The 3R Principles for Applying Biochar to Improve Soil Health

Mingxin Guo

doi:10.3390/soilsystems4010009

Abstract

Amending soil with biochar is a promising approach to persistently improve soil health and promote crop growth. The efficacy of soil biochar amendment, however, is soil specific, biochar dependent, and influenced by the biochar application programs. To maximize the benefits of biochar application, this paper proposes the 3R principles for applying biochar to soils: right biochar source, right application rate, and right placement in soil. The quality of biochar as a soil amendment varies significantly with the feedstock and the production conditions. Biochar products capable of everlastingly sustaining soil health are those with high stable organic carbon (OC) content and high water- and nutrient-holding capacities that are manufactured from uncontaminated biomass materials. Acidic, coarse-textured, highly leached soils respond remarkably more to biochar amendment than other types of soils. Soil amendment with particular biochars at as low as 0.1 mass% (equivalent to 2 Mg ha−1) may enhance the seasonal crop productivity. To achieve the evident, long-term soil health improvement effects, wood- and crop residue-derived biochars should be applied to soil at one time or cumulatively 2–5 mass% and manure-derived biochars at 1–3 mass% soil. Optimal amendment rates of particular biochar soil systems should be prescreened to ensure the pH of newly treated soils is less than 7.5 and the electrical conductivity (EC) below 2.7 dS m−1 (in 1:1 soil/water slurry). To maximize the soil health benefits while minimizing the erosion risk, biochar amendment should be implemented through broadcasting granular biochar in moistened conditions or in compost mixtures to cropland under low-wind weather followed by thorough and uniform incorporation into the 0–15 cm soil layer. Biochars are generally low in plant macronutrients and cannot serve as a major nutrient source (especially N) to plants. Combined chemical fertilization is necessary to realize the synergic beneficial effects of biochar amendment.

Highlights

IntroductionAs a legacy of ancient inhabitants who added biomass-derived char to barren land, Terra Preta
As a legacy of ancient inhabitants who added biomass-derived char to barren land, Terra Preta in the central Amazon basin remains more productive relative to the surrounding highly leached, strongly acidic Oxisols and Ultisols [1,2,3,4]
Biochar products vary significantly in composition and quality characteristics with the feedstock type and the production conditions. In particular those manufactured from wood debris and crop residues do not contain significant contents of plant nutrients (e.g., N and P) and, should not be expected to serve as a chemical fertilizer alternative and function as a major nutrient source to crops

Summary

Introduction

As a legacy of ancient inhabitants who added biomass-derived char to barren land, Terra Preta After applied as a soil amendment in crop production and abandoned land reclamation, different biochars exhibited noticeably dissimilar capabilities for improving soil health properties and promoting plant growth [17,18,19]. When the application rate was too low or too high (varying with the biochar source and the soil type), the biochar effect on modifying the soil health became either insignificant or even unfavorable [9,25,26,27], respectively. Biochar amendment to attain best soil health and crop productivity improvement effects warrants the scientific selection on the biochar source, application rate, and biochar placement in soil. This paper is to propose and elucidate the 3R (right source, right rate, and right placement) principles for applying biochar to persistently improve soil health, with an additional aim to provide a better understanding of biochar and biochar application

The Mechanisms for Biochar to Improve Soil Health

The Right Biochar Source

The Right Application Rate

The Right Placement in Soil

Findings

Conclusions