Near Yuma, an EPA report found 8 dry tons of biosolids per acre cut fertiliser needs to one-fourth in year 1; by year 3, no supplemental fertiliser was required

Farming in deserts can be challenging. Soil surrounding Yuma in Arizona is predominantly sandy, which tends to drain quickly and can have low organic matter, making careful management of nutrients and water important for crop growth. A study carried out by the US Environmental Protection Agency on the effects of using biosolids to replace chemical fertilisers in farmland reported that biosolid applications reduced the need for conventional fertiliser in the trials. According to the EPA's Biosolids Recycling: Practices and Benefits report, applying 8 dry tons of biosolids per acre to sandy irrigated soils near Yuma, Arizona, reduced chemical fertiliser requirements to about one-quarter of normal levels during the first year after application. The report further states that by the third year of biosolids use, no supplemental chemical fertiliser was required.

The functioning of the trial

As the EPA report notes, biosolids are not raw sewage. They are treated to reduce pathogens and stabilise the material so it can be applied to land under federal rules. Once treated, biosolids can provide organic matter and nutrients such as nitrogen and phosphorus to soil. Applying 8 dry tons per acre in the Yuma study added organic material to the sandy soil while also supplying nutrients for crop growth. This helped reduce the need for conventional fertiliser during the trial.


The timing of the effect was important. During the first year after application, the EPA report says only about one-quarter as much chemical fertiliser was needed. By the third year of biosolids application, the report states that no supplemental chemical fertiliser was required. The report does not, however, conclude that the soil had become permanently self-sustaining. Instead, the results show that biosolids supplied nutrients and organic matter in a way that reduced the need for additional chemical fertiliser over the period described.

What the Yuma trial means for soil and gardens

The Yuma trial also illustrates why organic matter can be important in sandy soils. Because sandy soil allows water to move through quickly, nutrients can be lost from the root zone more readily than in soils that hold water and nutrients more effectively. Adding organic matter can help improve the soil's ability to retain moisture and nutrients while also contributing nutrients as it breaks down.

The EPA trial does not mean that gardeners should copy the application directly. Biosolids used in the study were treated before land application, and their use is subject to federal requirements. For home gardens, compost, leaf mould and well-rotted manure are more familiar ways of adding organic matter to soil. The broader lesson from the trial is that improving the soil itself can reduce reliance on repeated fertiliser applications.

The results also show why soil changes should not be expected immediately. In the EPA report, the reduction in chemical fertiliser requirements developed over several years, with the first-year and third-year results showing different levels of dependence on supplemental fertiliser. Gardeners trying to improve sandy or nutrient-poor soil can therefore add organic matter gradually and use soil testing to determine whether additional nutrients are still needed.

What to know before using biosolids

The biosolids used in the Yuma study were treated before land application, in accordance with federal requirements intended to reduce pathogens and stabilise the material. This is different from raw sewage and is an important distinction when discussing biosolids as a soil amendment. Home gardeners should not assume that material described simply as sewage sludge is equivalent to the treated biosolids used in the EPA trial.

For most gardens, the practical takeaway is straightforward: adding suitable organic matter can improve the physical condition of soil while also contributing nutrients. Sandy soil such as that around Yuma may respond differently from clay-heavy or loamy soil, so the appropriate amendment and amount will vary. A soil test can help determine what nutrients are already present before additional fertiliser or amendments are applied.

The Yuma study therefore offers a more specific lesson than simply suggesting that biosolids can replace fertiliser. It shows how adding organic matter and nutrients to sandy agricultural soil can reduce the need for conventional fertiliser over several years. The result does not establish that every soil will respond in the same way, but it demonstrates the potential value of improving soil rather than relying solely on repeated fertiliser applications.