Biosolids Application Plan
Biosolids application rates (dry tons per acre) are calculated from the results of biosolids quality analysis, soil testing data, crop to be grown and yield estimates. The biosolids are continually monitored at the District's treatment facility. Soils are monitored after the last crop has been harvested and before each biosolids application. These application sites, 320-acre maximum size, are designated with Destination Code numbers (DCs). See Map 1 in Section I for all the DCs on the METROGRO Farm Property. The soil samples are one or two feet deep dependent upon what crop is to be grown and are composites of at least 16 core samples per DC or field(s) therein.
The Colorado Biosolids Regulation No. 64 recently underwent its triennial review. During the review process, an agronomic loading rate policy was developed. The new policy along with the changes to the Regulation went into effect April 1, 2007. The agronomic rate policy was developed in an effort to streamline the various biosolids appliers across the state and how they calculate their loading rates. The new policy uses the Plant Available Nitrogen (PAN) method. An example using the PAN method to calculate an agronomic rate is shown below:
Biosolids PAN = (m) (Organic N) + (v) (NH4-N) + (NO3-N)
Where: PAN = Plant Available Nitrogen (lbs. per dry ton of biosolids)
m = Mineralization factor
v = Volatilization factor
Organic N = Organic nitrogen in biosolids (%)
NH4-N = Ammonia-nitrogen in biosolids (%)
NO3-N = Nitrate-nitrogen in biosolids (%)
Nutrient loading rate tables in the Colorado State University Guide to Fertilizer Recommendations for the crop to be planted, yield goal and results of the soil tests, dictate the nitrogen (N) loading rate for each DC or field(s) therein. The recommended fertilizer rate (lbs. N/acre) is then used together with the available N to calculate how many truckloads of biosolids will be taken to the DC and how many dry tons of biosolids will be applied to each acre of land in the DC.
Biosolids application at the METROGRO Farm has been significantly reduced for the last two years. Reasons for the reduction include the long drought that Colorado has experienced. Crops are not producing as much as expected so the nutrients are staying in the soil profile. This limits the amount of fertilizer that can be applied in subsequent years. Another reason is the Metro District’s commitment to its private farmers. With several thousand acres of private farmland, the District is able to land apply its biosolids to sites closer to the Central Treatment Plant (CTP) and to irrigated sites. More sites can be applied to in a shorter amount of time because travel time to and from the CTP is reduced. Land applying biosolids to more irrigated sites is beneficial to the District because the nutrient loading rates are much higher than those of dryland sites. In order to satisfy the crop nutrient requirements, more dry tons of biosolids can be applied to an acre of irrigated land than to an acre of dryland farm property.
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