What do insiders think about the substrate? They see the substrate as a reservoir for moisture, a channel for gas, a buffer for the root zone, and a base for plants. Different substrate products will have multiple types of water reservoirs, diverse air channels, slowly changing microenvironments, and moderate solidation. Although plants have been planted in the soil since ancient times, the substrate, an artificial soil made with modern technology, has replaced soil and entered the high-end stage of factory-based agriculture.

In order to prepare substrate products that can meet the needs of customers, substrate preparation companies need to start from the four functions of building and improving the products. According to the needs of planting plants and cultivation techniques, they can only prepare professional substrates with various functions of ventilation, water permeability, buffering, and anchoring after in-depth research on different substrate materials, modulation processes, and functions.

The substrate is ​​a low-density porous material, and it must be a low-density porous material. Otherwise, it is impossible to achieve a professional substrate that can both ventilate and store water and can buffer and fix roots. The function is determined by the materials and the preparation process. However, raw material properties are not equal to substrate properties, which are not a simple superposition of raw material properties.

Therefore, an expert looks at a substrate and looks at the selection of raw materials, the construction of a structure, the modulation of properties, and the addition of biological agents or biologically active ingredients. Because biological agents or biologically active ingredients can not only stimulate plant growth but also avoid or reduce the occurrence of diseases. The substrate with added biological agents or active ingredients is the highest level of substrate today.

1. Selection of raw materials
There are many materials that can be used as raw materials for substrates, but all of them are low-density porous materials, but the density and pores of the raw materials are different. There are both organic and inorganic materials, and there are materials that are neither organic nor inorganic.

Organic raw materials include moss peat, herbal peat, cocopeat, coconut chunks, wood fiber, fermented wood chips, fermented bark, rice husk, and peat moss.

Inorganic raw materials include perlite, vermiculite, pumice, volcanic rock, sand, and expanded clay.

Other raw materials include peanut shells, distiller grains, vinegar grains, cocoa shells, walnut shells, biochar, phenolic foam, and polyurethane foam.

Although there are many raw materials that can be used for substrates, the properties of different materials are obviously different. Some are well-ventilated, and some are strong in water absorption. Some are stable in structure, while others decompose quickly. Some have large buffer capacity, and some have strong structural effects. Although these raw materials should complement each other's advantages, minimize the use of imported resources and actively expand the use of domestic resources. However, after a comprehensive analysis of the advantages and disadvantages of various materials, the best, most efficient, most stable, and safest materials are none other than moss peat and herbal peat. This is because peat has both large pores for ventilation and small pores for water storage. It has the advantages of low pH, low conductivity, and low nutrition, as well as low unit weight and low density. In addition, peat is rich in humic acid, has a strong acid-base, nutrient buffering, storage, and replacement capabilities, and can maintain a long-term stable pH, electrical conductivity, and nutrient microenvironment of the substrate. At present, there is no material in the world that can completely surpass the advantages of peat.

2. Construction of the structure
The core of the substrate is ​​a reasonable pore structure because the pore structure determines the capacity of the water storage, ventilation, buffering, and solidation functions, and it is determined by the type and the characteristics of the raw material. Most of the inorganic raw materials only rely on surface activity to retain water, while organic raw materials can retain water on the surface of the raw materials as well as in the interior. There are also raw materials that do not have much water retention capacity themselves, but adding these raw materials can increase the aeration of the final product. Therefore, in the preparation of the substrate, the priority of which raw material to choose is the effect of it on the construction of the final structure.

In order to build a good structure, in addition to scientifically selecting the type of raw materials, it is also necessary to pay attention to the particle-size spectrum of them because different particle sizes will form different pores. Large particle-size materials can build large pores, thereby expanding the air pores of the substrate. Small particle size materials can build small pores, which can increase the water porosity although it reduces the air porosity. In the water pores, due to the different particle sizes of the raw materials, small pores with different water suction are formed. When the suction force of pores to water is between 1 and 10 kPa, the suction force of plant roots to water is greater than that of substrate pores, and this part of water can be absorbed by plant roots, which is effective water in the substrate. When the suction force of the substrate pores to water is greater than 10 kPa, the suction force of the substrate to water is greater than that of plant roots, and the water stored in this part of the pores cannot be used by plants and becomes ineffective water. Excessive amounts of ineffective water in the substrate not only cannot be absorbed and utilized by plants but also hinders the entry of air, which affects the oxygen absorption of plant roots and inhibits root expansion.

The effect of the materials on the water storage, ventilation, buffering, and solidation functions of the substrates also depends on the processing methods. The particle size distribution of fermented bark varies depending on how the bark is processed, aged, composted, and screened. Untreated moss peat has a rich porous structure, but when processed into fine particles, its air-void ratio and water-holding capacity will be significantly reduced. These are the direct effects of different processing methods of raw materials on the properties of the substrates.

The modern preparation process for substrates is the construction process of the substrate structure. However, since the large particle size will contain the small particle size in raw materials, the volume of 1 liter of large particle size raw materials and one liter of small particle size raw materials mixed together is often less than 2 liters, indicating that the structure construction process is far from 1+ 1 is as simple as that. If the different properties of different raw materials are considered, the difficulty of building the substrate structure will be further increased. In the past, the formula screening method used in structure construction research was too few, slow, and expensive. The future direction is to use digital technology to solve the problem during structure construction. At that time, substrate structure construction will be a digital, intelligent, and automated process.

There is also a common problem in the preparation of substrates in China, which is that the stability and continuity of raw materials are ignored, and some companies arbitrarily change raw materials and suppliers. Different raw materials, or even the same material, have different processing methods between different companies, and the technical indicators of the obtained materials will have differences that are visible or invisible. Using raw materials from different sources, even if it is based on the exact same formula, the technical indicators and quality levels will be significantly different, resulting in uneven and unstable function and quality between different batches, which greatly reduces the reliability and safety of the enterprise's products, thereby affecting the market reputation of the enterprise.

3. Modulation of properties
After the construction of the structure is completed, the next step is to enter the modulation stage of acid-base, conductivity, nutrients, and wettability.

Depending on the type of raw materials, the basic pH and conductivity values ​​are significantly different. The basic pH value and conductivity of some raw materials are very low, which leaves more room for modulating the properties of the substrate, and it is easier to adjust the pH value and conductivity of the substrate to the appropriate range of plants according to different plant needs. However, some substrate raw materials have high pH value and conductivity values, leaving too little space for substrate modulation, and it is difficult to meet the pH value and conductivity requirements of specific plants, so the properties and quality of the prepared substrate will not be very ideal.

pH is an index that characterizes the acid-base strength of the substrate. The ideal pH range for general purposes is between 5.2–6.2 and should be around 5.8 after wetting with water. The ideal pH for seeding and rooting substrates should be between 5.0 - 6.0, and the pH should be 5.6 after substrate wetting. Although the pH value of the substrate is slightly lower, the pH value of the substrate may increase due to fertilization or increase due to the high alkalinity of the irrigation water during the use of the substrate, so the pH value of the substrate is usually 5.6-5.8 is reasonable.

The suitable pH value of plants is mostly around 5.6-5.8, and the pH value of moss peat is mostly between 4.0-5.0, which is lower than the suitable space for plants and requires reasonable adjustment during substrate preparation. To adjust from pH 4.0-5.0 to pH 5.6-5.8, just mix well with the appropriate amount of dolomite and limestone powder. If herbal peat with pH 5.0-5.5 is used to prepare the substrate, the amount of lime will, of course, be reduced because the basic pH value of herbal peat is close to the target value of 5.6. If the basic pH value of the selected substrate material is higher than the plant requirement of 5.6-5.8, only a dilute acid solution can be used to adjust the pH value of the substrate. Due to the high price of various inorganic acids, dilution and spray corrosion of equipment, high operational risks, and strict safety control, it is difficult to achieve industrialization. Therefore, the selection of low-pH raw materials is a prerequisite for modulation. The basic pH value of crop straw-fermented products mostly exceeds 7, so straw and green plant-fermented products cannot independently prepare substrates. The German standard stipulates that the additional amount of straw or green plant fermentation material in the substrate should be less than 20%, and the maximum should not exceed 40%.

The effect of raw material on conductivity modulation is similar to its effect on pH modulation. The conductivity represents the level of salt content in the solution and also the level of absorbable nutrients in the substrate. The ideal conductivity of the universal substrate is between 1.0-2.0 mS/cm, and the ideal EC value of the seeding and cutting-raising substrates is between 0.5 - 1.1 mS/cm. The conductivity of moss peat is between 0.1-0.3 mS/cm, which is far from the ideal conductivity, leaving a large space for adjustment. Substrate companies can safely add additives such as fertilizers, limes, or conditioners according to the adaptability of plants without worrying about the conductivity exceeding the standard. Some substrate materials, such as cocopeat and coconut chunks, may contain highly soluble salts and high electrical conductivity. If they are not washed sufficiently beforehand, the electrical conductivity of the substrate may exceed the standard and cause plant root damage. The situation for piling bark is similar to that of cocopeat. They will release soluble substances due to aging and stacking, increase the conductivity, and cause the conductivity of the substrate to exceed the standard. It is best to wash it in advance. The electrical conductivity of straw and green plant fermented products mostly exceeds 1.5mS/cm, and they cannot be washed. Excessive addition of substrates will lead to excessive substrate electrical conductivity, so the usage must be limited.

4. Addition of bacterial agents
The global fourth-generation substrate is the addition of bacterial agents or biologically active components on top of the customized substrate. The use of biological agents or active microbial ingredients in combination with the substrate can effectively improve plant yield, quality, and output value and prevent crop diseases and insect pests. Some Bacillus inhibits root diseases caused by Fusarium, Pythium, and Rhizoctonia and protects plants from certain insects, such as toadstools and thrips, by inhibiting their food sources, which would otherwise pupate to be passed on to the next generation. Bioactive ingredients in the substrate can replace chemical fungicides to control root diseases. Some fungicides are beneficial fungi, which can form a symbiotic relationship between natural fungi and plants, improve water and nutrient absorption by expanding root growth, reduce transplant shock, and increases plant stress resistance. The addition of biological agents is the highest stage of modern substrate preparation technology, and it is also the development direction of the future substrate preparation technology in China.

For more information:
Meng Xianmin
China Humic Acid Industry Association Peat Industry Branch