pH and base saturation percentage (BSP) are two important soil chemical properties which have implications both for soil fertility [1] and taxonomy [2] [3] . For examples in Chinese Soil Taxonomy [2] , pH is used in the definitions of Sulfuric horizon (pH < 4.0), Sulfidic materials (pH may < 4.0), “Acidic” soil subgroup (pH < 5.5); BS is used in the definitions of Mollic epipedon (BSP ≥ 50%), Umbric epipedon (BSP < 50%), Agric horizon (BS is higher or much higher than that of underlying layers), L. C. of purplish sandstones and shales (Base saturated), L. C. of carbonate rocks (BSP ≥ 50%), and “Rutrophic” soil subgroups of Ferralosols and Ferrosol (BSP ≥ 35%).

The determination of BSP needs to measure total amount of exchangeable base (Ca²⁺, Mg²⁺, K⁺, and Na⁺) and cation exchange capacity (CEC) of soil samples in the laboratory [4] [5] [6] [7] . It is more laborious and time-consumptive for the measurement of BSP compared with that of pH which is easily and simply determined by potentiometer [4] [5] [6] [7] . Some studies have proved that there is significant correlation between BSP and pH [8] [9] , so it is possible to infer BSP directly from pH. Meanwhile and more importantly, nowadays pH can be rapidly measured in situ by portable pH meter, so it is helpful for soil survey staffs to preliminarily identify soil types in field more accurately, thus can promote the quality of the description on the formation factors and morphological characteristics of soil profiles. However, BSP is a dynamic soil property, which is affected by climatic, geochemical, and environmental conditions [10] . In soil science it is common that a model established in one scenario often does not work well in other ones, new model has to be set up for higher predication accuracy.

Red soils, as udic Ferrosols, Argosols and Cambosols in Chinese Soil Taxonomy [3] , is widely distributed in the hilly area of tropical and subtropical south China, mainly derived from weathered materials of quaternary red clay, tertiary red sandstone, granite, basalt, phyllite and limestone, etc. and under different land use types [11] [12] . Subjected to intensive weathering and leaching, red soil is characterized by high acidity, clayey texture and poor structure, etc. [13] [14] . There are literatures on the relation between BSP and pH for red soil series in county scale [8] [9] , but so far little information is available of red soils series in province scale. Therefore, the objective of the current study is to setup the optimal correlation between BSP and pH for red soil series in south China.

Since significant correlation is existed between pH and BSP, here we only compare pH values in different circumstances. Low pH values of red soil series are normal because red soils in south China are generally acid due to the high precipitation and intense weathered process [11] [12] , the leaching losses of K, Na, Ca and Mg in south China could be higher than 80% [17] [18] . Higher pH value (5.13 ± 0.63) of red soil series in Fujian than that of Guangdong (4.88 ± 0.55) could be attributed to the higher annual temperature and precipitation in Fujian (17˚C - 21˚C, 1400 - 2000 mm) than Guangdong (19˚C - 24˚C, 1300 - 2500 mm).

For soil types, pH of Ferralosols (4.92 ± 0.51) and Ferrosols (4.83 ± 0.45) are significantly lower than those of Argosols (5.20 ± 0.69) and Cambosols (5.04 ± 0.72), which may be resulted from the further development of Ferralosols and Ferrosols due to the higher temperature and precipitation than Argosols and Cambosols [17] [18] , it could be supported further by the definitions of these four kinds of soil types in Chinese Soil Taxonomy [3] and Chinese Genetic Classification [11] [12] . For land use types, it is common in south China that farmers often used lime or dolomite powders to weaken soil acidity of farmlands [19] , thus, it is reasonable for pH of dry land (5.30 ± 0.63) significantly higher than that of forest (4.90 ± 0.63). Meanwhile, soil samples of forests most are from Pinus massoniana and Pinus masoniana, it is well known that Pinus massoniana

could exacerbate soil acidly [20] , and more chemical fertilizers are used for rapid growth of Eucalyptus robusta, which could also intensify soil acidity [21] . Similarly, pH of basalt (5.36 ± 0.84) is highest could be attributed to its highest ratio of dryland samples (56.5%), the lowest of pH of sandstone (4.93 ± 058) attributed to its highest ratio of forest land samples (81.31%).

Significant correlation between BSP and pH were found in many studies, usually in linear and quadratic models, but the parameters of models are different [8] [9] [22] [23] [24] . Our studies also showed that quadratic model is optimal in most scenarios in describing the correlation between BSP and pH for red soil series of south China.

According to the definition of BSP, it is decided by exchangeable base caions (K⁺, Na⁺, Ca²⁺ and Mg²⁺) and the exchangeable H⁺ and Al³⁺ simultaneously, because these cations are influenced by temperature, precipitation, parent materials, land use type etc. [1] [11] [12] , thus an optimal model should be established according to a specific scenario [25] , which is further proved by our studies based on different regions, parent material and land use type or soil types. Because our study is mainly to give out the optimal models between BSP and pH, we will discuss further about the correlation between BSP with the exchangeable cations in red soil series in our next work.

As mentioned above in the introduction, 35% and 50% are two key thresholds of BSP in Chinese Soil Taxonomy [3] , based on the model established in our study of total red soil series (y = 6.84x² − 45.86x + 81.52, R² = 0.494^**, n = 394, Figure 1, right), the corresponding pH were inferred as 5.33 and 5.93for BSP = 35% and 50%, respectively. But it is should be noted that such inferred pH values are only feasible to identity soil taxonomy type preliminarily in the field, the final soil taxonomy type must be decided by the measured values of BSP in the laboratory.

By using the database of soil pH and BSP of red soil series of Fujian and Guangdong provinces, this study discloses that there is significant correlation between BSP and pH, and the optimal models are most in quadratic form, but the optimal model and the accuracy are different with different regions, parent materials, soil types and land use types. For all red soil series, pH < 5.33 or <5.93 could be used roughly to judge BSP < 35% or <50%, respectively.

This study was supported by projects of the National Natural Science Foundation of China (Grant No. 41877008) and the National S & T Basic Special Foundation Project (No. 2014FY110200). We would like to express thanks to the contribution from all the participants in the data preparation and treatment.

The authors declare no conflicts of interest regarding the publication of this paper.

Wang, A.Q., Ju, B. and Li, D.C. (2019) Predicting Base Saturation Percentage by pH. Agricultural Sciences, 10, 508-517. https://doi.org/10.4236/as.2019.104040