Effects of Organic Compound Fertilizers on Soil Water Infiltration Characteristics

Jianqi LAN Shiyou XIE

Abstract In order to study the effects of organic compound fertilizers on soil water infiltration characteristics， an indoor one-dimensional soil column water infiltration test was carried out. Six soil sample treatment groups were set， namely marshy solonchak （control check， CK for short）， bamboo charcoal-mixed marshy solonchak， Difuyuan-mixed marshy solonchak， salined flavo-aquic soil （CK）， bamboo charcoal-mixed salined flavo-aquic soil and Difuyuan-mixed salined flavo-aquic soil. After 120 min of infiltration， compared with the CK groups of marshy solonchak and salined flavo-aquic soil， the cumulative infiltration volumes of the bamboo charcoal and difuyuan treatment groups increased by -18.78%， -3.93% and 25.77%， 6.53%， respectively; and the displacement increased by -18.93%， 1.64% and 22.6%， 12.5%， respectively. The relationship between the wetting front displacement and time conformed to a linear function. The effects of organic compound fertilizers on the initial infiltration rates of marshy solonchak and salined flavo-aquic soil were significant， and the relationship between infiltration rate and time conformed to a power function. The vertical distribution of soil moisture under the application of organic compound fertilizers showed that the moisture content changed relatively small within 5 cm of the surface soil layer， then decreased slowly within 5-10 cm of the soil layer， and decreased drastically below 10 cm of soil layer; and relative to the CK group， the application of organic compound fertilizers was beneficial to increase the moisture content of salined flavo-aquic soil. The organic compound fertilizers reduced the water infiltration capacity of marshy solonchak and improved the water infiltration capacity of salined flavo-aquic soil. Compared with Difuyuan， bamboo charcoal reduced the water infiltration capacity of marshy solonchak better and improved the water permeability of salined flavo-aquic soil more significantly.

Key words Organic compound fertilizer; Wetting front; Cumulative infiltration volume; Infiltration rate; Soil moisture

Organic compound fertilizers refer to fertilizers for agricultural production that mix organic fertilizers and inorganic fertilizers in a certain proportion and contain a variety of effective auxiliary microorganisms and high-energy compounds[1]. Organic fertilizer， also known as "farm fertilizer"， mainly comes from plants or animals. After a period of fermentation and decomposition or processing， it is applied to the soil and provides carbon-containing materials for plants. It mainly includes human manure， animal manure， compost， green manure， cake fertilizer， biogas fertilizer， etc. Inorganic fertilizer， also known as "chemical fertilizer"， does not contain organic matter and is composed of inorganic matter. It mainly includes elemental fertilizers such as nitrogen fertilizer， phosphate fertilizer， and potash fertilizer and compound fertilizers. Organic fertilizers and inorganic fertilizers are used in combination to complement each other. In recent years， organic compound fertilizers have attracted much attention as a new type of chemical fertilizer and improver. Compared with inorganic compound fertilizers， organic compound fertilizers have obvious advantages in improving crop yield， reusing domestic waste and production waste， improving soil， conserving soil and water and even protecting environment[1]. Infiltration is the physical process of surface water infiltrating into soil， also the link among groundwater， surface water and soil water， and is an important part of the process of runoff formation and water circulation. Guo et al. found that the main factors affecting soil moisture infiltration are soil texture， soil structure， initial soil moisture content， soil organic matter content， soil bulk density， soil temperature， skinning， density， sodium ion content and profile characteristics， etc.[2-5] Wang et al.[6] showed that the addition of chemical modifiers has played a role in reducing soil permeability. Liang et al.[7] reported that gypsum can effectively reduce soil infiltration and water conductivity. Song et al.[8] adopted the pot experiment method to study the improvement effect of different proportions of peat and weathered coal in saline-alkali soil and the effect on crop yield increase. The preliminary research by Wang et al.[9] on the composite modifier of coastal saline-alkali soil showed that the combination of organic fertilizer and calcium-containing materials in a suitable ratio has a very good effect in improving coastal saline-alkali soil.

Studying the effects of organic compound fertilizers on soil water infiltration is of great significance for improving soil， especially improving saline-alkali soil， making crops grow better， increasing yield and income， and retaining fertilizer and water. In this study， the vertical one-dimensional infiltration method was applied to analyze and investigate the water infiltration performance of the soil after applying organic compound fertilizer. Infiltration rate， cumulative infiltration volume， and wetting front are important characteristic values describing the infiltration process. For specific infiltration conditions， acquiring above characteristic quantities is of great significance for the evaluation of regional runoff and sediment production， farmland irrigation， farmland drainage， groundwater recharge， etc.[10].

Materials and Methods

Experimental materials

The test soil samples were taken from Dongying marshy solonchak and Binzhou salined flavo-aquic soil in the Yellow River Estuarine Wetland. The soil sampling depths were 0-60 and 0-30 cm， respectively. After sampling， impurities were removed， dried naturally in the dark， and sieved with a 2 mm sieve. The initial moisture contents of the soil measured by the drying method were 1.34% and 1.21% respectively. Two main types of organic compound fertilizers were selected in the experiment， namely bamboo charcoal organic compound fertilizer and Difuyuan organic compound fertilizer， both of which were black granules. After rolling， they were sieved with a 2 mm sieve. Bamboo charcoal has a loose and porous structure， a hard texture， high porosity， and contains a part of minerals needed for biological growth. It can maintain a good nutritional balance and is very suitable as a carrier of soil microorganisms and organic nutrients， thus serving as a good soil improvement material. Difuyuan organic compound fertilizer has the effects of adjusting the physiological function of plant cells， improving the water absorption and fertilizer absorption of crop， adjusting the physical and chemical properties of soil， improving the root environment of plants， and promoting plant growth， early maturation and high yield.

Experimental methods and observation items

A one-dimensional soil column water infiltration test was carried out. The soil columns had an inner diameter of 6 cm， a height of 32 cm， and a cross-sectional area of 28.27 cm2， and were made of transparent plexiglass. The water supply level was controlled. The Markov flasks used had an inner diameter of 5.4 cm and a cross-sectional area of 22.9 cm2. The height of water head on the soil surface was 1 cm. According to the proportion of organic compound fertilizer applied in actual farming， the bamboo charcoal and Difuyuan organic compound fertilizers were designed to be uniformly mixed with Dongying marshy solonchak and Binzhou salined flavo-aquic soil at 87.4 g∶1 000 g. The mixtures were loaded into the upper 5 layers of the soil columns to a depth of 10 cm， respectively. When loading soil into a column， 81.10 g was filled in one layer according to the soil bulk density of 1.4 g/cm3， 2 cm/layer， following the principle of mixing evenly and pressing into the plexiglass soil column to form a uniform soil column. A layer of filter paper was laid on the top and bottom， respectively， such that the soil particles would not be dispersed under the impact of water and affect water infiltration.

At the beginning of the infiltration， a stopwatch was employed to count the time， and the change in the water level of the Markov flasks and the advance distance of the wetting front with time were recorded. Within 5 min after the start of the test， the data was recorded once every 30 s. In brief， the depth of the wetting front was recorded once by the scale marked outside the plexiglass tubes， and the change in the height of infiltration water was recorded once by the scale marked outside the Markov flasks. In the period of 6-10 min after the start of the test， a total of 5 min， the data was recorded once every 1 min. From 11 to 30 min after the start of the test， a total of 20 min， the data was recorded once every 2 min. In 31-90 min at the beginning of the test， a total of 30 min， recording was performed once every 3 min. From 91 to 120 min at the beginning of the test， a total of 30 min， the data was recorded every 5 min. The experiment was set with six soil sample treatments， namely marshy solonchak （control check， CK for short）， bamboo charcoal-mixed marshy solonchak， Difuyuan-mixed marshy solonchak， salined flavo-aquic soil （CK）， bamboo charcoal-mixed salined flavo-aquic soil and Difuyuan-mixed salined flavo-aquic soil. After the infiltration was completed， each layer of soil sample was taken out， and the moisture content of each layer of soil sample was measured by the drying method. Each treatment was repeated 3 times， and finally the average value of the three tests was calculated. The total number of the test soil columns was 18.

Results and Analysis

Effects of organic compound fertilizers on cumulative infiltration

Cumulative infiltration is the total amount of water that permeates into the soil through the surface area per unit time after infiltration begins. The cumulative infiltration amount is the integral of the infiltration rate with respect to time， so this value is closely related to the infiltration curve[11]. The equation for instant infiltration amount is Instant infiltration amount=The difference between the readings of the Markov flask×The cross-sectional area of the Markov flask/The cross-sectional area of the soil column， and the cumulative infiltration amount is the sum of the instant infiltration amount in the cumulative time.

It could be seen from Fig. 1 that in the CK groups （without applying organic compound fertilizers） and the groups applied with bamboo charcoal or Difuyuan， the cumulative infiltration volumes of marshy solonchak and salined flavo-aquic soil increased over time， but the values of cumulative infiltration in different treatment groups were significantly different.

It could be seen from Table 1 that during the first 5 min， the marshy solonchak treatment groups were not much different in cumulative infiltration amount. It might be due to that the effects of the organic compound fertilizers were small at the beginning of the experiment， the soil surface layer was close to the ponding infiltration area， and the effect on soil moisture was dominated by ponding water. With the extension of the infiltration time， the soil infiltration depth increased， and the organic compound fertilizers re-combined with the surface soil and played a full role， resulting in different cumulative infiltration amounts. At 120 min when the infiltration was completed， compared with the CK group， the cumulative infiltration amounts of the bamboo charcoal and Difuyuan treatment groups decreased by 18.78% and 3.93%， respectively. After 5 min， the cumulative infiltration amounts of the various treatment groups at the same infiltration time roughly ranked as CK>Difuyuan-mixed application group>bamboo charcoal-mixed application group. That is， under the same infiltration time， the cumulative infiltration amounts of the treatment groups applying bamboo charcoal and Difuyuan were less than that of the CK without applying organic compound fertilizers， and the cumulative infiltration amount of the treatment group applying bamboo charcoal was lower than the Difuyuan treatment group except for the first 5 min. Because the soil used in the experiment was marshy solonchak which contains a lot of silty sand and has a high sand content， few clay particles and large intergranular space， which lead to easy water infiltration， fast internal drainage， small water storage capacity and strong evaporation and water loss， water vapor diffused from the large pores to the soil surface and was lost[12]. The test soil had good ventilation and permeability， but poor water and fertilizer retention capacity. The application of organic compound fertilizers improved the soil structure， and made the large particles dispersed in the soil stick or dissolved into small particles， which blocked the pores of the soil， reduced the soil water conductivity， and kept more water in the soil layer. Therefore， organic compound fertilizers could be applied to marshy solonchak which is slightly sandy to improve the soil water retention capacity. The infiltration reduction effect of bamboo charcoal on marshy solonchak was stronger than that of Difuyuan.

According to the test data， when the infiltration duration of salined flavo-aquic soil was 87 min， the cumulative infiltration volumes of the CK and the Difuyuan treatment group were the same， both of 4.90 cm. When the infiltration duration was less than 90 min， the cumulative infiltration of the CK was lower than that of the Difuyuan treatment， but after 90 min of infiltration， the cumulative infiltration of the CK was lower than that of the Difuyuan treatment， which was due to the high viscosity of salined flavo-aquic soil. In the early stage， Difuyuan played a small role， and when the depth of soil infiltration increased， Difuyuan re-combined with the surface soil and fully exerted its effect， resulting in the cumulative infiltration volume being higher than that of the CK after 90 min. The cumulative infiltration volume of the bamboo charcoal treatment group during the same infiltration time was always higher than those of the CK and the Difuyuan treatment group， which was because bamboo charcoal has loose and porous structure with hard texture， high porosity and strong water absorption performance， and can activate saline-alkali soils and increase water infiltration capacity. At the end of the 120-min infiltration， compared with the CK， the cumulative infiltration volumes of the bamboo charcoal treatment group and the Difuyuan treatment group increased by 25.77% and 6.53%， respectively. The salined flavo-aquic soil used in the experiment was coastal salined flavo-aquic soil. China has a long coastline， and the coastal sediments are on the beaches and coastal plains in bays， among which there are many clayey sediments. Clayey soils have a high content of fine particles （especially clay particles） and a very low content of coarse particles （sand particles， coarse powder particles）， and usually have a solid framework with tight adhesion. The pores of clayey soils are often occupied by water， so they have poor ventilation and permeability， and strong water and fertilizer retention capacity[12]. The application of organic compound fertilizers can improve soil structure， increase the number of cohesive soil pores， and improve soil hydraulic conductivity， so the application of organic compound fertilizers can improve the water infiltration capacity of salined flavo-aquic soil. Compared with Difuyuan， bamboo charcoal had a better effect in improving the water infiltration capacity of salined flavo-aquic soil.

Effects of organic compound fertilizers on infiltration rate

Infiltration rate represents the amount of water that permeates into the soil through the surface per unit time. It reflects the soil infiltration performance， and is affected by soil texture， soil bulk density， soil structure， initial moisture content and other related factors. In the infiltration of the initial air-dried soil， the soil matrix potential is very large at the beginning， and the infiltration rate is very large at this time. With the extension of the infiltration time， the soil matrix potential continues to decrease， and the infiltration rate shows a decreasing trend. When the infiltration time is infinite， the soil matrix potential approaches zero， and at this time， the infiltration rate is a stable value， that is， the stable infiltration rate[12]. The infiltration rate can reflect the soil infiltration capacity. The commonly used indicators in soil science are the initial infiltration rate， the final infiltration rate （stable infiltration rate）， the infiltration rate 1 h after the infiltration starts， and the cumulative infiltration volume[13].

It could be seen from Fig. 2 that with the extension of time， the infiltration rate continued to decrease and eventually tended to be stable. The soil columns of different treatment groups had different infiltration rates when the infiltration time was equal. The fitting curves of the infiltration rate with time for the CK， bamboo charcoal treatment group and Difuyuan treatment group were all power functions， with r2 all above 0.99， and the fitting degrees were very high. Therefore， the dynamic change process of infiltration rate could be simulated by the power function relationship.

In order to facilitate the analysis， the infiltration rates at several specific time were taken to analyze the effect of mixed application of organic compound fertilizers on the infiltration rate of marshy solonchak. It could be seen from Table 2 that the initial infiltration rates of the treatment groups with the application of organic compound fertilizers were generally higher than that of the CK. The stable infiltration rates according to the application of different organic compound fertilizers were roughly as follows： CK>Difuyuan treatment group>bamboo charcoal treatment group. At the initial infiltration rate test of each treatment group （T=0.5 min）， the initial infiltration rates of the CK， bamboo charcoal treatment group， and Difuyuan treatment group were 1.860， 2.030， and 2.000 cm/min， respectively， and the difference between the maximum value and the minimum value was 0.170 cm/min. At 120 min， the infiltration rates （stable infiltration rates） of the CK， bamboo charcoal reatment group and Difuyuan treatment group were 0.085， 0.070， and 0.081 cm/min， respectively， and the difference between the maximum value and the minimum value was 0.015 cm/min. The differences between the treatments were not significant. By comparison， the effects of the organic compound fertilizers on the initial infiltration rate of marshy solonchak were more significant.

According to the dynamic change in the infiltration rate of each treatment group of salined flavo-aquic soil in Table 2， the initial infiltration rates of the treatment groups with organic compound fertilizers were generally lower than that of the CK. The time for various treatment groups to reach the stable infiltration rate was also basically the same. The stable infiltration rates were roughly arranged according to the application of different organic compound fertilizers： bamboo charcoal treatment group>Difuyuan treatment group>CK. At the beginning of the test （T=0.5 min）， the initial infiltration rates of the CK， bamboo charcoal application treatment group， and Difuyuan treatment group were 2.110， 1.860， and 1.380 cm/min， respectively， and the difference between the maximum and minimum values was 0.730 cm/min. At 120 min， the infiltration rates （stable infiltration rates） of the CK， bamboo charcoal application treatment group， and Difuyuan treatment group were 0.046， 0.058， and 0.049 cm/min， respectively， and the difference between the maximum and the minimum values was 0.012 cm/min. The differences between the treatments were not obvious. By comparison， the effects of organic compound fertilizers on the initial infiltration rate of salined flavo-aquic soil were more significant.

Effects of organic compound fertilizers on wetting front

Wetting front refers to the maximum depth of water infiltration[6]. As can be seen from Fig. 3， in the treatment groups with different organic compound fertilizers， the depth of the wetting front increased with the extension of infiltration time. For marshy solonchak， under the same infiltration time， the order of the wetting front depths was roughly Difuyuan treatment group>CK>bamboo charcoal treatment group. In the case of salined flavo-aquic soil， under the same infiltration time， the wetting front depths roughly ranked as Difuyuan treatment group>CK>bamboo charcoal treatment group. The fitting curves of the infiltration rate of the CK， the bamboo charcoal application treatment group and the Difuyuan treatment group with time were all linear functions， with r2 all above 0.9， so the fitting degree was high. Therefore， it could be concluded that the depth of the wetting front and the infiltration time is in a linear relationship.

It could be seen from Table 3 that for marshy solonchak， when the test was carried out for 5 min， the wetting front advance distances of the CK， bamboo charcoal treatment group and Difuyuan treatment group were 5.95， 5.60 and 6.23 cm， respectively. In the early stage of infiltration， the mixed application of organic compound fertilizer had little effect on the advance of the wetting front. With the passage of time， by 60 min， the wetting front advance distances of the various treatments were 17.20， 14.20 and 17.70 cm， respectively. The difference between the maximum value and the minimum value was 3.50 cm. The depth of the wetting front of the Difuyuan treatment group was slightly higher than that of the CK. The bamboo charcoal treatment group was lower than the CK and the Difuyuan treatment group， and the differences were significant. By the end of the test （T=120 min）， the wetting front advance distances of the various treatment groups were 24.30， 19.70 and 24.70 cm， respectively， and the difference between the maximum and minimum values was 5.00 cm. The Difuyuan treatment group was still slightly higher than the CK， with a little difference， while the depth of the wetting front in the bamboo charcoal treatment group was significantly lower than the CK and the Difuyuan treatment group， and the differences were more significant. According to the experimental observation and data analysis results， it was found that the differences in the advance distance of the wetting front between the treatment groups were small in the initial stage， but significant in the later period. It was speculated that the reason might be that the organic compound fertilizers played a small role in the early stage of the test， and the wetting front showed little differences. With the passage of time， the water in the soil increased， and the organic compound fertilizers reacted with the soil and began to play a role. Applying organic compound fertilizers to marshy solonchak reduced the cumulative infiltration volume and infiltration rate， thereby affecting the moving speed of the wetting front. Compared with the CK， by the end of the infiltration test （T=120 min）， the bamboo charcoal treatment group and Difuyuan treatment group increased the displacement by -18.93% and 1.64%， respectively. It could be seen that bamboo charcoal had an obvious effect of reducing the infiltration of marshy solonchak.

Table 3 shows the wetting front depths in different treatment groups of salined flavo-aquic soil at different time periods. It could be seen that when the experiment was carried out to 5 min， the wetting front advance distances of the CK， bamboo charcoal treatment group and Difuyuan treatment group were 5.15， 6.00 and 5.20 cm， respectively. The difference between the maximum value and the minimum value was 0.85 cm， that is， in the early stage of infiltration， the mixed application with each organic compound fertilizer had little effect on the movement of the wetting front. With the passage of time， by 60 min， the advance distances of the wetting front in the different treatments were 10.75， 12.25， and 11.60 cm， respectively， and the difference between the maximum value and the minimum value was 1.50 cm. The depths of the wetting front in the Difuyuan treatment group and the bamboo charcoal treatment group were slightly higher than that of the CK， while the Difuyuan treatment group was lower than the bamboo charcoal treatment group， with a large difference. By the end of the test （T=120 min）， the wetting front advance distances of the various treatment groups were 13.95， 17.10， and 15.70 cm， respectively， and the difference between the maximum and minimum values was 3.15 cm. The depth of the wetting front in the control group was significantly lower than those in the bamboo charcoal treatment group and the Difuyuan treatment group. The application of organic compound fertilizer to salined flavo-aquic soil resulted in a reduction in the cumulative infiltration volume and infiltration rate， thereby producing an impact on advance speed of the wetting front. Compared with the CK， at 120 min， i.e.， at the end of the infiltration period， the bamboo charcoal treatment group and Difuyuan treatment group increased the displacement by 22.6% and 12.5%， respectively. It could be seen that the organic compound fertilizers improved the water infiltration capacity of salined flavo-aquic soil， and the effect of applying bamboo charcoal on the salined flavo-aquic soil was better.

Effects of organic compound fertilizers on soil moisture

Soil moisture content is the amount of moisture in the soil， also known as soil moisture， which is the basis for studying and understanding soil moisture movement and changes and its movement and changes in various aspects[13]. It could be seen from Fig. 4 that the surface soil moisture content was the largest， and the moisture content at the wetting front was the lowest. It could be seen from Table 4 that the wetting front depths of the marshy solonchak control group， bamboo charcoal treatment group and Difuyuan treatment group at the end of infiltration were 24.30， 19.70， and 24.70 cm， respectively， and the wetting front depths of the salined flavo-aquic soil control group， bamboo charcoal treatment group and Difuyuan treatment group at the end of infiltration were 13.95， 17.10 and 15.70 cm， respectively. On the whole， the water content decreased with the increase of the depth of the soil layer， and the water content within 5 cm of the soil surface layer decreased with the increase of the depth of the soil layer， but did not change much. The 5-10 cm soil layer was the slowly decreasing zone of soil water content， and the moisture content of the soil layer below 10 cm dropped dramatically. The water content of the soil layer is related to the amount of infiltration water[6]. The water contents of the organic compound fertilizer treatments in the 10 cm soil layer were higher than that of the control groups （the organic compound fertilizers were only applied in the 1-10 cm soil layer）， while the water contents of the organic compound fertilizer treatment groups in the soil layer below 10 cm decreased faster. For salined flavo-aquic soil， the moisture contents of the bamboo charcoal treatment group and the Difuyuan treatment group were generally higher than that of CK at the same soil depth， indicating that the application of organic compound fertilizers increased the moisture content of salined flavo-aquic soil.

Discussion

The saline-alkali soil field is a kind of low-yield field. When organic compound fertilizer is applied， the soil water infiltration characteristics will change， which can provide a reference for the research on improving saline-alkali land. Most scholars have explored the effects of different types of modifiers on the infiltration characteristics of saline-alkali soils through experiments[6-9]， but no one has yet discussed the effects of the two organic compound fertilizers bamboo charcoal and Difuyuan on the infiltration of saline-alkali soil. In this study， the effects of these two organic compound fertilizers applied to two different types of saline-alkali soils on the characteristics of water infiltration were compared. Dongying marshy solonchak and Binzhou salined flavo-aquic soil were selected to carry out the indoor one-dimensional soil column water infiltration experiment. Under the condition of certain soil bulk density， different organic compound fertilizers were mixed and applied and the dynamic changes of cumulative infiltration volume， infiltration rate and wetting front with time and the dynamic change of soil moisture content with soil depth were compared and analyzed， so as to analyze their laws.

In this study， it was found that under the application of organic compound fertilizers， the cumulative infiltration volume and wetting front displacement of marshy solonchak in the bamboo charcoal group increased by -18.78% and -18.93%， respectively， indicating that the organic compound fertilizer had an infiltration reduction effect on marshy solonchak. It could reduce the loss of soil water and improve the soils ability to retain water， which is more consistent with the results of Wang et al.[6-7]. It was also found that the organic compound fertilizers improved the water infiltration capacity of salined flavo-aquic soil， which shows that different modifiers might not necessarily reduce the infiltration of different types of saline-alkali soils， that is， different modifiers have different effects on soils， which is more similar to the results of Dong et al.[15-16].   The effects of the organic compound fertilizers on the initial infiltration rates of marshy solonchak and salined flavo-aquic soil were more obvious， but the effects on the stable infiltration rates were smaller， which is similar to the results of Wang et al.[16]. The vertical distribution of soil moisture under the application of organic compound fertilizers showed that the water content changed relatively small in the soil surface within 5 cm， slowly decreased in the 5-10 cm soil layer， and decreased sharply in the 10 cm soil layer， which is inconsistent with the research results of Wang et al.[6] on the vertical distribution characteristics of soil moisture， probably because the type and dosage of the modifiers used are different， and the duration of the tests is also different. The organic compound fertilizers investigated in this study was not only used as a fertilizer， but also as an improver. From the point of view of soil moisture infiltration characteristics， we found that the application of organic compound fertilizers was beneficial to increase the moisture content of salined flavo-aquic soil， indicating that the organic compound fertilizers could improve the soil and increase the soils ability to retain water， which is similar to the results of Wang et al.[9，14].   Therefore， to study the effect of organic compound fertilizers on soil water infiltration not only helps to understand the significance as crop fertilizers， but also can provide some theoretical support in improving soil. In the future， when developing organic compound fertilizers， in addition to considering its significance as a fertilizer， they should also play a role as an improver for different types of soil. In the future， when developing organic compound fertilizers， in addition to considering their significance as a fertilizer， they should also play a role as an improver for different types of soil. Different types of saline-alkali soils should be applied with organic compound fertilizers having better effects. In this study， it was found that the improvement effect of bamboo charcoal organic compound fertilizer was better than that of Difuyuan organic compound fertilizer. Marshy solonchak has good air and water permeability but poor fertilizer and water retention capacity， and the application of organic compound fertilizers could reduce its infiltration and enhance its water retention capacity. Therefore， in agricultural activities， choosing bamboo charcoal for such marshy solonchak is more conducive to soil improvement. Salined flavo-aquic soil has poor air and water permeability and strong fertilizer retention ability， and bamboo charcoal had a better effect of improving its water infiltration capacity than Difuyuan. Therefore， when selecting organic compound fertilizers for marshy solonchak and salined flavo-aquic soil， the improvement effect of bamboo charcoal organic compound fertilizer is better.

For indoor simulation tests， due to the differences in the test method， device， the accuracy of loading soil columns and the groundwater recharge method， recharge amount and observation frequency， there will be a certain impact on the test results， and the changing conditions have certain differences in the actual soil environment in saline-alkali land area. The research on the application amount and effect of organic compound fertilizers and the research on other types of soil are still limited in space scale， and need to be further deepened.

Conclusions

Different organic compound fertilizers have different effects on the cumulative infiltration volume， infiltration rate and wetting front depth of different types of soil. At the end of the infiltration （t=120 min）， compared with the marshy solonchak control treatment group， the cumulative infiltration volumes of the bamboo charcoal and Difuyuan treatment groups increased by -18.78% and -3.93%， respectively， and the wetting front displacements increased by -18.93% and 1.64%， respectively; and compared with the salined flavo-aquic soil control treatment group， the cumulative infiltration volumes of the bamboo charcoal and Difuyuan treatment groups increased by 25.77% and 6.53% respectively， and the wetting front displacements increased by 22.6% and 12.5%， respectively. The wetting front displacement conformed to a linear function with time. The effects of organic compound fertilizers on the initial infiltration rates of marshy solonchak and salined flavo-aquic soil were significant， but they had little effects on the stable infiltration rate. The relationship between infiltration rate and time conformed to a power function.

The vertical distribution of soil moisture under the application of organic compound fertilizers showed that the moisture content changed relatively small within 5 cm of the surface soil layer， then decreased slowly within 5-10 cm of the soil layer， and decreased drastically below 10 cm of soil layer. Compared with the CK， the application of organic compound fertilizers was beneficial to increase the moisture content of salined flavo-aquic soil.

The organic compound fertilizers reduced the water infiltration capacity of marshy solonchak and improved the water infiltration capacity of salined flavo-aquic soil. Bamboo charcoal had a better effect in reducing the infiltration of marshy solonchak. Compared with Difuyuan， bamboo charcoal improved the water infiltration capacity of salined flavo-aquic soil better， and improved its water permeability more significantly. Therefore， when selecting organic compound fertilizers for marshy solonchak and salined flavo-aquic soil， the improvement effect of bamboo charcoal organic compound fertilizer is better. Therefore， in actual agricultural production， different organic compound fertilizers should be applied according to different soil types， so as to exert the maximum effect of differ-ent types of organic compound fertilizers， which is conducive to the growth of crops.

References

[1] XU H. Research progress on production and application about new type of organic compound fertilizer[J]. Guangzhou Chemical Industry， 2011， 40（13）： 32-34. （in Chinese）

[2] GUO CH， JIANG J. Factors influencing soil water infiltration[J]. Shanxi Science and Technology， 2011， 26（5）： 39-40. （in Chinese）

[3] LI XF. Experimental research on main factors affecting soil water infiltration characteristics[J]. Shanxi Hydrotechnics， 2008， 20（1）： 38-41. （in Chinese）

[4] LI Z， LIU YH， YANG Q. Review of the research on the influence mechanism of soil moisture infiltration[J]. Journal of Irrigation and Drainage， 2011， 30（5）： 124-127. （in Chinese）

[5] KOU XH， WANG W， ZHENG GQ. Soil water infiltration mechanism and experiment： A review [J]. Guangdong Forestry Science and Technology， 2013， 29（4）： 74-77. （in Chinese）

[6] WANG CX， WANG QJ， LYU YB， et al. The studies of infiltration characteristics on sandy saline alkali soil by chemical amelioration[J]. Journal of Soil and Water Conservation， 2014， 28（1）： 31-35. （in Chinese）

[7] LIANG JP， SHI WJ， WANG QJ， et al. Effects of gypsum on soil water infiltration characteristics[J]. Bulletin of Soil and Water Conservation， 2016， 36（6）： 160-165. （in Chinese）

[8] SONG X， DU LP， ZHANG CC， et al. Studies on amelioration effect of salinization soil with the organic matter[J]. Journal of Henan Agricultural Sciences， 2004， 16（8）： 58-60. （in Chinese）

[9] WANG SJ， ZHAO LW， SU YX， et al. Preliminary study on the compound improver for coastal saline-alkali soil[J]. Science and Technology of Tianjin Agriculture and Forestry， 2010， 12（2）： 4-7. （in Chinese）

[10] CHENG DJ， ZHANG YL， et al. Experiment guide of soil physics[M]. Beijing： China Water Conservancy and Hydropower Press， 2012： 58-61. （in Chinese）

[11] WANG H， WANG QJ， YAO BS. Effect of PAM dosage and application method on the characteristics of vertical infiltration of ponding water[C]∥The Proceedings in Commemoration of the 30th Anniversary of the Establishment of the Chinese Society of Agricultural Engineering and the 2009 Annual Conference of the Chinese Society of Agricultural Engineering. Jinzhong： Shanxi Agricultural University， 2009： 1115-1118. （in Chinese）

[12] HUANG CY， XU JM. Soil science[M]. Beijing： China Agriculture Press， 2010： 118-119. （in Chinese）

[13] LIN DY， XIE YH. Soil science[M]. Beijing： China Forestry Publishing House， 2011： 76. （in Chinese）

[14] CAI AX， JIANG QA， CHANG YC， et al. Study of effects of biogas fertilizer in improving alkaline soil and increasing yield increase[J]. Chinese Journal of Soil Science， 1999， 30（1）： 4-6. （in Chinese）

[15] AN D， LI XP， ZHANG YH， et al. A study on the effects of different soil amendments on alkalic halosols[J]. Agricultural Research in the Arid Areas， 2010， 28（5）： 15-118. （in Chinese）

[16] WANG Y， YANG JS， WANG XP， et al. Study on the law of soil infiltration characteristics of coastal farmland[J]. Journal of Irrigation and Drainage， 2015， 34（8）： 39-43. （in Chinese）

[17] YI HP， ZHANG LB， ZHANG PY. Biogeography[M]. Beijing： Science Press， 2012： 38-39. （in Chinese）

[18] GUAN H， LI JB. Hydrology[M]. Beijing： Science Press， 2010： 43-48. （in Chinese）

[19] ZHOU Y， WU JC. Research status， problems and prospects of soil improvers[J]. Journal of Henan Agricultural Sciences， 2010（8）： 152-155. （in Chinese）