Estimating lime requirements for tropical soils: Model comparison and development

Acid tropical soils may become more productive when treated with agricultural lime, but optimal lime rates have yet to be determined in many tropical regions. In these regions, lime rates can be estimated with lime requirement models based on widely available soil data. We reviewed seven of these models and introduced a new model (LiTAS). We evaluated the models' ability to predict the amount of lime needed to reach a target change in soil chemical properties with data from four soil incubation studies covering 31 soil types. Two foundational models, one targeting acidity saturation and the other targeting base saturation, were more accurate than the five models that were derived from them, while the LiTAS model was the most accurate. The models were used to estimate lime requirements for 303 African soil samples. We found large differences in the estimated lime rates depending on the target soil chemical property of the model. Therefore, an important first step in formulating liming recommendations is to clearly identify the soil property of interest and the target value that needs to be reached. While the LiTAS model can be useful for strategic research, more information on acidity-related problems other than aluminum toxicity is needed to comprehensively assess the benefits of liming.

Liming effects of poultry litter derived biochar on soil acidity amelioration and maize growth

Crop production in acid soils is facing enormous challenges due to low soil quality associated with an increase in the acidification rate and aluminum toxicity. Despite comprehensive prior work with biochar application on nutrient availability and crop productivity in acid soils, little information is available about the recommendation or standardization of biochar application rates that are more suitable for soil fertility improvement under different soil environments (physico-chemical properties) for maximizing the benefits of biochar applications and minimizing the potential environmental risk. Thus, the objective of this study was to investigate the effectiveness of poultry litter (PL) and poultry litter biochar (PLB) in ameliorating the fertility of acid soils through incubation and pot experiments. The soil was amended with different materials as follows; lime (1 g kg^-1), PL (5, 10 and 15 g kg^-1) and PLB (5, 10 and 15 g kg^-1) along with control (non-amended). A pot experiment was also conducted using similar treatments to observe the responses of maize crop to the different amendments. The results indicated an increase in the pH and a decrease in exchangeable acidity in lime, PL and PLB amended soils. Lower soil pH, base cations and soil available phosphorus (P), and higher exchangeable acidity were found in control than the amended soils. Compared to PL and lime, PLB achieved greater increase rate in soil pH and reduction rate in soil exchangeable acidity with increased soil exchangeable base cations. An increase in soil available calcium (Ca) was observed in the lime treatment, while in PL and PLB treatments, there was an increase in soil available Ca, magnesium (Mg), potassium (K) and P. Application of the amendments increased availability of nitrogen (N), P, K, Ca and Mg relative to the control for maize in the pot experiment. When PL and PLB amendments were compared, it was found that the PLB was the best choice for the amelioration of acid soils as well as nutrient uptake by maize plants. It is suggested that application of PLB at the rate of 15 g kg^-1 is suitable for maize growth in acid soils.

Soil acidification in Chinese tea plantations

Soil acidification is a major problem in intensive agricultural systems and is becoming increasingly serious. Most research has reported the soil acidification of cereal crops, forests, and grasslands. However, there is no information about soil acidification under tea cultivation on a national scale. Therefore, we conducted a nationwide survey of soil acidification in the major tea-planting areas of China and used two nationwide surveys in three Chinese counties to evaluate changes in soil acidity over the past 20-30 years. Finally, the acidity of soil from forests and traditional and organic tea plantations was compared to evaluate the effects of agricultural management on soil acidification in tea plantations. Our results show that: (1) the average soil pH was 4.68 nationally and ranged from 3.96 to 5.48 in different provinces. Overall, 46.0% of the soil samples had a pH <4.5, which is too acidic for tea growth and only 43.9% had a soil pH of 4.5-5.5, which is optimal for tea growth. (2) In the past 20-30 years, the greatest soil acidification was observed in tea plantations; the pH decreased by 0.47 to 1.43, which is much greater than the decrease seen in fruit and vegetable systems (0.40 to 1.08) and cereals (0.30 to 0.89). (3) Compared with forests, tea cultivation with chemical fertilizer application caused serious soil acidification, while no significant acidification was observed at organic tea plantations. In conclusion, serious soil acidification occurs nationally in China, and organic management is an adaptive choice for sustainable tea growth.

Peanut straw biochar increases the resistance of two Ultisols derived from different parent materials to acidification: A mechanism study

The mechanisms for increasing soil pH buffering capacity (pHBC) and soil resistance to acidification by peanut straw biochar were investigated by undertaking indoor incubation and simulated acidification experiments using two Ultisols derived from tertiary red sandstone and quaternary red earth. The biochar increased the pHBC and resistance of the two Ultisols to acidification. The addition of 3% biochar increased the pHBC of the two Ultisols by 76% and 25%, respectively. The increased resistance of the soils to acidification led to the inhibition to decrease in soil pH and the activation of soil Al during acidification. The protonation of carboxyl groups on the biochar surface was the main mechanism responsible for resisting acidification of the Ultisols when the pH was between 4.5 and 7.0. The higher soil pH (>6.0) after biochar application and the large number of carboxyl groups on the biochar surface were essential if biochar was to significantly increase the resistance of soils to acidification.