in an unsaturated soil. Spatial variabilities of soil water content and concentration were analyzed based on the field data and stochastic models. During the 33-day field experiment under constant leaching, the soil water movement showed an approximately steady-state flow. The effect of soil variability on soil water distributions was relatively small. On the other hand, the concentration distribution exhibited a profound variability in the field, especially within the zones with peak values in the vertical profiles. Immobile water content calculated from the experimental data was about one-third of the total water content. Stochastic and deterministic models were used to simulate solute transport through the vadose zone. Comparison of the simulation results with the field experimental data showed that the models described the mean concentration reasonably well without considering the pore scale dispersivity. However, the pore scale dispersivity had a significant impact on the estimation of the concentration variance.

Effects of soil variables and season on the production and consumption of nitric oxide in oxic soils

NO production rates, NO uptake rate constants, NO compensation points, and different soil variables were determined for various soil types and different soil horizons, and checked for mutual correlations. NO production was detected in all, and NO comsuption in most soils tested. Only soils in a very early state of soil genesis showed no NO consumption activity. NO consumption was positively correlated with soil water and NH 4 + contents. NO production rates were not correlated with any soil variable. Both NO production and NO consumption tended to decrease from the upper organic to the deeper mineral horizons in different climax soils. The seasonal variation of NO production and NO consumption in a calcic cambisol and a luvisol showed highest rates in summer. The rates of NO production and NO consumption were correlated with a few of the soil variables, but showed no uniform, theoretically comprehensible pattern. However, NO production in samples of the calcic cambisol was stimulated by fertilization with NH 4 + , but not with NO 3 − and was inhibited by nitrapyrin, indicating that NO was produced by nitrification. NO production made up about 3% of the nitrification rates. In the luvisol, in contrast, NO production was not affected by the addition of NH 4 + or NO 3 − . Nitrification was also undetectable in this acidic soil, except for a few patches where NO production was also detected.

Effect of Soil Variability on Response to Tillage of an Atlantic Coastal Plain Ultisol

AbstractSubsoiling and chisel plowing are deep tillage practices which have increased crop yields on soils containing tillage pans. The relationship between crop response to deep tillage practices as influenced by soil property variability is not well understood. This study was conducted to obtain information about the response of corn (Zea mays L.) to selected tillage practices as influenced by soil variability. The effects of selected tillage practices on soil physical properties and corn production were investigated during 1981 and 1982 in a field with a tillage pan. The surface textural class of the Norfolk series (fine‐loamy, siliceous, thermic Typic Paleudults) in the field ranged from loamy sand to sandy loam. Three distinct soil units were delineated on the landscape based on depth to the B horizon. Four tillage treatments were investigated: disking, bedding, subsoiling plus bedding, and chisel plowing. Soil and plant parameters monitored at various times during the study were in situ mechanical impedance, bulk density, corn height, leaf area, barren stalk population, and grain yield. Significant interactions between tillage and soil unit for grain yield indicate a variable response to tillage due to depth to the B horizon. Mechanical impedance and bulk density in the conventional disk and subsoil treatments were negatively correlated with depth to the B horizon, implying that relatively less root restriction occurs when the B horizon is deeper. Grain yield response to deep tillage (subsoiling and chisel plowing) was significant during 1981 but not in 1982; this behavior was attributed to differences in rainfall distribution between the two years. Barren stalk population was significantly decreased by deep tillage both years.