7.3 What is the application rate for biosolids?

The application rate for biosolids refers to the quantity of biosolids that are applied to a specific area of land. This rate is typically measured in units such as dry weight (tons per acre) and is determined based on the following three essential factors:

• Crop type: Different crops have varying nutrient requirements.
• Soil Characteristics: Soil properties such as texture, organic matter content, pH (maintained at 6.5 or above), and nutrient status influence the capacity of the soil to absorb and retain applied biosolids. Requirements may be included in regulatory standards.
• Nutrient Content of Biosolids: Biosolids contain nutrients such as nitrogen, phosphorus, potassium, and micronutrients, which contribute to soil fertility and crop growth. The nutrient content of biosolids determines the amount needed to meet crop requirements.

Additionally, the following aspects need to be kept in mind:

• Regulatory Guidelines: Regulatory agencies may establish guidelines or limits on biosolids application rates to protect soil and water quality, prevent nutrient overloading, and ensure public health and safety.
• Environmental Considerations: Environmental factors such as slope, proximity to water bodies, and risk of nutrient runoff or leaching may influence the application rate to minimize environmental impacts.

Let’s look at a specific example.

FS-based Compost

As with biosolids, the application rates for faecal sludge-based composts vary with local soil conditions and crops. The rates can range between 5 and 25 tonnes per hectare.

Only matured (well-composted) FS-derived compost should be applied to soils in order to prevent possible negative effects on crop growth.

The application varies depending on the condition of the compost. Pelletized compost, when applied directly to plants, should be placed at least 5 cm from the base of the plant, either on the soil surface or buried at a depth of about 5 cm. Non-pelletized compost can be mixed with the soil, for example, one week before the intended planting.

On very poor (sandy) soils, biosolid-based compost can have a high impact by alleviating many soil structure- and soil fertility-related shortcomings, however, it can only contribute to some extent to immediate crop nutrient requirements.

The following table provides application rates for a normal and a nitrogen-enriched sludge-based compost, based on crop nutrient needs. These guidelines are published by the Ministry of Food and Agriculture in Ghana and need to be adapted to local soil conditions and crops in different geographical domains.

Crop	Quantity required of an enriched (3% N) FS-based co-compost (t/ha)	Quantity required of a not enriched FS-based co-compost (t/ha)	Possible additional P and K fertilizer needs depending on soil fertility
Cabbage	3.4	10.8	–
Maize	3.0 – 4.0	10.0	–
Carrot	2.5	–	P
Watermelon	2.0	–	P, K
Tomato	3.2	8	K
Onion	2.5	–	–
Garden egg	3.7	7.5 – 9.5	K
Cucumber	2.0	–	–
Lettuce	4.3 – 5.0	12.5	–
Banana	3.7	–	–
Rice	3.2	7 – 8	–

A nitrogen-enriched compost will, for some crops, require an extra application of other nutrients, like phosphorus (P) and potassium (K). This might, for example, be the case on sandy or highly weathered soils.

Some countries may have specific application limits that need to be respected aside from soil and crop needs. For example, Bangladesh defines application limits of

• < 3 t dry substance sewage sludge per ha in 3 years, and
• < 10 t dry substance sludge compost per ha in 3 years.

Additionally, the Fertilizer Recommendation Guide can be consulted, which provides information on soil and crops in Bangladesh (Link).

For further information, please click on the Materials tab at the top of the page.