The serious obstacle caused by secondary salinization of soil to crop production has become a roadblock to the sustainable development of current agricultural production, which is particularly evident in greenhouse vegetable production. If there is a lack of scientific management, as the cultivation period increases, the severity will gradually deepen, ranging from reduced yield to unsustainable greenhouse production. Excessive use of chemical fertilizers can lead to secondary salinization of soil, and producers generally have a clear consensus on this (which will not be repeated in this article). Based on this consensus, organic fertilizer application is often increased to reduce dependence on chemical fertilizers. However, in fact, improper selection of organic fertilizers can also lead to intensified secondary salinization of greenhouse soil. Producers often have unclear understanding of this fact and lack corresponding guidance, resulting in production misunderstandings and losses.

What are the manifestations of secondary salinization of soil? What are the reasons for secondary salinization of soil caused by organic fertilizers? How to deal with secondary salinization after it occurs? How to scientifically select organic fertilizers?

In this article, the author will provide answers to the above questions based on actual production, in order to provide reference for producers to scientifically respond to soil secondary salinization.

1、 The manifestation and harm of soil salinization

Firstly, distinguish between two similar concepts:

Soil salinization (salinization): refers to the process in which the salt in the bottom layer of soil or groundwater rises to the surface with capillary water, evaporates, and accumulates in the surface soil.

Secondary salinization of soil (also known as secondary salinization): It is a process of soil salinization caused by unreasonable cultivation and irrigation.

The difference between the two is that the former is caused by the natural environment, while the latter is caused by improper human agricultural operations, one of the main reasons being improper application of organic fertilizers.

1. The manifestation of secondary salinization of soil

① Soil performance

The inherent characteristics of soil can be judged by some visible symptoms, such as red soil often having low organic matter content and high viscosity; Black soil often has a high organic matter content and good soil permeability, water retention, and fertilizer retention. When secondary salinization occurs in soil, its severity can also be judged by some external characteristics. Based on the severity, the apparent judgment criteria are as follows:

Warning: The originally loosely structured soil begins to frequently form clumps and curl up on the surface, which is the initial manifestation of secondary salinization of the soil and should be given sufficient attention. Due to the low severity level, it is necessary to adjust the fertilization structure to prevent further deterioration.

Mild: With further accumulation of salt (which is chemically classified as salt), the color of the soil surface will appear green. This green color is due to the fertilizer applied to the soil not being fully absorbed and utilized by plants, accumulating in the soil and exceeding the soils ability to retain and adsorb. These excess nutrients are free between soil particles and formed under the action of irrigation, providing sufficient nutrition for green algae. Under suitable humidity conditions, green algae will reproduce in large numbers, leading to the production of green moss.

The negative impact on plant growth at this stage is not particularly evident, and as a result, it has not been taken seriously, leading to further deterioration of soil conditions.

Moderate: With further increase in salt accumulation, the soil structure will become more solid. When moist, the soil surface will appear dark green, which is not suitable for the reproduction and growth of green algae. When the soil loses water and becomes dry, a layer of white frost will appear. The reason for the formation of white frost is the chemical reaction between excessive cations such as Ca, Mg, Na in the soil and anions such as Cl and SO4.

At this stage, the normal physiological activities of the soil have been significantly affected, and there will be significant difficulties in improving yield and quality. It is necessary to adjust the fertilization structure as soon as possible to ensure normal production.

Severe: When the soil is moist, there is a layer of gelatinous green and red alternating mucosa on the surface. When the mucosa loses water and wrinkles or appears powdery red during drought, it indicates that the degree of secondary salinization of the soil is already high. The main reason for the formation of red and gelatinous mucosa is caused by red purple algae.

If soil remediation treatment is not carried out at this stage, it will no longer be suitable for the growth of crops such as vegetables.

② Plant performance

Excessive salt content in soil solution can cause difficulties in absorbing water and nutrients by roots, resulting in low production of new roots, short and thick new roots, and less root hairs. A well grown root system presents a bright white color, but under secondary salinization conditions, it is difficult for the root system to extend, resulting in not only small root clusters, but also red, yellow, and even necrotic black color of the roots. The leaves are prone to wilting, and wilting is exacerbated under good lighting conditions, and recovery is slower.

2. Secondary salinization hazards

① Causing physiological drought, resulting in poor growth, flower and fruit drop

Physiological drought refers to the presence of water around the root system, but the high salt content in the water makes it difficult to be absorbed and transported into the plant body for growth and development needs. Water is the carrier of material and energy transport in plants. The deficiency of water in plants is directly reflected in the inhibition of transpiration. In order to cope with the insufficient water supply, plants will reduce the surface area (wilting) of their leaves to reduce water evaporation, which leads to a decrease in the area of photosynthesis and affects the growth of plant leaves and the development of flowers and fruits. For example, the proportion of fallen flowers and fruits in fruits and vegetables such as eggplant and cucumber has increased, or the proportion of malformed fruits has increased.

② Causing a decline in product quality

Excessive salt content can lead to a large accumulation of nitrate in soil and plant bodies. For acidic soils in the south, while mineral elements such as iron and manganese accumulate significantly, there is insufficient absorption of elements such as zinc; For the alkaline soils in the western region, it leads to insufficient absorption of elements such as zinc and iron. The ultimate result is causing nutritional imbalance in plants. Moreover, nitrate content is an important negative indicator that needs to be controlled for the formation of vegetable quality, and it is often a factor in the failure of vegetable export quality inspection to meet standards.

2、 Analysis of the causes of soil salinization caused by commodity organic fertilizers

1. Two types of organic fertilizers with different sources and vastly different uses

One thing is clear: organic waste such as livestock manure, straw, and fallen leaves from household farming can be safely applied according to the standard (2000-4000kg/mu) as long as they have been fully decomposed. However, commercial organic fertilizers cannot be used because the salt content in commercial organic fertilizers is generally too high.

Before explaining the reasons behind it, it is necessary to distinguish the categories of organic waste and commercial organic fertilizers from their sources:

Organic waste sources: decaying branches and fallen leaves, fermented crop straw, household raised "vegetable cattle", pig and other livestock excrement.

The main sources of organic fertilizers for commodities are pig, cow, chicken, sheep and other breeding farms.

2. Why does commodity organic fertilizer become a hidden danger of secondary salinization?

Those with abundant wealth and fur are not counted. "African swine fever" and "avian influenza" are shocking, and almost all farms fear livestock and poultry diseases like tigers. Therefore, sterilization and disinfection of farms are an important part of daily management, which is a major reason for the high salt content in livestock and poultry manure. The author takes commodity chicken manure and commodity cow manure as examples to explain.

① Taking commodity chicken manure as an example

Chickens have a short gut in terms of physiological structure, and incomplete food digestion leads to high levels of "nutrient rich" mineral salts in their excrement. Another important reason is that chicken farmers disinfect chicken manure, which is prone to bacterial growth, using caustic soda, which is a highly corrosive chemical. After disinfection, there will be a large amount of salt residue in the chicken manure. Research data shows that the salt content in chicken manure in China ranges from 10-20 pounds per cubic meter, and for soil in vegetable greenhouses, a salt content of more than 20 pounds per acre can cause serious production obstacles. Continuous application without sufficient absorption can lead to significant accumulation, often resulting in secondary salinization within just a few years.

② Taking commodity cow manure as an example

Similar to chicken farms and cattle farms, caustic soda is also commonly used to disinfect excrement, which is one of the reasons for the high salt content in excrement. Furthermore, cattle farms often add salt to their feed in order to promote cow feeding. In order to increase milk production, dairy farms also require cows to drink a large amount of water. To achieve this goal, breeders add more salt to the feed. In addition to a small amount absorbed by cows, most of this salt is excreted with feces, ultimately leading to excessive salt content in the feces, which exacerbates soil secondary salinization when applied to the soil.

Therefore, for commercialized organic fertilizers, they must be used with caution or not. However, organic fertilizers produced by individual farming in rural areas can be safely applied according to standards because there is no practice of sterilization, disinfection, or salt feeding.

3、 Solution for secondary salinization of soil

1. Soil testing formula fertilization

Seeking help from agricultural machinery promotion departments or professional institutions such as agricultural colleges to determine soil fertility, salt content (EC value), acidity and alkalinity (pH value), and other indicators, to achieve a thorough understanding of soil conditions, and to apply fertilizers in a targeted manner. This operation is not complicated. If the example is far away, you can communicate with relevant departments and follow the "five point sampling method" to take soil samples and send them. The cost of sampling one piece of land is about 500 yuan. After the results are available, the structure and quantity of fertilizer application can be scientifically guided based on data, thereby achieving on-demand fertilization and avoiding excessive application that exacerbates secondary salinization.

2. Using water pressure salt

Flatten the ground, raise high ridges around to block water, and introduce a large amount of fresh water for flooding, using water pressure and salt. The principle is that salt dissolves in fresh water, and during the process of water infiltration, it carries the salt in the soil to the deep layers, purifying the surface soil where the roots extend.

For example, Changyuan County in Henan Province was once a heavily affected area of saline alkali land with low grain production. In the 1980s, the Yellow River water was diverted for flood irrigation and salt suppression transformation. After the transformation, the soil salinization situation in Changyuan County improved significantly and became a rice production area.

3. Biological desalination

Planting plants with fast growth rate, strong salt tolerance and absorption, Poaceae plants have strong salt absorption ability and large growth volume, and can consume a lot of salt, such as corn, sorghum, Sudan grass (with more prominent salt removal effect), etc. Summer is the off-season for greenhouse production. After rotary tillage the plot, high-density sorghum or corn seeds can be spread to achieve high salt removal effect for the purpose of increasing its biological yield.

Sudan grass

4、 How to choose organic fertilizer correctly

Due to the consideration of rural environment, family farming may gradually withdraw from the historical stage. So, how to solve the problem of organic fertilizer sources? The approach can be considered from the following perspectives:

1. Fermented and decomposed straw

The utilization of waste straw is a universal practical problem facing current agricultural production. For agricultural producers, the role of organic fertilizers in fertile soil and improving product quality is irreplaceable. Producers can recycle the straw nearby (usually only need to transport it by truck, and the straw itself does not require any cost), and then dig a biogas digester to crush the straw. After adding urea and fast-growing bacteria, it can ferment completely in about a month when the temperature is suitable in summer

2. Soybean meal and other grain and oil leftovers

This organic fertilizer has sufficient fertility and good fertilizer efficiency, and has a significant effect on improving the flavor of fruits and vegetables such as watermelon, melon, and strawberry. It is now widely used in places such as Rushan in Shandong and Dandong in Liaoning, with good results.

These organic fertilizers not only have low salt content themselves, but also have higher organic matter content. After decomposition, they are converted into colloids such as fulvic acid, which adsorbs excess salt in the soil and has a good effect on reducing the impact of secondary salinization on plants.

Conclusion

The EC value (an indicator of soil salt content) in Chinas commodity organic fertilizers is 1.47 times higher than that of organic waste, and the EC value in commodity pig manure is 1.74 times higher than that of pig manure. In the future trend of agricultural development that increasingly emphasizes quality and health, commercial organic fertilizers will gradually be marginalized. Combined with the difficulty of straw processing, in the future, fermented and decomposed straw will be a promising source of organic fertilizer while solving the transportation cost of straw; Organic fertilizer sources with higher organic matter content, such as soybean meal, have been proven to perform well in the cultivation of high-quality sweet melons, strawberries, and tomatoes.