What is 'legacy' phosphorus

Posted 3/22/21

Agriculture, urban development and Mother Nature all played a part in building up phosphorus in the soil as well as in the waterways of South Florida.

Legacy of dairies“LaMartin’s …

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What is 'legacy' phosphorus


Agriculture, urban development and Mother Nature all played a part in building up phosphorus in the soil as well as in the waterways of South Florida.

Legacy of dairies
“LaMartin’s History of Okeechobee County” noted in 1945, Okeechobee County had only a single 109-cow dairy farm. By the late 1950s dairy-men had discovered that the area offered productive land at reasonable prices. By 1972 there were 26 dairies in operation with about 25,000 cows.

By 1972, Okeechobee County was the leading dairy production county in the southeastern United States and accounted for 45 percent of Florida’s milk production.

In the 1950s, it was an accepted practice to clean the milking barns by washing waste into nearby canals or waterways. The heyday of dairy farming left a legacy of nutrient load from manure. That nutrient load from dairies was addressed decades ago.

FDEP was so sure of the dairy-phosphorus link that in 1986, a law was passed, the FDEP Dairy Rule, which put strict limits on the phosphorus content of runoff from dairies. The rules were so strict that many dairies simply could not meet them. More than half of the dairies moved out of the watershed, taking advantage of a dairy buyout program. The dairies that stayed found ways to recycle the waste and keep it out of the runoff and/or use berms and retention
ponds to keep the water on the property.

Legacy of sewage dumping
Some of the phosphorus attributed to “agriculture” comes from urban areas.

Land spreading of biosolids is still a common practice in Florida, and it is commonly done in remote, rural areas zoned for agriculture. But the nutrient load in the materials dumped are many times — sometimes hundreds of times — the amount needed for use as fertilizer. The primary purpose of land spreading of biosolids is to get rid of the waste as cheaply as possible.

While other methods of disposal of sewage such as incineration and bioenergy are available, according to FDEP most Florida utilities dispose of biosolids by land application, dumping the treated waste in a landfill or processing it to a marketable standard and distributing it as fertilizer. Municipalities usually choose the least expensive method available. As of 2013, the state banned the practice in the Lake Okeechobee watershed.

Legacy of fertilizer
Golf courses, vanity lawns - Fertilizers used to keep the non-native grasses used for golf courses and urban lawns green year round can also contribute to the problem. Some homeowners associations require property owners to use and maintain St. Augustine grass. A Florida law passed in 2009 sought to support homeowners’ efforts to use “Florida-friendly” plants has not stopped them from filing lawsuits against owners who challenge their rules, and most residents don’t have the resources to fight the HOAs in court.

Legacy phosphorus in the soil
Florida soil contains a lot of natural phosphorus, as evidenced by the phosphate mines in Polk County. According to the Florida Industrial and Phosphate Research Institute website, “A blanket of phosphate deposits covers much of peninsular Florida. In the areas that are considered economical to mine, the matrix layer, which consists of approximately equal parts phosphate rock, clay and sand, averages 12 to 15 feet in thickness. The matrix is buried beneath a soil overburden that is typically 15-30 feet deep.” Much of the watershed that feeds Lake Okeechobee is naturally high in phosphorus, and when it was dredged for navigation and flood control, water started to flow more quickly, changing the way phosphorus moves from soil into waterways.

In 1998, Pat Hogue of the Okeechobee County Extension Office was among those who took soil samples from the spoil along the Kissimmee River channel — the dirt dug out of the riverbed to create a straight, deep channel to speed the flow of water from the Kissimmee River basin south to Lake Okeechobee. Taking soil samples is a routine part of an extension agent’s job. When the samples were sent to the University of Florida for testing, the phosphorus readings spiked “off the chart,” as high as 448+ parts per million. The + indicates that at this point there is so much phosphorus, further testing is irrelevant, he explained.

According to Mr. Hogue, for Bahia grass pastures, anything below 15 parts per million might need additional phosphorus in fertilizer. In the average backyard garden, phosphorus levels of 30-50 parts per million are desired, depending on what you are growing, Mr. Hogue explained. Mr. Hogue said he and his colleagues took their findings to the South Florida Water Management District, but the governing board was not interested. “They accused us of not knowing how to take a soil sample,” he recalled. The board ridiculed extension agents with decades of experience taking soil samples, accusing them of not knowing how to do one of the most basic parts of their job.

Over the past 30 years, others have tried to get SFWMD to consider the phosphorus loading that could be coming from the flood control channels and from the spoil left on the canal banks, with no success.
Mr. Hogue tried the soil tests again in 2009. The results were the same — with the phosphorus readings “off the scale.” He published his findings in December 2010 in an article titled “Impact of Cattle Grazing in South Florida.”

Not only did digging the channel leave soil high in phosphorus on the banks, but the Kissimmee River restoration project pushes the soil back into the channel, said Mr. Hogue. Multiple times since the restoration work started, storm events have caused that phosphorus-rich fill to be washed out of the channel and into Lake Okeechobee. In 2017, Hurricane Irma damaged a section of the restoration in an area that had just been filled prior to the storm.

Back-pumping and backflow
The legacy of back-pumping water from canals south of Lake Okeechobee back into the lake started with concerns about water supply. In 1968 the U.S. Army Corps of Engineers proposed using pumps to send water the wrong direction — from the EAA back into Lake Okeechobee — to prevent water shortages. While farmers use little fertilizer in the Everglades Agricultural Area because the “black gold” soil is high in natural fertilizer, nitrogen and phosphorus from the muck itself came with that pumped water, In 2006, a federal district judge in Miami ruled that the South Florida Water Management District must comply with the Clean Water Act. And on June 15, 2007, a federal court issued an injunction requiring it to apply for pollution permits to engage in pumping dirty water into the lake. Today, backpumping is still allowed but only to prevent flooding in the cities on the lake’s south shore. Similar to backpumping, backflow means the water is going the wrong direction, from the south or from the east into the lake. When the lake level is lower than the water level in the canals on the south and eastern shores, water can flow back into the lake, bringing with it excess nutrient loads.

Commercial farming
While much of the blame for phosphorus load has been placed on cows and sugar cane, other agricultural sources also contribute to the load. Runoff from row crops, sod farms, chicken farms and other agriculture throughout the watershed – which starts at Orlando – contributes to the phosphorus load. (Sugar cane is actually a “nutrient sink” which removes phosphorus from the watershed.)

A legacy of development ...
According to the U.S. Census, Florida is the fourth-fastest growing state in the country with more than 21 million residents. The South Florida Water Management District is home to more than 8 million people.

When the current flood control system was designed, the population of South Florida was about 2 million. In addition to full time residents, millions more live in South Florida in the winter or vacation here. In 2017, about 72 million people visited at the top of the watershed that flows into Lake Okeechobee. Every building roof, street and parking lot means more runoff and less green space available to allow rain to soak back into the earth, That runoff carries with it fertilizer and pesticides from urban landscapes, oils and coolant spilled on roadways, trash and debris.