Mineral soil conditioner requirement and ability to adjust soil acidity

Reagents and equipment

Para-nitrophenol, analytical grade; triethanolamine, analytical grade; calcium acetate (Ca(CO₂CH₃)₂), analytical grade, China National Pharmaceutical Group Co., Ltd. Potassium chromate (K₂CrO₄), analytical grade; hydrochloric acid (HCl), analytical grade, Beijing Chemical Works. Calcium chloride (CaCl₂·2H₂O), analytical grade, Shanghai Macklin Biochemical Co., Ltd. Sodium hydroxide (NaOH), analytical grade, Xilong Chemical Industry Incorporated Co., Ltd. pH meter, Thermo Fisher Scientific, USA. ICP-OES 730, Agilent. X ray diffractometer (XRD), Bruker D8, Germany.

Soil conditioner: Developed by Tianjin Cement Industry Design and Research Institute Co., Ltd. Specific preparation method: The raw materials were K-feldspar (produced in Inner Mongolia, with ({mathrm{KAlSi}}_{3}{mathrm{O}}_{8}), ({mathrm{NaAlSi}}_{3}{mathrm{O}}_{8}), and SiO₂ being the main components), CaCO₃, and CaMg(CO₃)₂, which were crushed and ball-milled in order to get the sizes that could pass through an 80 μm sieve, before being mixed in appropriate ratios. Next, these were sintered by using an alumina crucible placed in a box furnace at 1270 °C for 60 min, then naturally cooled inside, and finally ground to approximately 80 μm to obtain the MSCs.

Soils samples

14 typical acid soils were listed in Table 1, which are from three provinces in China: Hunan (No. 1–5), Sichuan (No. 6–10), and Guangdong (No. 11–14). Soils were sampled in 0–20 cm, with all vegetation residues in the surface layer removed, then were placed indoors, naturally air-dried, and passed through a 2 mm sieve. A portion of each sample was used for testing and analysis, and the remained was used for the culture test.

The evaluation of relationship between L_R from schematics of ΔpH and soil acidity by using SMP-DB method

Mclean’s improved SMP-DB method was used to calculate L_R. The calculation principle for the double buffer method is shown in Fig. 1, while the specific operating method and calculation principles of the experiment are as follows.

Based on the schematics of the double buffer method and in accordance with the isosceles triangle principle, the proportional relationship was established as shown in Eq. (1):

Equation (2) was obtained after conversion of Eq. (1):

According to Fig. 1, Eqs. (3) and (4) were obtained by making (Delta {pH}_{1}=7.5-{pH}_{1}) and (Delta {pH}_{2}=6.0-{pH}_{2}), respectively. ({left(frac{Delta x}{Delta y}right)}^{^circ }) is the milligram equivalent (meq) of H⁺ that must be depleted to increase the pH of the buffer solution by one unit. It is determined based on the standard curve of the buffer solution.

Equations (5) and (6) were obtained after conversion of Eqs. (3) and (4):

These were substituted into Eq. (2) and then integrated to obtain Eq. (7):

Equation (7), intended for theoretical calculations, was derived from the mathematical relationships among the various parameters. (d) is the equivalent acidity of the soil neutralized by the buffer solution when the soil–buffer solution was at the ideal pH (6.5). The L_R required to neutralize 5 g acid soil to pH 6.5 could then be extrapolated based on the measured data and the aforementioned equation.

Since the molecular weight of 1 mol of CaCO₃ is 100, there is a clear conversion relationship between lime and CaCO₃. For calculation convenience, L_R is commonly expressed as the mass of CaCO₃ needed to deplete the H⁺ present in 100 g of soil. In other words, ({mathrm{L}}_{R}= {text{meq CaCO}}_{3}/100 ,mathrm{g}=20mathrm{d}). After comparing the results obtained via the SMP-DB method and the Ca(OH)₂-titrated acidity method, Mclean found that Eq. (8), a revision of the earlier equation, had a better correlation with the actual situation.

Under normal circumstances, the weight of a 20 cm thick layer of ploughed soil would be 2250 t per hectare. For one hectare of soil, the L_R of CaCO₃ could be calculated using Eq. (9), with the unit being tons per hectare. This amount is expressed in meq CaCO₃/100 g soil; to obtain approximate rates in metric tons per hectare (0–20 cm), it can be multiplied by 1.125.

SMP-DB buffer performance

Preparation of buffer solution²⁷

800 mL distilled water was poured into a 1 L beaker, and then 1.8 g of para-nitrophenol, 2.5 mL of triethanolamine, 3.0 g of K₂CrO₄, 2.0 g of Ca(CO₂CH₃)₂, and 53.1 g of CaCl₂·2H₂O were added into the beaker; the mixture was then stirred and mixed. NaOH 40% (w/w) or HCl 50% (v/v) was used to adjust the pH to 7.5 before the buffer solution was transferred to a 1 L volumetric flask. The beaker was rinsed for 3 times with 50 mL of distilled water, and the rinses were transferred to the volumetric flask. The eventual constant volume was 1 L.

Test method for buffer standard curve titration

1 mL of 0.05 M HCl was added into a 50 mL beaker with 10 mL of buffer solution in it and the pH of the solution was measured after stirring for 1 min. This operation was repeated 8 times and the corresponding pH was recorded to plot a titration curve.

The standard curve of the prepared buffer solution is shown in Fig. 2. It has a pH of 5–8 and its standard curve is linear, which could be fitted using a proportional function. The fitting yielded the linear equation y = −7.19x + 8.05, r² = 0.998, which was highly significant. The buffering performance of the buffer solution was calculated based on the fitting equation, and the specific calculation process is stated below.

Two points (x₁, y₁), (x₂, y₂) were selected and substituted into the fitting equation to obtain Eqs. (10) and (11):

The two equations were subtracted to obtain Eq. (12):

Let (Delta y={y}_{1}-{y}_{2}, Delta x=-left({x}_{1}-{x}_{2}right)), then Eq. (13) would be established.

When the pH increased by one unit, ∆ y = 1 was substituted into Eq. (13) to obtain Eq. (14):

In other words, 0.139 meq of H⁺ must be depleted per unit increase in the pH of the buffer solution.

During the process of the buffer titration test, the pH of the buffer was adjusted from 7.5 to 6.0. It was shaken again to determine pH₂. Titration was performed using 0.05 M HCl (1 mL of 0.05 M HCl = 0.05 meq HCl). When the pH of the buffer solution was adjusted from 7.5 to 6.0, the reduction of 1.5 units required 0.139 × 1.5 = 0.2085 meq HCl, which converted to 4.2 mL of 0.05 M HCl.

Test procedure for buffer titration²⁸

Soil pH was measured using a glass electrode pH meter. 5.00 g soil sample was weighed and placed in a 50 mL beaker. Deionized water was added at a 1:1 water-to-soil ratio, and the beaker was shaken for 10 min at 250 r min⁻¹. After standing for 30 min, the pH (suspension) was measured. 10.00 mL of the SMP buffer solution was added and the mixture was shaken again for 10 min and then allowed to stand for 30 min. The suspension’s pH was measured to obtain pH₁.

After measuring the pH and pH₁, 4.2 mL of 0.05 M HCl was added to the suspension. This was the equivalent amount needed to adjust the buffer solution’s pH from 7.5 to 6.0 and was calculated according to the buffer solution’s standard curve. The mixture was shaken again for 10 min, and stand for 30 min before the pH of the soil suspension (pH₂) was measured. The steps were repeated for 3 times.

ICP-OES measurement

The MSC main elemental contents were determined using ICP-OES. The operating parameters are presented in Table 2.

0.200 g of each MSC was weighed and put into a 30 mL platinum crucible, and 1.500 g of molten agent was added (the mass ratio of sodium carbonate to sodium tetraborate was 2:1). After the molten agent and samples were mixed, the crucible was placed in a muffle furnace and its temperature was raised to 950 °C for 60 min to melt the contents. The crucible was taken out of the furnace after cooling and the sample inside was leached using 70 mL of HCl (3 + 7) to reach a constant volume of 100 mL. This solution was directly used to determine the Ca, Mg, Ba, Ti, and Mn content. Next, the solution was diluted 10 times to determine the high concentrations of K, Al, Si, Na, and Fe content.

XRD measurement

The MSC samples were ground to 0.045 mm (300 mesh) using an agate mortar and uniformly distributed inside sample frames. These were then pressed, flattened, and compacted using glass slides before being placed on the sample stage of the XRD sample chamber for analysis. The powder XRD patterns were obtained using a Bruker D8 Advance powder diffractometer working at 40 kV and 40 mA, using monochromatized Cu Kα radiation (λ = 0.154056 nm). The measurement was performed in the range angle 2θ = 15°–70°. Before the XRD test, all the samples were ground to 80 μm. The MDI Jade 5.0 software package (USA Materials Data Inc.) was used for qualitative analysis of the XRD spectra being tested.

Soil culture experiment

The soil samples were mixed with the MSCs and cultured for 30 days. Changes in the soil pH values were used to calculate the MSCs’ pH adjustment capacity and MSC_R. The test treatments involved the addition of 0, 0.2%, 0.4%, 0.8%, 1.2%, or 1.6% of MSCs (total six levels) to the 14 soil samples and have 3 replications. The specific operating steps were as follows: 50 g of each soil sample and MSCs were mixed in each plastic cup uniformly, and water was added to 60% of the field moisture capacity. The cups were then sealed with plastic film to prevent excessive evaporation. The soil pH was measured after 30 days (1:1 water-to-soil ratio, measured after 10 min of shaking)²⁹. The measurements were repeated twice. The measured pH and actual MSC_R were subjected to regression analysis, and the regression equation was used to determine the MSC_R required to neutralize the soil pH to 6.5 (the ideal pH for this study).

Source: Ecology - nature.com