Department Name

 

Leach-M Performance in Sandy Soils of North Florida

Jaime.F. Sánchez and Rao S. Mylavarapu
Soil and Water Science Department
University of Florida, Gainesville, Fl 32611

Abstract

The use of models to evaluate the impact of agricultural practices in the contamination of water and soils is now a common practice. Leach-m is a model that uses the Richard’s equation to describe the water movement through the profile. The basic input to the model was obtained from water retention studies carried out in different farms, under specific fertilization programs, obtaining the Campbell’s parameters as well as bulk density, hydraulic conductivity and particle size distribution. In a selected profile, we compared the nitrate and ammonia contents observed in the field with the levels calculated by the model. Leach-m had the tendency to underestimate the ammonia content through the profile and overestimate the nitrate in the top layers. The probable reasons may be how Leach-m computes the movement of water in the soil profile. The parameters a and b of the Campbell’s retention function do not yield good results although the goodness of fit was high. However a best estimates of this parameter could be obtained using multivariate analysis that permit evaluate all the variables at the same time.

Objectives

- To evaluate the nitrate movement predicted by Leach-m and compare with the nitrate levels observed in the field in north Florida soils

- To evaluate the goodness of fit of the Campbell’s parameters a and b through the soil profile

- To determine soil physical characteristics of soils from north Florida under poultry manure management

Materials and Methods

Six points, describing a transect, were defined in a spray area from a poultry farm located in the Lafayette county, Florida. The most representative site was selected for this poster.

Undisturbed soil samples were obtained per site, each 10 cm up to 1.5 meters depth. The samples obtained were used to establish the water retention curves, bulk density and saturated hydraulic conductivity.

The soil is classified as a loamy, silicieous, thermic, Arenic Paleudult.

Information was collected from the farm to establish the management practices that includes poultry manure application, bahia grass and grazing.

At the end of the cycle nitrates content were evaluated for each depth and each site.

Leach-m was evaluated for each profile and nitrate yields were obtained for each particular depth.

Discussion and Conclusions

In the spray area, the farmer is applying 294 kg of N ha-1 and 148 kg P ha-1. But the loadings are greater if we consider the grazing.

From June to August, there is a period where rainfall is higher than evapo-transpiration. Application of poultry manure during this period may be avoided.

Campbell’s parameters were adjusted with higher goodness of fit at the top layers of the soil. In the deeper layers (> 60 cm) the performance of the Campbell’s equation was poorer, suggesting that other models should be evaluated to describe the water movement in that soil.

Bulk density is stable between 1.60 and 1.70 t m-3. Although the sand content reduced with depth (data not shown), lower bulk densities were observed in the surface layers.

As expected, the water retention at –200 kPa of suction increased with depth.

Leach-m over-estimated the nitrate content and under-estimated the ammonia content in the soil solution. This result confirmed that it is necessary for these soil types to use a different approach to adjust the Campbell’s parameters to improve the nitrate and ammonia prediction by Leach-m.

Bibliography

Zavattaro, L.; C. Grignani. 2001. Deriving hydrological parameters for modeling water flow under field conditions. Soil Science Society of America Journal 65(3):655-667.

Hutson, J.L.; A. Cass. 1987. A retentivity function for use in soil-water simulation models. Journal of Soil Science. 38:105-113.

Campbell, G.S. 1974. A simple method for determining unsaturated conductivity from moisture retention data. Soil Science 117(6):311-314.

Hutson, J.L. 1986. Water retentivity of some South African soils in relation to particle size criteria and bulk density. South African Journal of Plant and Soil 3(4):151-155