Phosphorus And Compost Use Dynamics

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Courtesy of BioCycle Magazine

Organic forms of phosphorus, such as biosolids and compost products, contain low to very low levels of water extractable phosphorus, but increasingly are regulated like inorganic P sources.

Phosphorus is one of the 16 essential plant nutrients, and is considered one of the three major plant nutrients (along with nitrogen and potassium). Phosphorus is not only important in root development, but also in encouraging rapid root growth during establishment of turf, landscapes and many other plants, improving flower and seed development, and hastening maturity in food crops. Furthermore, phosphorus is an essential component of adenosine triphosphate (ATP), which is involved in most biochemical processes in plants and enables them to extract nutrients from the soil. Finally, it plays a critical role in cell development and DNA formation (Cornell Co-op Extension, 2005).

Phosphorus (P) or phosphate fertilizers are sold in the marketplace by their amount of pentavalent P in the material, which is calculated as phosphorus pentoxide (P2O5). On a fertilizer label, P2O5 is listed as “available phosphate.” The sum of the water-soluble and citrate-soluble phosphate is considered to be the amount of phosphate available to the plant, and is the amount guaranteed on the fertilizer label. However, P is actually absorbed by plants in the orthophosphoric acid (orthophosphate) form, generally as H2PO4- or H2PO42-. The amounts of these ions in the soil solution are determined by soil pH (Thomason, 2002). Note that products such as biosolids are often tested (because of regulatory requirements) for their content of total phosphate, which can be converted to P2O5 by multiplying the total P by 2.29.

In most cases, keeping soils at a neutral (7.0) or slightly acidic pH will keep P in more water soluble forms. However, inorganic P can become unavailable when it reacts with oxides of iron, aluminum, manganese (in acid soils), or calcium (in alkaline soils) to form phosphate minerals (Cornell Co-op Extension, 2005). This makes the P less available for plant uptake and leaching.

Rock phosphate is the raw material used in the manufacture of most commercial phosphate fertilizers on the market. In the past, ground rock phosphate itself had been used as a source of P for acid soils. However, due to low availability of phosphate in this native material, high transportation costs, and small crop responses, very little rock phosphate is currently used in agriculture (Rehm, et al, 1997). Today, most commercial phosphate fertilizers (inorganic) begin with production of phosphoric acid from the phosphate rock, and culminate with production of orthophosphoric acid (orthophosphate), which is concentrated into liquid or dry forms of fertilizer. Inorganic forms of phosphate are essentially derived from orthophosphate, which is highly mobile as it contains a large percentage of P in “water extractable” form, also know as water extractable phosphorus (WEP). Since WEP testing measures the portion of P that is water-soluble, inorganic forms of phosphate pose the greatest risk to water quality.

Organic phosphate fertilizers have been used for centuries as the phosphate source for crops. Even with the advent of phosphate fertilizer technology processes, organic phosphate sources from animal manures, composts and biosolids (sewage sludge) are still very important (Rehm, et al, 1997). The phosphate contained in organic phosphate sources is actually a combination of both inorganic and organic phosphate.

The Problem

The excessive application of animal manure over time, and in some cases, the excess application of P-based fertilizers, has led to P migration into waterways and even ground water. Excess amounts of phosphate in fresh water streams and lakes can cause algae blooms to occur; algal blooms comprised of blue-green alga (cyanobacteria) can produce toxins that contaminate drinking water. Further, when algae die, their decomposition results in oxygen depletion, which can lead to the death of aquatic plants and animals. This process is called eutrophication (Cornell Co-op Extension, 2005), an incredibly serious problem that must be managed through greater and more responsible nutrient management and erosion control.

To better protect ground water, the application of both animal manures and biosolids will likely be governed by the P requirements of the crop to be grown, and not just the nitrogen (N) requirements. Although both manure and biosolids typically contain more N than P, the overall ratio is shallow, which means that P is applied in excess (luxury) amounts when these materials are applied to meet the N demand of the crop. Excess P application does not typically harm plants, but it can be an environmental concern.

Based on U.S. EPA data, there are 50,000 impacted waterways/water bodies in the U.S. that surpass their allowable Total Maximum Daily Loads (TMDL) for pollutants (e.g., nutrients, sediment, etc.). Further, there are many “hot spots” around the county where overproduction of animals, often in confined animal feeding operations (CAFO), has led to over application of manure to such a degree that N and P leach out of the soil and into groundwater. The causes of these hot spots, however, are much different than the cause of most of the impacted surface waters. These surface waters are primarily impacted by sediment loading, whereby erosion carries soil particles (sediment) and the nutrients (and other contaminants attached to it) to surface waters.

Thus, there are two major problems — contamination by erosion and by overfertilization — which must be managed in very different ways. Most groundwater contamination problems must be managed by better limiting the over application of manure, while most surface water contamination problems must be managed by applying appropriate erosion control measures.

Phosphorus Regulation

With significant environmental problems already existing, and future issues looming, many states have created regulations restricting the application of phosphate fertilizer to some degree. The concept of enacting regulation to reduce over or improper fertilization is a good thing, especially creating proper setbacks from surface water, avoiding application of fertilizer on frozen or saturated land, or getting it on paved or impervious surfaces.

However, implementation of the enacted regulations (and the products targeted within them) is often greatly problematic. These new regulations not only significantly impact chemical or “inorganic” phosphate usage, but in many cases, also the use of recycled organics-based products (e.g., biosolids and manure based dried fertilizers, and various composts). It should be noted that several states have exempted recycled organics-based products from nutrient management regulations, but many have not.

Based on data compiled by the Association of American Plant Food Control Officials (AAPFCO), 16 states (Connecticut, Delaware, Florida, Illinois, Maine, Maryland, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, Vermont, Virginia, Washington and Wisconsin) have enacted P fertilizer restrictions for turf. Generally, these regulations primarily impact turf managed by landscapers and homeowners. Interestingly, in most states, these regulations exempt agricultural land in production, golf courses, sod production and gardening. Many regulations allow use of phosphate fertilization on P deficient turf (if appropriately soil tested) and allow usage in turf establishment.

In response to state concerns, AAPFCO has developed recommended language regarding “Fertilizer Restrictions for Urban Landscapes,” as well as other related SUIPs (Statements of Uniform Interpretation and Policy) to assist states in developing science-based regulation. Unfortunately, many states have been overzealous in their regulation, e.g., almost eliminating even maintenance applications of P on turf, while some have not dealt with the “real” primary causes of their nutrient contamination (e.g., over fertilization or excess manure application on agricultural land, lax enforcement of NPDES Phase II regulation on sediment control during construction). Other states have unfortunately ignored the science, regulating all phosphate sources the same (i.e., ignoring their actual mobility).

Data On Compost And Phosphorus Movement

A great deal of university research data exists illustrating the fact that the majority of biosolids-based products, regardless of their form (e.g., dewatered, composted, dried/granulated), contain low to very low levels of water extractable phosphorus (WEP). Therefore, these products pose lower risk to the environment. It should be noted that biosolids treated through a biological phosphorus removal (BPR) or similar processes, possess higher levels of WEP, since they concentrate P into the solid fraction of biosolids and in a more water soluble form. This same research also illustrates that:

  • P movement and WEP levels are much greater in chemical or inorganic forms of P fertilizer.
  • WEP levels in manure products are greater than biosolids, but much less than inorganic P fertilizer.
  • Testing products for total P content is much less reliable than testing for WEP content when attempting to predict P movement (leaching).
  • Noticing that less data existed on the WEP content of compost products, and facing new P regulations in Massachusetts, Agresource, Inc. collected a series of compost samples and had them tested through The Penn State University testing laboratory. The test results are summarized in Table 1. Samples are presented in order from lowest to highest levels of P content on a dry weight basis. The following are interpretation of the analytical data provided by Dr. Geoff Kuter, Agresource, and Dr. John Spargo, The Pennsylvania State University.

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