Biochar compound fertilisers increase plant potassium uptake 2 years after application without additional organic fertiliser.

Biochar compound fertilisers (BCFs) are an emerging technology that combine biochar with nutrients, clays and minerals and can be formulated to address specific issues in soil-plant systems. However, knowledge of BCF performance over consecutive crops and without re-application is limited. This study aims to assess the residual effect of organic BCFs soil-plant nutrient cycling 2 years after application and without additional fertiliser inputs. We applied BCFs and biochar with organic fertiliser amendments and established a crop of ginger and a second crop of turmeric (Curcuma longa) without re-application or additional fertilisation. All treatment formulations included bamboo-biochar and organic fertiliser amendments; however, two novel BCFs were formulated to promote agronomic response in an intensive cropping system. We report here on the effect of treatments on soil and plant macronutrient and micronutrient cycling and turmeric growth, biomass and yield at harvest. Both BCFs (enriched (10 t ha-1) and organo-mineral biochar (8.6 t ha-1) increased foliar K (+155% and +120%) and decreased foliar Mg (-20% and -19%) concentration compared with all other treatments, suggesting antagonism between K and Mg. Plants were limited for K, P and B at harvest but not N, Ca or Mg. Foliar K was dependent on the biochar formulation rather than the rate of application. Biochar-clay aggregates increased K retention and cycling in the soil solution 2 years after application. Clay blended BCFs reduced K limitation in turmeric compared to biochar co-applied with organic amendments, suggesting these blends can be used to manage organic K nutrition. All formulations and rates of biochar increased leaf biomass and shoot-to-root ratio. Novel BCFs should be considered as an alternative to co-applying biochar with organic fertiliser amendments to decrease application rates and increase economic feasibility for farmers. Applying BCFs without re-application or supplementary fertiliser did not provide sufficient K or P reserves in the second year for consecutive cropping. Therefore, supplementary fertilisation is recommended to avoid nutrient deficiency and reduced yield for consecutive organic rhizome crops.

Effects of biochar, compost, and biochar-compost on soil total nitrogen and available phosphorus concentrations in a corn field in Papua New Guinea.

Replenishing soil nutrient particularly total nitrogen (TN) and available phosphorus (P) is important to sustain soil health for food production. Organic amendments such as compost and biochar are commonly applied to improve soil nutrient retention especially N and P. In farms, biochar is usually applied once followed by applying other organic amendment applied in their full rates. Both form and rate of organic amendments can affect soil nutrient concentrations particularly in short term. This study aimed to examine the effects of compost and mixture of compost with biochar (both at full rates) on soil nutrient concentrations in short term. A randomised complete block filed experiment with eight replicates was used for this study. The effects of biochar (5 t/ha) only, compost (at the rates of 10 t/ha, 25 t/ha and 35 t/ha) and biochar mixed with compost (5 t/ha and 10 t/ha, respectively) on soil nutrient concentrations compared with control were explored in a corn field. Compost treatment at the rate of 35 t/ha had significantly higher TN, available P, calcium (Ca) and iron (Fe) compared with other treatments and control. Soil potassium (K) levels remained unchanged among all treatments. Biochar only treatment had significantly higher available P and Ca concentrations compared with biochar mixed with compost treatment. Compost application at higher rate (35 t/ha) proved best practice to significantly increase TN and available P concentrations in short term. Significantly higher available P concentration in biochar only treatment compared with the biochar mixed with compost treatment could have been associated with stimulation of P immobilisation when biochar was mixed with compost. Our results indicated that the form and rate of organic amendments in short term cropping systems are important to be considered while applying to a volcanic soil to ensure N and P availability for plants are not compromised.