Liming is one of the key agronomic practices to improve crop yields in acid soils because, among other things, it reduces aluminum toxicity and creates favorable conditions for crop growth. For an effective liming program, the methods to determine lime requirement should be as precise as possible. This paper reviews the existing lime requirement methods and discusses the potential of a new one suitable for routine use in the laboratory to test most agricultural soils. The most widely used lime requirement methods can be categorized into four groups: titration, incubation, buffer, and field methods. Other methods such as spectroscopy method or the use of empirical equations have also been adopted. Although some methods are highly reliable, they are not optimal for routine use because they are inconvenient during the laboratory procedures or cannot be validated for all conditions. Based on the linearity between soil pH and the added base in the pH range from 4.5–6.5 in most agricultural soils, a titration‐based method on 1:1 soil:0.01 M CaCl2 slurry of a single sample appears to be a promising candidate for routine use. In further studies, this generally applicable method should be evaluated to provide a better comparison to established methods for lime requirement determination.
In recent decades, mangroves have been seriously devastated by shrimp farming development in the Vietnamese Mekong Delta. As a result, integrated mangrove-shrimp farming has emerged as a potential solution to culture shrimps and protect mangroves. The present study aims to understand whether mangrove-to-pond cover ratios influence shrimp yields in an integrated mangrove-shrimp farming system. Five integrated mangrove-shrimp ponds in the Tam Giang Commune, Nam Can District, Ca Mau Province (Southern Vietnam) were chosen for this study. The study estimated that the mangrove-to-pond cover ratios ranged from 42.00 % to 72.50 %. The total shrimp yield per year (kg.ha-1 yr-1 ) was generally high, ranging from 76.62 to 249.09 (including 37.93 to 108.64 for the black tiger shrimp (Penaeus monodon Fabricius, 1798), and 38.69 to 140.45 for other shrimps, namely Penaeus indicus Milne Edwards, 1837, Penaeus merguiensis de Man, 1888, Metapenaeus ensis (De Haan, 1844), and Metapenaeus lysianassa (de Man, 1888)). Moreover, a strong positive correlation between the mangrove-to-pond cover ratios and the shrimp yields were observed (r > 0.71, P < 0.05). In conclusion the mangrove-to-pond cover ratios have a direct impact on the total shrimp yield. The mangrove-to-pond cover ratios should be 50 % to enhance shrimp yields in this system.
This study examined the pH buffering capacity (pHBC) of haplic Acrisols under intensive cassava production in an upland area of Southeastern Vietnam where accelerated soil acidification has occurred. Soil samples (0–20, 20–40 and 40–60 cm) were taken at 12 sites under cassava and three sites under secondary dipterocarp forest as reference. The pH buffer curves were linear in the pHH2O range from 3.97 to 5.24, corresponding to a pHCaCl2 range from 3.74 to 5.20. Soil pHBC were low (1.16 ± 0.13 cmol/kg/pH) and correlated significantly with pH, Aluminium (Al) and Al‐related components. The results suggested that exchange acidity contributes significantly to soil's buffering capacity in acidic soils with low organic carbon. It also pointed to the possibility of using indicators of acidity to estimate soil pHBC. The low pHBC indicated a high risk for further acidification and also pointed to the possibility of using lime to remediate soils. Either acid buffering capacity or lime buffering capacity not pHBC in general should be considered, respectively, in acidification and liming studies. Factors and processes involved in soil acidification and liming need to be addressed as a background for soil remediation in the study area.
This paper clarified the characteristics of soil acidification of haplic Acrisols on ancient alluvial deposit under intensive cassava cultivation in Chau Thanh district, Tay Ninh province, Southeastern Vietnam. Soils were sampled at 3 intervals (0-20, 20-40, 40-60 cm in depth) in 12 sites of intensive cassava cultivation and geochemical parameters related to soil acidity were analysed. The haplic Acrisols under intensive cassava cultivation showed quite high levels of active and exchange acidity (pH H2O 4.40±0.11, pH KCl 3.98±0.07). The hydrolytic acidity and Al saturation level were also high (respectively 4.52±0.37 meq/100g and 57.64±6.41%) while the exchange alkali and alkaline earth cations were very low (Ca 2+ 0.76±0.25 meq/100g, Mg 2+ 0.88±0.85 meq/100g, K + 0.16±0.06 meq/100g in the top layer). This exhibited a limit for mineral nutrients and risk of Al toxicity to cassava plants. If the area for intensive cassava cultivation is expanded and the high-yield cassava varieties are applied, the risk of soil acidification will be expected to be highly serious. It is needed to clarify the processes involved and to establish measures to reduce soil acidification and stabilize cassava production in the study area and Tay Ninh province.
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