Combining Biochar with Compost or Manure Improves 9 of 16 Soil Properties Compared to Biochar Alone

Biochar has attracted considerable attention as a soil amendment. The material - produced by heating biomass at high temperatures with limited oxygen, a process called pyrolysis - is porous, chemically stable, and capable of improving soil water retention, nutrient cycling, and long-term carbon storage. It does not decompose quickly, which means its effects on soil can persist for decades. But field results have been inconsistent: biochar performs well in some soils and conditions and modestly or poorly in others.

A review published in the journal Biochar, synthesizing 28 field studies from multiple countries and climates, addresses a practical question: does adding biochar alongside conventional soil amendments - compost, manure, or mineral fertilizers - produce better outcomes than applying biochar on its own? The analysis found that in a majority of measured soil properties, the combined approach outperformed biochar alone.

What the combined approach changed

The review examined 16 soil health properties across the 28 studies, including physical, chemical, and biological parameters. Co-application - biochar plus another amendment - improved nine of those sixteen compared to biochar applied alone.

The numerical results were substantial in several categories. Soil phosphorus levels increased by as much as 76 percent in some comparisons when biochar was paired with organic amendments. Cation exchange capacity - the soil's ability to hold positively charged nutrients that would otherwise leach away with water - improved by more than 50 percent on average. Soil water content, aggregate stability (how well soil particles clump into stable structures that resist erosion), and hydraulic conductivity (how water moves through soil) also showed consistent improvements.

Biological responses were equally notable. Multiple studies reported increases in microbial biomass and enzyme activity, both of which signal active nutrient cycling. The proposed mechanism combines two complementary effects: biochar's porous structure provides physically protected microhabitats where bacteria and fungi can shelter from predation and desiccation, while the organic amendments supply the carbon and nitrogen that those microbes need as food.

Organic co-amendments outperformed inorganic ones

Not all amendment combinations performed equally. Biochar paired with organic materials - compost or manure - produced more consistent improvements than biochar combined with inorganic fertilizers alone. This likely reflects the microbial ecology explanation: organic amendments feed the soil biology that takes up residence in biochar's pores, whereas synthetic fertilizers do not contribute meaningfully to microbial habitat or long-term soil structure.

Application rate, soil type, and climate modulated results across studies, as expected in field research. Soils with initially low fertility showed larger gains than soils that were already nutrient-rich. Degraded or heavily weathered agricultural soils in tropical and subtropical regions appeared to benefit most from co-application, which makes practical sense given that these are also the contexts where soil health degradation is most severe and food security most precarious.

What the data cannot yet tell us

The most significant limitation is time. Many of the 28 studies ran for only a few years - too short to assess whether the observed benefits persist, whether they diminish as organic amendments decompose, or whether the combination creates any longer-term problems such as nutrient imbalances. Biochar itself can remain in soil for centuries, but its interactions with organic matter, microbes, and plant roots evolve over time in ways that short studies cannot capture.

Long-term field experiments are expensive, slow, and difficult to sustain institutionally - which is why they are rare. The review authors explicitly identify this as the critical gap. Understanding decade-scale dynamics will be necessary before co-application strategies can be confidently recommended as permanent components of soil management systems rather than as treatments that may need periodic renewal.

The practical pathway for farmers is clearer than the theoretical picture. Compost and manure are already common agricultural inputs; biochar is increasingly available as agricultural and forestry byproducts are processed through pyrolysis. Combining them does not require fundamentally different practices, only attention to application rates and timing. For operations already using organic amendments, adding biochar appears to amplify the benefits rather than create new costs.