Zeolites – A Potential Soil Amendment for Vegetable Crops in South Florida

Vegetable farmers in South Florida have many challenges; including, farming in the rocky or sandy soil with low nutrient and water holding capacities; and potential of leaching nutrients and agricultural chemicals into ground water.  The Biscayne aquifer is very shallow, and during the rainy summer months ground water is located just a few feet below the soils surface. Some growers use cover crops during the summer time, between vegetable production seasons, to improve soil properties and remove nutrients left in the soil after the growing season. Land leasing contracts may limit the use of cover crops.  In many cases, growers are leasing the farm land only for short period if time, just for a few months of the growing season, so they are not obligated to plant cover crops after the end of production season.  A different grower may farm the same field in the coming fall/winter production period.

Other methods of improving soil properties may include applications of compost.  Unfortunately, the closest reputable compost facility is located in Palm Beach, about 100 miles north; therefore, transportation and application costs are too high to make this practice feasible for farmers.

Another possible soil amendment considered for use in the crop production is zeolites. Natural zeolites, sometimes called "la roca magica", are alumonisilicate minerals characterized by a very large surface area and high cation and anion exchange capacities, allowing absorption and release of plant nutrients and moisture without any change in the nature of zeolite.  For example, 0.5 kg of zeolite material has surface area as that of a football field.  Zeolites have both negative and positive charges and this ability is widely used in industry and agriculture.  Natural zeolites, found in the volcanic sedimentary rocks, have been used in construction since Roman times.  Some other uses include, filler in paper and uptake of the radioactive substances from nuclear waste, dietary supplements in animal diets, and ammonia filters in kidney-dialysis equipment. Benefits of using zeolites as soil amendment reported from several studies include: improving soil water and nutrient holding capacity; reduction in N and P leaching from soils and soilless media; and improving nutrient availability to plants and yield increase.  Currently, the use of zeolites in agriculture is more common in Japan and Europe than in the US.

South Florida's Krome, very gravelly loam soils, with a very low cation exchange capacity and strong potential to leach farm chemicals into the shallow Biscayne aquifer, were a good candidate to evaluate effectiveness of zeolites to reduce ammonium and nitrate leaching.  In the 2003-2004 vegetable growing season, a group of researchers from the Tropical Research and Education Center (TREC) in Homestead, lead by Yuncong Li and one of his graduate student, in cooperation with a Miami-Dade County Extension vegetable agent and a local sweet corn grower, conducted a series of field studies with tomatoes and sweet corn. These studies were followed by laboratory studies evaluating abilities of zeolites to prevent leaching of phosphorus, ammonium, and nitrate in agricultural soils in the C-111 canal basin. This research was sponsored by the South Florida Water Management District.

Two tomato trials were conducted at TREC, and two sweet corn trials were conducted in the growers' field in the 2003 and 2004 growing seasons.  In order to monitor water quality, a zero-tension pan lysimeters with two major parts, pan and water collector, were installed in both locations.  Zeolites obtained from several producers were applied as 5000 kg/ha and 0 kg/ha to tomato beds prior to fumigation and placement of plastic mulch. The same amount of zeolites per acre was applied to the sweet corn field after corn germination.  In both trials, zeolites were incorporated into the soil.  In addition to zeolites, application of phosphorus fertilizer (0, 50 and 100 kg P/ha) were essential components of corn trials.  Lysimeter water was collected and tested four times from each study. Soil and tissue samples were collected from both fields. Water and soil were analyzed for pH, electrical conductivity, NH4-N, NO3-N, ortho-P, and total P.

Results from the experiments conducted in 2003 indicated that zeolite increased yield of tomato, but there was no improvement in the sweet corn yield.  High concentration of soil P found in the corn field, prior to seeding of corn, resulting from many years of P fertilizer applications, could be a reason for lack of the effect of zealots on P in this particular case. In the tomato trial, leachate NO₃-N and ortho-P concentrations were lower from the zeolite treated plots then from control. There was no difference between treatments for concentrations of NH₄-N and total P. The results from these field studies showed some beneficial effects of zeolites as soil amendment for crop production and improvement of water quality. More studies are needed for making solid conclusions and future recommendations for growers.

Teresa Olczyk
Extension Agent, Miami-Dade County
Vegetarian, 05-04