University of Florida

Gainesville, Florida

Effects of organic and conventional farm management regimes on soils were studied on two Florida farms during a two-year period. There are two goals of the project: (1) to determine the effects of different farm management systems on soil quality and (2) to relate the ratio of product output and energy input to the efficiency of the management systems. The two farms were selected because each has Typic Quartzipsamments and a cropping regime of watermelons and peanuts. The histories of the two farming systems are similar because both utilize organic matter incorporated from an eight year cover crop of bahia grass.

This long-term cover crop is used for its high lignin content and slow decomposition rate and for control of annual weed populations. This is a traditional method of watermelon farming on Florida sandy soils. This cover cropping system equally affects soils of both the organic and conventional farms. Organic management includes fertilizing with four tons per acre of composted chicken manure, rotation of rye grass cover crop, and mold board tillage. The conventional scheme includes applications of synthetic fertilizer, fungicide, herbicide, and lime. Mold board tillage, black plastic mulch over watermelons, and some use of cover crops are used in the conventional system.

Physical, chemical, and biological properties were used to quantify soil quality. Respectfully, these properties were represented by morphological descriptions, moisture holding capacity, organic carbon content and microbial carbon. Samples were taken from six different sites including the following: a control site under natural vegetation, pasture of bahia grass, conventional and organic watermelon and peanut fields. Samples were taken at two month intervals from January to September 1996. These samples represented two growing seasons including the first year crop of watermelons followed by the second year crop of peanuts.

In the short-term, improvement of soil properties with conventional practices were indicated by the results. For instance, microbial content was consistently higher through the first growing season for the conventionally grown watermelon. This may be attributed to management, which included a black plastic mulch. The dark plastic most likely increased soil temperature and maintained moisture content. The recent turnover of organic matter in the form of bahia grass contributes the third factor that readily enhances soil microbial growth. The application of nitrogen in the conventional field contributed to a ripe environment for microbial growth. Upon harvest, microbial measurements were higher for the organic management systems. This indicates that additional nitrogen applications artificially fed the soil microorganisms under the conventional system. No statistical differences were measured in moisture holding capacity and organic matter content of cropped fields. In the long-term, both systems are predicted to sustain soil quality if fields are returned to pasture for an extended number of years.

The question of sustainability of soil quality and farm production was addressed through energy analysis of each farming system in terms of output product to energy input. Typically in conventional systems, 11% of energy use comes from fossil fuels contributing to synthetic fertilizers. Theoretically, it is predicted that this amount of fossil fuels can be eliminated in organic systems, although fossil fuels are still necessary in production because of tractor operations. Synthetic fertilizers were not used in the organic system and its microbial biomass was larger in the long-term than the first year conventional system. Therefore, it is suggested that addition of fertilizers is not necessarily the only way to effectively sustain soil organisms. Still, we are discovering that lower input systems are not necessarily the highest yielding systems and therefore not calculated to be efficient systems.

This exemplifies the problem that often faces the optimization of organic farming systems especially in Florida. Questions about quantity of organic matter, planting times, plant access to water, and plant spacing are significant issues for Florida organic and conventional watermelon farmers to perfect, in order to improve soil quality and farm production. The quality and quantity of agriculture inputs used to sustain Florida soil quality are as timely and fragile as the sandy soils themselves.

References

Doran, J. W., M. Sarrantonio, M.A. Liebig. 1996. Soil health and sustainability. Advances in Agron. 56;1-54.

Fauci, M.F. and R.P. Dick. 1994. Soil Microbial Dynamics; Short- and Long- Term effects of inorganic and organic nitrogen. J. of Soil Sci. Am. 58;801-806.

Fluck, R. C. 1996. In Swisher, M. E. and A. Todd-Brockarie(eds.), Energy, the hidden input. Cooperative State Research, Education, and Extension Service. Special project no. 94-EATP-1-0053.

MacRae, R.J., S.B. Hill, G. R. Mehuys, J. Hennings. 1990. Farm-scale agronomic and economic conversion from conventional to sustainable agriculture. Adv. in Agron. 33;155-198.

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