Agriculture and Science 2023/2/3

A method for calculating the amount of converter slag and slaked lime needed to control root-knot disease from the clay and carbon content of paddy soil.

Tohoku Agricultural Research Center
Field Crops and Horticulture Research Area New Vegetable Production Group
Chihito Yamaguchi

Introduction

　The demand for staple rice has been declining for a long time, and it is desirable to introduce vegetable crops, including cabbage for processing and commercial use, to paddy farming in the Tohoku region. On the other hand, the introduction of vegetable crops into converted paddy fields has been problematic due to the high incidence of soil-borne diseases. For example, Brassica root-knot blight, a soil-borne disease that is difficult to control, tends to occur under humid conditions1), so it is necessary to take special measures in converted paddy fields, which are prone to high humidity due to poor drainage.

　Since the activity of filamentous fungi such as root-knot fungi is suppressed when soil pH is high (alkaline), it is recommended that soil pH be raised to about 7.5 for the control of Brassica root-knot disease. To adjust soil pH, converter slag or slaked lime is used as an alkaline material. Converter slag is a byproduct of the steelmaking process and is an eco-friendly alkaline material made from industrial waste. Because alkaline soils tend to cause micronutrient deficiencies in plants, converter slag, an alkaline material containing micronutrients, is used.

　When correcting soil pH to control root-knot disease, it is necessary to know the amount of alkaline material needed to bring soil pH to 7.5. However, each soil has a different buffering capacity, and it is difficult to predict the degree of increase in pH in response to the application of alkaline materials. Therefore, until now, the amount of alkali material to be applied has been estimated by drawing a soil pH buffer curve using soil from each field2) . 2) On the other hand, the creation of a soil pH buffer curve requires a shaker, pH meter, and other equipment, and is time-consuming. In this report, we propose a formula3) to determine the amount of converter slag and slaked lime to be applied using parameters (clay content and total carbon content of the soil) that can be extracted from field measurements and Web information, assuming that they will be used in the field.

2. addition of alkaline materials to test soil and measurement of soil pH

　A total of 234 paddy soils (crop soils)4) with known grain size composition were sampled from various locations in eastern Japan and Hokkaido. The classification of the soil samples used in the analysis according to the first draft of the Comprehensive Soil Classification5) is shown in Table 1. The maximum, minimum, mean, median and standard deviation of the particle size composition (ISSS method), total carbon content and original pH of the test soils are shown in Table 2. The particle size composition of the soils was measured by the pipette method4) . The total carbon content of the soils was provided by the respective county personnel who sampled the soils. Soil samples were air-dried and passed through a 2-mm sieve before being used to generate pH buffer curves.

　For the preparation of the pH buffer curves, converter slag (granular, Kumiai converter by-product lime No. 2, Minex Corporation) or slaked lime (powdered, 70-proof slaked lime, Toa Sangyo Corporation) was used as alkaline material. The alkali content (sum of 0.5 mol L-1 hydrochloric acid soluble calcium and magnesium converted to calcium oxide) of the converter slag and slaked lime was 50% and 70%, respectively. To each 10 g of dry soil, 0.025, 0.05, 0.25, 0.5, and 1.0 g of converter slag or slaked lime was added in terms of alkali content, and mixed well. Then, 25 mL of distilled water (2.5 distilled water per 1 dry soil) was added, shaken for 1 hour, allowed to stand for 1 hour, and the pH (H2O) was measured by the glass electrode method6).

3. mathematical formulation of soil pH buffer curve

　The mathematical formulation of the soil pH buffer curve followed the method of Luo et al. (2015)6). Alkali materials were converted to alkali content
The pH when x (g/10g dry soil) is added at (1) is expressed by the equation (1).

pH = pHmax-pHintexp (kx) ..................(1)

　pHmax is the maximum value for each material, and pHmax-pHint is the soil pH without the material (original pH in Table 2). k is a constant related to the amount of alkaline material added. The maximum value of pHmax was defined as the pH when 1.0 g of converter slag or slaked lime in terms of alkali content was added to 25 mL of distilled water. The pHmax of converter slag = 12.73 and that of slaked lime = 12.89. Equation (1) was fitted to the data by finding the value of k using the method of least squares, which minimizes the sum of the squares of the difference between the measured value and the value obtained from the equation. Equation (1) shows that when the amount of alkali material applied is 0 g, the test soil shows the original pH. As the alkali input approaches infinity, the pH buffering capacity of the soil becomes negligible and approaches the pH of the soil when the alkali material is added to water (pHmax).

　Figure 1 shows examples of pH buffer curves for four soils, including those with the largest and smallest k values among the samples used in this study. k value is an index of the pH buffering capacity of a soil, and the slope of the pH buffer curve increases with the absolute value of k. The slope of the pH buffer curve varies with the soil and each alkaline material. The slope of the pH buffer curve was different for different soils and different alkaline materials, but it could be calculated by changing the k value and pHmax, respectively, using Equation (1).

4. index k of soil pH buffering capacity expressed in terms of soil clay content and carbon content

　It has been reported that the pH buffering capacity of soil is strongly correlated with the cation exchange capacity (CEC)7, 8, 9) .) Therefore, we performed multiple regression analysis using the k-values obtained from equation (1) as objective variables and clay content and total carbon content in soil as explanatory variables when converter slag and slaked lime were added, respectively (Table 3). The coefficients of determination of the single regression equation obtained from the k values estimated by the equation using both clay content and total carbon content as explanatory variables and the measured k values were 0.333 for converter slag (adjusted coefficient of determination: 0.327) and 0.429 for slaked lime (adjusted coefficient of determination: 0.424) (Figure 2). The clay content and total carbon content in the soil explained about 30 to 40% of the variance in k (Figure 2).

Comparison of measured pH values after addition of converter slag and slaked lime with values estimated from clay content and total carbon content

　For each soil sample, k was calculated using the equation in Table 3, and the obtained k value was substituted into Equation (1) to estimate pH. The measured values on the horizontal axis and the estimated values using Equation (1) on the vertical axis for the cases where 0.025, 0.05, 0.25, 0.5, and 1.0 g of converter slag and slaked lime were added in terms of alkali content were plotted, and the estimated soil pH values agreed well with the measured values up to the slightly alkaline range (Figure 3).

Based on the above, the following estimation equation is recommended for estimating the amount of alkali material applied to correct soil pH in the range up to weak alkalinity.

Converter slag: pH = 12.73-pHintexp {(0.0140 x clay (%) + 0.1437 x total carbon content (%) - 2.686) x (g/10g dry soil)}

Slaked lime: pH = 12.89-pHintexp {(0.1833 x clay (%) + 1.0729 x total carbon content (%) - 21.668) x (g/10g dry soil)}

6. a simple method for calculating the amount of alkali material to be applied

　The equation in Figure 4, which is a conversion of the estimation equation, is a formula for calculating the amount of converter slag and slaked lime required to correct the soil to the target pH. The equation uses the initial pH, clay content, and total carbon content as soil information. The clay content and total carbon content of the soil can be determined by touch and color, respectively, and the initial pH of the soil (soil pH without alkali materials) can be measured using a portable pH meter to simply calculate the approximate amount of application. In the future, it may be possible to determine the clay content and total carbon content of the soil of a target site from a digital soil map12) such as e-Soil Map II, and this estimation method will be effective in determining the amount of alkali material applied using the data from the soil map.

7. Precautions in applying this method

　The recommended soil pH for the purpose of soil disease control varies among municipalities. The grain size of converter slag varies by product and lot, and the difference in solubility of the material caused by the difference in grain size greatly affects the pH buffer curve. In addition, soil pH varies depending on the moisture content of the soil after application, etc. Therefore, the calculated amount of alkali material to be applied is a theoretical value and can only be used as a rough guide.

Acknowledgements

　We would like to thank the agricultural research institutes in each prefecture that provided soil samples and analytical data for this study, as well as the Tohoku Agricultural Research Center of the National Agricultural Research Organization (NARO) for their cooperation in conducting this study.

References

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　　Soil preparation for soil remediation. p.44-56, Noubunkyo, Tokyo. pp.44-56, Noubunkyo, Tokyo.
2）Keiichi Murakami and Itsuo Goto 2008. Simplified application of converter slag for control of Brassica root-knot disease in vegetables.
　　Determination method. Bulletin of the Kansai Disease and Insect Research Society, 50, p. 97-98.
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　　A Method for Estimation Based on the Abundance and the Carbon Content. Dohi Journal, 92, p. 174-181.
(4) Takahashi, T., Nakano, K., Nira, R., Kumagai,E., Nishida, M. and Namikawa, M. 2020.
　　Conversion of soil particle size distribution and texture classification from ISSS system
　　to FAO/USDA system in Japanese paddy soils. Soil Sci. Plant Nutr. 66, 407-414.
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(6) Soil Environmental Analysis Methods Editorial Committee, 1997. p.195-197, Hakuyusha, Tokyo.
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No Soil - Part 19
　　Until the absorbed nitrogen becomes protein.
　　-Plants are self-sufficient in all the amino acids they need.

Hokkaido Branch Office, JCM Agri Co.
　Teruo Matsunaka Technical Advisor

　Plants select and absorb only the nutrients they need from the water in the soil (soil solution) by a well-developed mechanism. The absorbed nutrient ions support the nutrition of the plant by either becoming components of substances that help to nourish the plant or by participating in various reactions in the plant body.
　This month, as an example, we will look at how plants use nitrogenous nutrient ions such as ammonium and nitrate ions and carbohydrates produced by photosynthesis in leaves to make proteins.

1. how amino acids are synthesized from ammonium ions

　Ammonium ions (NH4+) nitrogen (N) and hydrogen (H) are components of proteins and important plant nutrients. However, when large amounts of ammonium ions are introduced into the above-ground parts of plants, they have a detrimental effect on the plants, for example, by interfering with photosynthesis. For this reason, many ammonium ions are immediately synthesized into amino acids and detoxified once they are taken up into the root cells. Figure 1 shows this pathway.

　Ammonium ions taken up by plant root cells are first combined with an amino acid called glutamic acid to form an amino acid called glutamine. This reaction is the function of glutamine synthetase. This glutamine is converted into two glutamic acids by reacting with 2-oxoglutaric acid, an organic acid that is an intermediate product of the breakdown of carbohydrates produced by photosynthesis through plant respiration. This reaction is the function of glutamate synthase.

　Of the two glutamic acids, one is again combined with ammonium ions to make glutamine. The other reacts with various organic acids, which are intermediate products of the breakdown of photosynthetic products by plant respiration, to produce the necessary amino acids. In this case, the reactions are also carried out by enzymes that assist in each reaction. In this way, all necessary amino acids are self-sufficient, and proteins are synthesized from them.

2. how amino acids are synthesized from nitrate ions

　Ammonium ions are used to synthesize amino acids through the pathway shown in Figure 1. However, ammonium ions are converted to nitrate ions by the action of microorganisms in the soil under conditions where the ammonium ions are easily exposed to oxygen in the air (oxidative conditions), as in a field. This is the nitrate conversion effect. Therefore, nitrogen as a nutrient absorbed by field crops is mainly in the form of nitrate ions. In this case, how are amino acids synthesized and used as materials for proteins?
What is it? Again, plants have a clever mechanism. In order for nitrate ions absorbed by plants to become raw materials for amino acids, nitrate ions must be converted to ammonium ions and incorporated into the mechanism of amino acid synthesis from ammonium ions (called the GS-GOGAT system) as shown in Figure 1. The GS-GOGAT system performs this function.
They are nitrate reductase and nitrite reductase (Figure 2). Both are enzymatic reactions that work together to reduce nitrate ions to ammonium
Monium ions are converted to monium ions.

　Many nitrate ions are absorbed by the roots and then translocated in their intact form through the ducts to the leaves. This is because leaves are well exposed to sunlight and can easily obtain the light energy necessary for this enzymatic reaction. The nitrate ions transferred to the leaves are converted to nitrite ions by the action of nitrate reductase. Nitrite ions are further converted to ammonium ions by nitrite reductase. The nitrite ion then enters the GS-GOGAT system, an amino acid synthesis pathway (Fig. 2), and the necessary amino acid synthesis is carried out.

3. mechanism to prevent excessive accumulation of ammonium ions

　The issue here is the relationship between the rate of the enzymatic reaction that converts nitrate to ammonium ions and the rate at which the resulting ammonium ions are incorporated into the GSGOGAT system for amino acid synthesis. If the rate of the former exceeds that of the latter, the ammonium ions produced by this enzymatic reaction will accumulate in the leaves. However, this must be avoided. The accumulation of ammonium ions is detrimental to the plant. To avoid this, nitrate reductase has a function that prevents the accumulation of ammonium ions.

　Nitrate reductase activates an enzymatic reaction when nitrate is absorbed, producing nitrite ions.
However, when nitrite is converted to ammonium ion by the action of nitrite reductase, nitrate reductase inhibits its own enzymatic activity until it is incorporated into amino acid synthesis and detoxified in the cell. In other words, nitrate reductase self-regulates its enzymatic activity so that it does not unnecessarily produce ammonium ions. This maintains a balance between the rate at which nitrate ions are converted to ammonium ions and the rate at which ammonium ions produced by the enzymatic reaction are converted to amino acids. Enzymes like nitrate reductase, which have the ability to regulate their reaction activity to suit the situation, are called adaptive enzymes or inducible enzymes.

　The fact that nitrate reductase is an adaptive enzyme is especially important for many plants that mainly absorb nitrate as a nitrogen nutrient source, in order to avoid the danger of excessive accumulation of ammonium ions.

4. the only external raw material required for amino acid synthesis is ammonium ions

　From what we have seen so far about the mechanism of amino acid synthesis, we can notice something. The only raw material that plants need to acquire from outside in order to synthesize amino acids is ammonium ions. The other raw material, photosynthetic products (carbohydrates), is synthesized by the plant itself. This is made possible by two enzymes that synthesize glutamine and glutamic acid. It is thanks to the action of these two enzymes that plants are able to synthesize complex amino acids from simple substances such as ammonium ions.

5. plants do not contain essential amino acids

　We are not self-sufficient in all the amino acids needed for protein synthesis. Therefore, we need to acquire the necessary amino acids from the food we eat. These are the essential amino acids. Plants do not have essential amino acids because they are self-sufficient in all the amino acids necessary for protein synthesis. We should avoid the misconception that plants have substances like the essential amino acids of animals.