Soil Testing and Testing Technology 1

(a) soil survey Soil testing is mainly to determine the effectiveness and content of certain soil nutrients, and various soil factors that affect the effectiveness of these nutrient elements such as soil texture, cation exchange capacity, bulk density, redox potential, etc. Not determined in soil tests. The analysis of these soil factors is a measure of the basic properties of the soil and is to be carried out through soil surveys.

The purpose of the soil survey is to understand the local soil types, land use and quality assessment and various obstacle factors, and propose soil improvement measures. Prepare soil maps, land use and evaluation maps, agrochemical maps, and soil improvement maps. When serving farmer households, a land archive with farmer households should be established, and farmland area, utilization status, yield level, cultivation measures, and fertilization level should be recorded year by year as the basic data for recommended soil fertility tests.

The content of the soil survey report should describe the soil conditions in the survey area in addition to the description of environmental conditions, such as topography, slope, drainage, soil texture, soil thickness, surface and subsurface gravel, general fertility, and soil acidity. With salinization status and so on. The cation exchange capacity, salt saturation, and organic matter content associated with soil weathering and soil parent materials have the most important soil characteristics that reflect the natural fertility of the soil.

Considering the great influence of soil moisture on soil fertility and fertilizer effects, in soil surveys, some factors that focus on soil-water interactions, such as field capacity, permanent wilting point, and infiltration rate, are very important. necessary.

(B) field soil sampling technology Soil sampling is the key to successful soil testing, but often the most easily overlooked. Due to the heterogeneity of the soil, the sampling error in the field is often much larger than the laboratory analysis. At present, there is no set of accepted methods for field soil sampling in China. The sampling method is very confusing, which seriously affects the precision of soil testing and fertilization. Correct field soil sampling is a primary link in the recommended fertilization system.

1. Distribution characteristics of soil nutrients in field Soil and Fertilizer Research Institute of Soil and Fertilizer, Shaanxi Academy of Agricultural Sciences Lu Dianqing et al. (1990) conducted a field-sampling field study on 100 hectares of farmland in Xiguan Village, Fufeng County with a square grid area of ​​0.67 hectares. Total nitrogen, organic matter, dissolved nitrogen and available phosphorus were analyzed. The results of the study showed that due to the increase in fertilizing amount and fertilization techniques, the “concentric circle distribution rule” where the soil nutrient content was proposed in the 1970s did not exist with the village as the center, and soils at different distances from the village were 0-20, 20-40. There was no significant difference in nutrient content between centimeters, and the existence of a relatively balanced distribution provided a scientific basis for the possibility of large-scale soil fertigation. However, due to the different ways and forms of various nutrients in the soil, their mobility in the soil is also different, which results in local uneven distribution of nutrients in the field. Figure 10-5 shows the distribution of soil available P in a farmland in the village, each representing 0.67 hectares.

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5.7

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0.7

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14.3

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7.6

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4.3

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4.7

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8.5

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15.2

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4.1

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14.7

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4.9

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13.2

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10.9

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11.0

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20.8

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13.2

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22.8

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10.5

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8.7

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12.5

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28.8

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24.2

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12.1

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Distribution of Soil Available Phosphorus (P 2 O 5 ) in Tuxiguan Village

( 0~20 cm, mg/kg)

As can be seen from the figure, the variation of available phosphorus content in the same farmland soil can range from 0.7 to 28.8 mg/kg, which varies widely. The variability of soil organic matter, total nitrogen, alkaline dissolved nitrogen, and available phosphorus can be seen in Table 10-4. The data in Table 10-4 shows that there are varying degrees of variability in soil nutrient content. Among them, the variability of available phosphorus is the highest, and the coefficient of variation is as high as 60% or more. The content of organic matter is the lowest in the opposite sex, ranging from 7.9% to 11%. Other nutrients are between the two. With the increase of sampling area, the coefficient of variation of each nutrient content increased significantly.

Table 10-4 Variability of Soil Nutrient Distribution in Farmland

Soil layer (cm)

Nutrient type

13.3 hectares n = 20

33.3 hectares n = 50

X

CV (%)

X

CV (%)

0~20

Organic matter (%)

Total nitrogen (%)

Alkaline nitrogen (mg/kg)

Available Phosphorus (mg/kg)

1.1709

0.0799

58.172

9.120

7.90

9.06

21.80

50.7

1.1460

0.0810

62.48

9.193

8.20

9.90

21.90

53.40

20~40

Organic matter (%)

Total nitrogen (%)

Alkaline nitrogen (mg/kg)

Available Phosphorus (mg/kg)

0.8810

0.0667

50.15

2.76

4.20

10.50

10.7

33.8

0.9067

0.0692

49.11

3.03

7.5

13.0

11.2

41.2

Soil layer (cm)

Nutrient type

66.6 hectares n = 90

100 hectares n = 148

X

CV (%)

X

CV (%)

0~20

Organic matter (%)

Total nitrogen (%)

Alkaline nitrogen (mg/kg)

Available Phosphorus (mg/kg)

1.1440

0.0830

62.944

9.935

11.1

10.9

22.1

63.8

1.1370

0.0840

61.97

10.663

10.2

11.9

22.9

64.9

20~40

Organic matter (%)

Total nitrogen (%)

Alkaline nitrogen (mg/kg)

Available Phosphorus (mg/kg)

0.8936

0.0703

47.65

3.185

9.4

12.8

13.9

69.25

0.8851

0.0681

45.86

2.943

9.4

11.3

14.4

74.7

2. The determination of the number of sampling points for determining the number of soil samples in different areas is based on the degree of soil nutrient variability. Therefore, according to the principle of error analysis, they analyze and analyze the results of different areas of sampling analysis. The standard deviations (S) of soil total N, organic matter, alkali-hydrolyzed N, and available P all decreased with the increase of sampling points. The change rule was subject to y = ae b/x. Figure 10-6 shows the nutrient content of 33.3 hectares of plots. The relationship between the standard deviation and the number of sampling points, according to the error distribution law, can be used to estimate the number of sampling points required for different nutrients in different areas.

Based on the above studies, the sampling points of the soil in Guanzhong Plain Irrigation District can be seen in Table 10-5.

Table 10-5 Sampling Points Estimated for Standard Deviation of Different Area

Area (ha)

0.13~0.26

13.3

33.3

66.6

100

Sample points

8~12

15~20

20~25

25~30

30~40

* The sampling points for each nutrient are the same.

In the North China Plain, if 13.3 hectares is used as a soil sampling unit for soil fertigation, 15 to 20 points of soil samples should be taken from this unit, and 1 kilogram of soil should be taken for analysis after mixing. If the area of ​​33.3 hectares is used as a sampling unit, the sampling points should be increased to 25 to 30.

Feng Gongyan (1983) studied soil sampling techniques for paddy-type paddy soils in the lower reaches of the Yangtze River. In the 2124 m2 field, 88 samples were taken in a rectangular grid to analyze the total nutrients and available nutrients. The results are shown in the table. 10-6.

Table 10-6 Variation of nutrient levels distribution in paddy soils (n = 88)

project

Total nitrogen

(%)

Phosphorus

(%)

Total potassium

(%)

Organic matter

(%)

pH

Hydrolyzed nitrogen

(mg/kg)

Available phosphorus

(mg/kg)

Potassium

(mg/kg)

X

S

CV(%)

0.15

0.01

6.67

0.0710

0.0045

6.3400

2.070

0.18

7.09

2.54

0.18

7.09

7.790

0.061

0.790

105.4

12.2

11.6

17.1

6.9

40.5

127.0

13.8

10.9

Note: X - average, S - standard deviation, CV (%) - coefficient of variation.

The data in Table 10-6 indicate that the coefficients of variation of available nutrient values ​​are all above 10% in crops with evenly growing plots. Among them, the coefficient of variation of available P is large (40.5%), which is related to phosphorus in the soil. The mobility is weak. From this it can be seen that incorrect sampling will lead to errors in soil testing. A non-representative soil sample may not be sampled.

Soil sampling techniques include sampling area, sampling depth, samples of mixed soil samples, sampling time, and so on. All localities should determine the sampling area according to local conditions. Generally speaking, the area of ​​the plot is large, the soil type is relatively uniform, the fertility level is relatively uniform, and where the field management is relatively uniform, the sampling area can be larger, otherwise it should be smaller. Wherever there are large differences in crop conditions, different slopes, drainage conditions, different types of cultivation, and soil type changes, separate sampling should be performed.

The sampling depth varies depending on the density of crop roots. Field crops generally have a sampling depth of 0-20 centimeters, sometimes 30 centimeters, rice fields generally 0-15 centimeters, and grassland sampling depths of 0-15 centimeters.

The number of mixed soil samples was related to the degree of soil fertility within the sampling unit. In the above study by Feng Gongyan, the authors concluded that the soil hydrolyzes nitrogen, available phosphorus, and available potassium in the soils of the tidal mud soils. For testing mixed soil samples, generally 10 samples are used to determine the number of mixed soil sample points for soil available phosphorus.

The sampling time is basically determined by the laboratory's work schedule, and it is necessary to ensure that there is enough time for the preparation of fertilizers and fertilization after sampling, analysis of tests, and presentation of recommended fertilization schemes. In two- or three-cropping areas of the year, soil sampling is usually carried out during the growth of the upper crop. At this time, attention should be paid to avoiding topdressing, and the sampling period should be at least 20 days after topdressing.

Because the soil nutrient content in farmland has a certain level of stability, it is not necessary and impossible to sample and analyze samples every year. In general, soil nutrient tests were conducted once every 3 years, and the trace element nutrient content was measured once every 5 years. However, the soil test data of the upper crops cannot be used for the next crop because they are analyzed in the same year. However, due to different sampling seasons, the soil nutrients in the upper crops are different, and the soil nutrient values ​​will be very large. difference.

The amount of soil nutrients absorbed during the entire season of crop growth is taken as the number of “actual” effective nutrients in the soil, which can be measured by the amount of nutrients absorbed by the soil that is not applied to the nutrient, and the amount of nutrients absorbed by the crop in one season. It is the most basic reference standard for extractant screening studies. Also available as reference standards are relative absorption, relative yield, etc., which are the ratio of nutrient uptake or yield of crops that are not applied to the nutrient plot to the nutrient uptake or yield of crops in the whole fertility zone. The isotope A value method is used to determine the effective nutrient A value of the soil, because there is generally a good correlation with the crop uptake, so the A value is sometimes used as a reference standard for a certain soil nutrient.

1. Determination of Available Nitrogen in Soil Because the transformation of nitrogen in soil is a biological process, the simulation of soil nitrogen release by various chemical testing methods is not very successful. So far, conventional soil testing techniques in countries all over the world do not include soil nitrogen tests. The amount of nitrogen fertilizer application is basically determined by experience. However, in the short-term diagnosis of crop nitrogen requirements, the test of soil available nitrogen is still very useful.

(1) Nitrogen in soil total nitrogen is basically organic nitrogen. Inorganic nitrogen, such as nitrate nitrogen, accounts for only a small part. Mineralization of soil organic nitrogen is affected by soil texture, moisture status, humidity, and pH. Therefore, soil total nitrogen content can only generally reflect the soil nitrogen fertility. In recent years, studies have shown that in dryland soils, when total nitrogen is below 0.15%, there is a good correlation between total nitrogen and soil nitrogen supply (Zhou Mingxuan, 1987). Therefore, the total nitrogen and soil moisture status are still analyzed in routine analysis. The consistency is the condition using total nitrogen as an indicator.

(2) Soil hydrolyzable nitrogen Soil hydrolyzable nitrogen can be divided into two methods: acid hydrolysis nitrogen and alkaline hydrolysis nitrogen. Research work has shown that the correlation with nitrogen uptake is generally higher than soil total nitrogen. For the southern paddy soil, alkaline hydrolysis of nitrogen is a good indicator; for the northern soil, due to the presence of nitrate nitrogen, the alkaline solution disperses when adding reducing agent, so it is called reducing alkali hydrolysis nitrogen, the reducing agent type is iron sulfate + Silver sulfate (Li Zhikai et al., 1983), zinc + potassium sulfate (Zhou Zucheng, 1985), Dahl's alloy, etc.

According to Gao Jiasheng (1981) on the identification of soil alkali-hydrolysed nitrogen distillate, it was clarified that the nitrogenous substances contained in the alkali-hydrolysed nitrogen are mainly exchangeable ammonium nitrogen, amide nitrogen, amino acid nitrogen, and other nitrogen-containing substances that are more easily decomposed. , accounting for about 10% of total nitrogen, and Chen Qiu (1987) conducted a study on soil available nitrogen tests in paddy soils in the Sanming area of ​​Fujian Province. It was demonstrated that the soil alkaline solution dispersal method measures the relative yield of soil available nitrogen and rice or nitrogen uptake by plants. There was a very significant correlation between the amounts, with r values ​​of 0.8595** and 0.7558**, respectively. Jilin Province Liu Chengxiang et al. (1985) determined the available nitrogen content of black soil, chernozem, and white soil by reducing alkalinity. The correlation between the measured value and the relative yield and nitrogen uptake of maize was very high. The r values ​​were respectively Reaching 0.805** and 0.714** is very significant. This method has been applied on a large scale in soil test recommended fertilization and has proven to be a simple and reliable test method.

(3) Soil culture A small amount of mineralized soil (5~10g) is kept in the thermostat at the optimum temperature (30°C or 35°C) for 1~2 weeks, and the ammonium nitrogen and nitrate nitrogen produced by the leaching are Soil culture mineralized nitrogen, it can be divided into aerobic culture method and flooded culture method, which is more mature with flooding culture method. Zhang Shoujing (1978) believes that the best estimate of nitrogen availability in paddy soils is to add the initial ammonium nitrogen to the ammonium nitrogen released after 1 week of flooding. The work of Zhou Mingxuan et al. (1976, 1986) also proved that the flooded cultivation method is very reliable for the test of available nitrogen in paddy soil. However, this method is not widely used due to the long test time and cumbersome operation when actually guiding large-area soil fertilization.

(4) Soil nitrate-N and soil inorganic nitrogen (Nmin) In the effective nitrogen test method of northern dryland soil, nitrate nitrogen and deep inorganic nitrogen are good indicators. Yang Di (1982) studied the correlation of nitrate nitrogen content in 120 cm soil layer with wheat yield on wheat in dryland in Shanxi Province and considered that it can be used for the estimation of nitrogen requirement in wheat. Tian Yuanren (1979) measured the nitrate nitrogen content in 50 cm soil layers three times during the growth of Hebei farmland wheat to formulate the amount of nitrogen fertilizer with good results.

The deep inorganic nitrogen (Nmin) method is applied to the diagnosis of wheat and corn topdressing in parts of Germany. Some research institutes and universities in China are conducting research in this area.

Because of the large sample depths required for both nitrate nitrogen and Nmin measurements, the nitrate nitrogen method requires the measurement of ammonium nitrogen and nitrate nitrogen in soils from 120 to 180 centimeters, which is difficult under current technical conditions in China. Promotion.

2. Determination of Available Phosphorus in Soil Extraction Phosphorus in soil is derived from the weathering of phosphorus-containing minerals. The distribution of inorganic phosphorus forms in major soils in China is closely related to the weathering zone. Jiang Baixi (1983) believes that in the highly weathered brick red soil and red soil in the south, the inorganic phosphorus forms in the soil are mainly closed-state phosphates coated with iron oxide films, most of which are iron phosphates, and are in non-closed storage. Phosphate is also dominated by ferric phosphates. In the soils developed from northern China's loess parent materials, soil inorganic phosphorus is dominated by calcium phosphate. Closed storage phosphorus accounts for only a small percentage of yellow brown earth in the transition between the north and the south. Brown earth, aluminum phosphate content is relatively high, showing a certain transitional nature. According to a large amount of research work, it can be assumed that the effective form of paddy soil is dominated by Fe-P, while the acidic upland soil in the south is dominated by A1-P, while in the north dryland soil is mainly Ca-P, and Fe-P and A1-P are still The role of organic phosphorus compounds in addition to the tropical forest grassland soil is not the main form of available phosphorus in soil. Phosphate form in soil is the basic principle that determines the selection of effective phosphorus extractants.

Some methods for extracting available phosphorus from soils, such as Gilsanoff, Malacca, and Truog, were introduced into China in the 1950s, but they lacked relevant research on biological verification. Since the 1960s, a large number of formal studies have been conducted on available phosphorus extractants for soil, and many research reports on the applicability of Olson, Bray-I, A1-Abbas, etc. have been made (Li Yaohui, 1963, Yu Wentao, 1965). , Lin Zhongyu, 1982, Xie Lichang, 1983, etc.).

(1) 0.5 m/l NaHCO 3 extractant (Olson, 1954) This extract extracts Ca-P and A1-P in soil, and also extracts part of Fe-P, so it has the widest applicability. Phosphorus extraction from neutral to calcareous soils in northern, northwest and northeastern parts of China can be applied to neutral to slightly acidic paddy soils in South China. However, the Olsen method is effective in the soils with strong acidity in the south. The correlation between the amount of phosphorus extracted and the amount of crop absorption is poor and the temperature conditions are more stringent. This is a drawback of this method, and the amount of available phosphorus for soil is also less than other methods.

(2) 0.025 ML/L HC1-0.03 MO/L NH 4 F extractant (Brei, 1945) The available phosphorus extracted by ammonium fluoride extractant is mainly A1-P, which can extract both Ca-P and Fe. -P. Therefore, this method is suitable for neutral, slightly acidic, and moderately acidic soils below pH 7.5.

Jilin Academy of Agricultural Sciences Liu Chengxiang et al. (1985) conducted analysis of the inorganic phosphorus composition of 40 representative soils of the province, and conducted a related study with the phosphorus uptake of maize plants. It was found that the soil inorganic phosphorus content of major soil types in Jilin Province was Ca- P is predominant, but calcareous soil is Ca-P> A1-P> Fe-P, non-calcareous soil is Ca-P> Fe-P> A1-P. Phosphorus uptake by corn plants was significantly correlated with A1-P and Ca-P, but was also A1-P> Fe-P> Ca-P. Therefore, the Bray I method is the same as the Olson method in this area. When the Bray I's soil to liquid ratio is raised to 1:50, it can be applied to soils with less lime.

(3) Acidic extractants are currently used in foreign countries where there are more than five acidic extractants. They are 0.1 m/l HCI, 0.2 m/l HCI Gilsanov, 0.025 m/l H 2 SO 4 -0.05 MoHCI's Mehlich bis acid method and pH4 0.1 mg/litre calcium lactate-0.1 mol/l calcium acetate-0.3 mol/l acetate CAL method. The domestic research work considers the correlation is better 0.1 Hm / L HCL method (Fujian red soil paddy soil, Chen Qiu z, etc., 1985), double acid method (Guangxi red soil, Zhou Qingxiang, 1985; Zhejiang red soil, Lu Yunfu, 1985, etc.) The Gilsanov method may be applied to strong acidified ash in the north.

(4) Alkaline extractant (A1-Abbas, 1965) 0.3 mol/l NaOH-0.5 moll Na 2 C 2 O 4 extractant to extract Fe-P in soil, mainly neutral to slightly acidic rice Soil is very suitable. According to researches by Zhou Yi and Ye Dexian (1985), the method of extracting soil available phosphorus from purple soil in Sichuan Province is the best. The correlation coefficient between the available phosphorus content and the relative phosphorus uptake of rice can reach 0.896, which is very significant. The same applies to paddy soil in the western plain of Sichuan and the Olson method.

(5) Soil phosphorus isothermal adsorption method and phosphate bit method Strictly speaking, these two methods do not belong to the determination of extractants, but in the test of soil phosphorus nutrient supply, they are very unique methods. Many foreign researchers have conducted research in this area and some have been applied to production practice.

In his review, Jiang Baixi (1983) pointed out that the characteristics of phosphorus adsorption in the tested soils of several major soil types in China conformed to the three adsorption equations of Langmuir, Freundlich, and Temkin, and the maximum difference in the amount of absorbed phosphorus in different soils was very different from that of soils. There is a high correlation between the pH and the concentration of calcium and magnesium in the soil solution. Huang Deming et al. (1982) thought that there was a certain correlation between the phosphorus concentration in the soil solution and the phosphorus uptake in the crop. The 0.3 mg/L phosphorus solution in the soil balanced solution could meet the requirements of wheat and other crops on the fluvo-aquic soil in different fertility levels. Phosphorus needs. Phosphate bit is the strength indicator for soil phosphorus supply. Lin Zhongyu (1983) studied the phosphate sites in paddy soils and garden soils of the latosolic red soils in South China, and measured the equilibrium phosphorus levels of the tested soils (1/2pCa+pH 2). The value of PO 4 ˉ) is 8.12~10.9 for paddy soil and 7.02~12.5 for vegetable garden soil. Compared with the A-value method, the phosphate bit is not suitable for the diagnosis of soil phosphorus in rice, but it is suitable for vegetable garden soil.

(6) Combined Extractant Mehlich proposed joint extraction agent No. III in 1982. Its composition is: 0.2 mol/l HOAc-0.25 mol/l NH 4 HO 3 -0.015 mol/l NH 4 F-0.013 mol/l HNO 3 - 0.001 MOP/liter EDTA. This extractant can simultaneously extract the effective contents of 9 nutrient elements including phosphorus, potassium, calcium, magnesium, sodium, manganese, iron, zinc and copper. Duan Xiutai (1982) conducted a comparison of M-III extractant with Bly-I method and Olson method. She found that when the soil acidity is stronger, the Olson extraction of soil available phosphorus results is low. The stronger the soil calcareous, the lower the results with the Ble-I method. Only the M-III method results can accurately reflect the soil P fertility over a wide pH range.

3. Determination of Available Potassium in Soil Determination Since the 1970s, soil potassium has been determined by common nitrite nitrous oxide method or volumetric method, so soil exchangeable potassium extractants are all sodium salts, such as 10% NaC1, NaOAc or NaNO 3 etc. The widely used sodium tetraphenylborate extractant is still a continuous process of this method. By the end of the 1970s, the flame photometric analysis method was widely used, and only 1 mol/liter neutral ammonium acetate extractor was used as the standard method for the determination of exchangeable potassium. For all dryland soils, whether north or south, acidic or calcareous, exchangeable potassium can be used as indicators of soil potassium fertility, but there are many research reports that it is not highly relevant to the relative crop yield ( Xie Gaochang, 1983). This situation is more pronounced for paddy soils because of the easier release of slow-release potassium from the soil under flooding conditions, and the ability of rice to use slow-potassium potassium is stronger. It can use up to 20 potassium uptake. %~40%.

There are three methods for extracting slow-acting potassium from the soil: 1 mol/L HNO 3 boiled for 10 min, 2 MOX/liter cold HNO 3 leaching, and 6 MO/L H 2 SO 4 leaching, in which 1 mol/L HNO 3 is boiled Law is commonly used. Slow-potassium refers to potassium fixed in layered clay minerals and a portion of hydromica. Since the content of slow-acting potassium is affected by the parent material and the weathering conditions, it is not as easy to change as exchangeable potassium. It can better explain the difference in potassium storage in different soil types and is the potential for different soil potassium supply. Good indicators.

Electro-ultrafiltration is a physicochemical method that combines electrodialysis with ultrafiltration. Chinese scholars have done a lot of work on electro-ultrafiltration in recent years (Shi Ruihe, 1983; Li Shukai, 1985; Lu Yunshen, 1987). It is considered that the application of an electric ultrafiltration device can perform a complete leaching in a short period of time under variable voltage and temperature conditions, and at the same time, the parameters such as the strength, capacity and supply rate of nutrients in the soil are measured. Satisfactory results were obtained on the test of potent potassium.

Xie Jianchang et al. (1987) proposed using the cationic resin bag method to determine the effectiveness of soil potassium. The potassium extract of the resin bags measured by them was very similar to the potassium intake of crops. The correlation between them reached 0.916, which would be a promising prospect. The effective potassium extraction method for soil.

4. Determination of Soil Available Calcium, Magnesium, Sulfur, and Silicon Extraction In general, the total calcium content in soil is less than potassium and magnesium, but exchangeable calcium is much more than exchangeable potassium and magnesium. A large part of the non-exchangeable calcium in the soil is a primary mineral containing calcium, such as anorthite, calcite, dolomite, and amphibole. In alkaline earths and salts, a considerable amount of calcium exists in the form of carbonic acid or gypsum. Since calcium is the most important exchangeable cation in most soils, the calcium saturation in the soil absorption complex directly affects the soil pH. After the soil became strongly acidic, calcium became the limiting factor for plant nutrition. Therefore, the test of available calcium in soil included the lime content requirement of soil and the determination of available calcium content in soil. In recent years, Chinese scholars have found that Chinese cabbage has physiological calcium deficiency in soils with pH above 7.5 in North China, and other crops such as peanuts have certain effects on the production of calcium fertilizers. These studies have proposed some new topics for effective soil calcium testing. .

(1) Soil pH determination Diagnosing lime requirement is the easiest method for determining the lime requirement through soil pH measurement. When pH is less than 7, lime can be used to adjust, but it does not need to be adjusted to completely neutral. According to the research of Power Supply et al. (1987), in the paddy field with a pH of 5.6-6.0, the activity of aluminum ions in the soil is little or disappeared. When applying sufficient fertilizer, the application of lime has more disadvantages. The same experience is also experienced in foreign countries. In the soil of the Midwestern United States, using lime to adjust the pH to more than 6 does not increase economic efficiency, but it is advisable to adjust the pH to more than 6.5 when planting. In the subtropics and the tropics, even lower soil pH does not affect crop yields, but for various peat and sapropel soils, neutralizing them above pH 5.2 does not increase yield.

(3) Determination of Exchangeable Calcium Extraction Soil exchangeable calcium can be extracted using 1 mol/L of NH 4 OAC. Actually, ammonium acetate extractant can measure soil cation exchange capacity (CEC) and exchangeable potassium, sodium, calcium, magnesium, etc. cation. It is possible that the proportion of exchangeable cations in the soil is much more important than the determination of single exchangeable calcium.

(4) Determination of Exchangeable Magnesium Extraction Soil-exchangeable magnesium was also extracted using 1 mol/l NH 4 OAC and atomic absorption spectrophotometry. Reports of magnesium deficiency in rubber trees in southern China were conducted (Lu Xingzheng, 1987). The cause of magnesium deficiency is, on the one hand, insufficient supply of magnesium in the soil, and on the other hand, a rich supply of potassium. Foreign reports suggest that potassium/magnesium is an important diagnostic criterion, and the weight ratio of potassium to magnesium should be less than 5 for field crops. For vegetables and sugar beets should be less than 3, for fruit trees and greenhouse crops should be less than 2, otherwise it is prone to magnesium deficiency.

(5) Extraction of available sulfur According to Liu Chongqun et al. (1983), the distribution of soil sulfur in China is affected by factors such as temperature, rainfall, and soil organic matter. The southeastern part is warm and humid, with organic sulfur in the soil as the main component, and drought in the northwest is less rainy. The content of inorganic sulfur in the soil is relatively high. Sulphur content, renewal rate and distribution of soluble forms in paddy soil and red-yellow soil in the South of China.

There are many methods for extracting available sulfur from soil. The recommended method for the international rice industry is 0.01 mol/l calcium phosphate monohydrate, the ratio of soil to liquid is 1:4, and shaking is 24 hours. The critical value for diagnosing sulfur deficiency is 10 mg/kg sulfur. After extraction is generally measured with BaSO 4 turbidimetry.

(6) The extraction of available silicon determines that plants contain a considerable amount of silicon, but silicon is not an essential nutrient for plants. There are different opinions, but the effects of different levels of silicon production on rice production in rice are reported in Heilongjiang and Fujian provinces. Wu Ying, 1983, Peng Jiagui, 1987). The method of extracting available silicon in soil is widely used in China and Japan, such as pH 4 acetic acid-sodium acetate buffer solution, the ratio of soil to liquid is 10:100, and the temperature is 40°C. After shaking for 5 hours, it is shaken and filtered to analyze the silica in the extract. content. Extractions with a value of 50 mg/kg silica have a certain effect, and silicas larger than 100 mg/kg silica generally do not lack silicon. The above indicators apply to paddy soils in the south. When Heilongjiang Province carried out the results of silicon application tests on white soil, meadow soil, meadow black soil and black soil, the effective silicon content of black soil was ineffective at 1500-2000 mg/kg silicon application, and the other three kinds of soil-based effective silicon content The lower, the silicon molybdenum blue colorimetric determination of 200 ~ 300 mg / kg, after the application of silicon production effect is obvious (Wu Ying et al., 1987).

5. Determination of cations, zinc, copper, manganese and iron in soil

(1) DTPA combined extraction method DTPA is a metal ion chelating agent proposed by Lindsay in the United States in 1969. It is applicable to the extraction of zinc, copper, manganese and iron in neutral, calcareous and slightly acidic soils. The research work of Pannenberg et al. (1987) proved that this extractant is also suitable for red soil with strong acidity. They used the sweet corn absorption of zinc, copper, iron and manganese in the acidic soil as the reference standard, and the results of the correlation analysis with the DT-PA extraction amount are shown in Table 10-7.

Table 10-7 Correlation analysis of trace elements extracted from DTPA on acid red soil

Correlation factor

Correlation coefficient (r)

DTPA - Zinc Absorption of Zinc and Sweet Corn

DTPA - Copper and Sweet Corn Copper Absorption

DTPA - Iron Absorption of Iron and Sweet Corn

DTPA - Manganese Absorption of Sweet and Sweet Corn

0.949**

0.828**

0.313

0.842**

The Chinese chemical name of DTPA is diethylenetriaminepentaacetic acid, and the extractant contains 0.005 mole/liter DTPA, 0.01 mole/liter CaCl 2 and 0.1 mole/liter triethanolamine and is buffered to pH 7.30. The liquid-to-liquid ratio is 25:50, and it is shaken at 20-28°C for 2 hours. After filtration, the four elements of Zn, Cu, Mn, and Fe are simultaneously measured by an atomic absorption spectrophotometer.

(2) Neutral salt extraction method Scholars from various countries have studied the trace elements in soils extracted from various neutral salts. They have used 0.1 mol/l MgSO 2 and 0.5 mol/l NH 4 OAC (Ph4.8). , 0.2 M/l MgSO 4 etc. It was reported that 1 mol/l NH 4 OAC, pH 4.8, soil-to-liquid ratio 12.5:50, filtration after shaking for 30 minutes, and good iron and iron availability in acidic and neutral soils can be obtained . In addition, the use of 1 mol/L NH 4 OAC with a 10:100 ratio of soil to liquid to exchange manganese in acidic soils is also good, but for calcareous soils, the DTPA method is required.

(3) Weak acid and strong acid extraction methods are mainly used for 0.1 mol/l HCl, 0.05 mol/l MgSO 4 -H 2 SO 4, 0.05 mol/l HCl-0.025 mol/l H 2 SO 4, etc. At most, the earliest application was 0.1 mol/l HCl extractant. This method is suitable for extracting effective zinc and copper in acidic soils, not for manganese and iron. At the time of extraction, the ratio of soil to liquid was 10:50, shaking for 1.5 hours, and determination by atomic absorption spectrophotometry.

6. Extraction and determination of boron and molybdenum in the soil anionic trace elements Boron-deficient areas, crop types, soil and climatic conditions are more extensive than those lacking any other trace elements. Liu Xi (1983) pointed out in his review of trace elements that there are two major areas of boron-deficient soil in China: one is red soil, red soil and brick red soil in the southeast, and the other is the development of alluvial deposits in the loess and the Yellow River. Soil, in addition to a small piece of boron-deficient soil, is distributed in Hubei, Henan, Heilongjiang and other places.

The distribution of molybdenum-deficient soils in China is very consistent with the lack of boron in soils, mainly in the above two regions. The lack of molybdenum in the soils of the northern loess and the Yellow River alluvium is mainly due to the low molybdenum content in the parent material, and the lack of molybdenum in the acid soils in the south. The main reason is that the soil acidity reduces the effectiveness of molybdenum.

The effective boron extraction is currently only a hot water extraction method, the ratio of soil to liquid is 20:40, and the colorimetric determination is performed using the curcumin method or the methylamine-H method.

The most popular method for extracting effective molybdenum from soil is pH 3.3 oxalic acid-ammonium oxalate reagent (Tamm solution). This reagent can simultaneously elute considerable amounts of iron oxide and aluminum, so it can extract more molybdenum than the effective amount. It is necessary to take soil pH into consideration when interpreting the test value of soil effective molybdenum. Oxalic acid-ammonium oxalate extracted molybdenum can be determined by molybdenum thiocyanate colorimetric method, and can also be determined by catalytic polarographic analysis. In 1972, Dawson et al. proposed anion exchange resin method to determine soil available molybdenum, and 46 kinds of soil resin extracted molybdenum and plant absorption. Between molybdenum, r = 0.861. For soils with low effective molybdenum content, this method may be the best method to determine the effective molybdenum in the soil. In addition, the concept of molybdenum value is also very useful, molybdenum value = pH + (effective molybdenum amount × 10), its indicators are: lower limit <6.2, moderate 6.2 ~ 8.2, upper limit> 8.2.

7. Relevant researches on the selection of extractants The various extractants for soil effective nutrients and their approximate ranges are described above. Each locality should select local extractants according to the local soil conditions. The extraction agent screening method is mainly to use different soils to conduct several biological tests on extractants. The extractable soil nutrient content of each extractant is compared with the biological reference standard and the correlation coefficient is calculated. The usual practice is to select a number of 20 soil samples in the test area. These soil samples should be representative of the soil fertility levels in the area, both low-yield and high-yield soil samples. The pot experiment was arranged in the test site, because the test can be used as seedlings, so smaller pots can be used, both for low-yield plots, and if the production is to be a result, a larger pot is used. The experiment consisted of 4 treatments including whole fertilizer group (NPK), nitrogen-free group (P and K), non-phosphorus group (N and K) and non-potassium group (N and P), and 3 to 4 replicates. For the trial crops, the main local crops and cultivars can be selected. 20 种土壤在装盆施肥前，先各取一个土壤基础样，盆栽幼苗长至一定高度（一般在40~60 天）后，收取植株地上部，测鲜重，烘干备用。如果进行土壤有效磷提取剂选择，可选用3 种左右提取剂对20 种土壤基础样本进行有效磷含量测定，同时对盆栽试验的全肥组、无磷组两组植株样本化验其含磷量，并测其干重，以无磷组植株干重/ 全肥组植株干重× 100 为相对产量（ % ）。 20 个基础土样的土壤有效磷测定植与盆栽试验植株干重或籽粒产量的相对产量值，植株吸磷量等数据统计它们之间的相关系数。选取相关数值高的土壤有效磷提取剂作为本地区土测磷的标准方法。

陆允甫等（ 1987 ）对红壤旱耕地有效磷测定方法的筛选进行了研究。在浙江金华、衢州两市共7 个县进行了较大规模的磷肥田间试验， 2 年共40 个点。供试土壤均属酸性， pH （ H 2 O1:1 ） 4.9~6.85 （二处pH 大于7 除外），有机质含量0.68%~2.72% ，全氮为0.04%~0.156% ，全磷为0.014%~0.045% ，黏粒含量（ <0.002 毫米） 15.31%~53.27% 。土壤分类属红壤亚类下面的几个代表性土属。试验设氮磷钾处理与氮钾处理，并施用硫酸锌； 4 次重复。指示作物为玉米（丹玉六号）。试验中取用：①玉米洱粒相对产量；②总干物质相对产量；③全株总吸磷量；④籽粒含磷量等四项作参比标准。

表10-8 不同方法速效磷测定（ logP ）与参比项的相关系数

method

玉米籽粒

相对产量

（ n ＝ 35 ）

总干物质

相对产量

（ n ＝ 35 ）

玉米全株

吸磷量

（ n ＝ 29 ）

玉米籽粒

含磷量

（ n ＝ 31 ）