Study on Improvement of Soil Porosity

Xiong YAN, Wenzhao LI*, Xuhui WANG, Qiuyan WANG, Ling YANG, Jun ZHAO

1.Zunyi Normal University, Zunyi 563006, China; 2.Key Laboratory of Soil Resources and Environment Characteristics in Northern Guizhou Province, Zunyi 563006, China; 3.Guizhou Qianbei Granary Meters Co., Ltd., Zunyi 563006, China

Abstract In order to further improve soil porosity and improve soil physical properties, the raw materials, formula and application methods of soil amendment were optimized, and the pH and porosity of yellow soil were compared and analyzed after application of soil amendment.The results showed that the application of amendments increased soil pH and porosity.The results will provide the scientific basis for the improvement of soil porosity.

Key words Soil, Porosity, Amendment

1 Introduction

Soil is a porous body, and there are pores between soil particles and soil aggregates as well as in the aggregates.Soil pores are channels for water and gas migration in the soil, as well as sites for activities of soil organisms, and play a key role in soil physical, chemical and biological processes[1-3].The percentage of soil porosity per unit volume is called soil porosity.At present, the soil porosity is mainly improved by applying soil amendments.Soil amendments refer to compounds that can improve physical, chemical and biological properties of soil[4], improve physical properties and promote nutrient absorption of crops[5], such as lime, mineral and industrial by-products and biochar[6].However, powder is a current product form of amendments, which is not suitable for mechanized application, but can only be broadcasted artificially, with uneven dosage.Therefore, in order to overcome the shortcomings of the existing amendments, this study intended to develop a three-dimensional soil amendment, in order to effectively increase the porosity of soil.

2 Materials and methods

Phosphogypsum, bentonite and vermiculite were selected as the raw materials of soil porosity amendment, then mixed with fungus bran or starch and processed into three-dimensional amendment.The optimal size was selected.

3 Results and analysis

3.1 Screening of amendment raw materialsAfter several screening, phosphogypsum was selected as the main raw material, supplemented with bentonite and vermiculite.

Phosphogypsum, mainly consisted of calcium sulfate dehydrate, is a by-product in the production of compound phosphate fertilizer, and it is a renewable resource that can replace natural gypsum.However, the comprehensive utilization rate of phosphogypsum in China is only 20% because of the large yield and limited comprehensive utilization.The calcium ion rich in phosphogypsum exchanges the sodium ion in the soil to regulate the acility and alkalinity of the soil.Meanwhile, phosphogypsum has little influence on the chemical properties of surface phreatic water, but only plays the role of irrigation and drainage.As a result, the surface phreatic water is diluted, and the salinity of phreatic water is reduced by 39.62%, which can effectively improve the porosity of soil.In addition, the utilization of available nutrients such as sulfur, phosphorus and magnesium contained in phosphogypsum can effectively improve the physical and chemical properties of soil, and achieve the purpose of increasing crop yield and income.

Bentonite has a water absorption rate of 100%-240%, and its volume can increase by 10-30 times with the water absorption.Applying bentonite to soil can increase the number of soil aggregates, reduce soil bulk density, and increase soil porosity.Soil aggregate is the basic unit of soil structure, affecting the porosity and water holding capacity of soil, and its composition and stability directly affect the soil fertility and the growth of crops.Therefore, bentonite can improve the water holding and air permeability of soil.

Vermiculite can be used as the soil amendment.Because of good cation exchange and absorbability, it can improve soil structure, store water and moisture, improve soil permeability and water content, and make acid soil become neutral soil.Vermiculite can also act as a buffer, preventing the rapid change of pH, causing the slow release of fertilizer in the medium of crop growth, and allowing slightly excessive use of fertilizer without harm to plants.Besides, vermiculite can also provide K, Mg, Ca, Fe and trace elements such as Mn, Cu, Zn to crops.Because of water absorption, cation exchange and chemical composition, vermiculite plays the role of fertilizer retention, water retention, water storage, air permeability and mineral fertilizer.

3.2 Preparation method of amendmentBy mixing the three components of phosphogypsum, bentonite and vermiculite, the three-dimensional soil amendment was made, and its size was ensured to be 3-7 cm.Modified phosphogypsum was used as the basic framework, and a small amount of bentonite and vermiculite were mixed and distributed on the basic framework.There were several cone-shaped bumps on the surface of the three-dimensional soil amendment, which were made of fungus bran and adhered to the surface of soil amendment by adhesive.The adhesive was composed of 5-10 copies of attapulgite clay powder.The rough formula of soil amendment was as follows: 200-500 copies of modified phosphogypsum, 50-100 copies of bentonite, 30-50 copies of vermiculite and 30-50 copies of fungus bran, according to the weight of raw materials.In specific preparation, modified phosphogypsum powder, bentonite powder and vermiculite particles were mixed with water and processed into three-dimensional form.Then the block fungus bran was adhered to the three-dimensional surface by adhesive to get rough billets.The processed rough billets were dried at 90-120 ℃ until the moisture content was less than 10% to get soil amendment.The modified phosphogypsum was prepared by breaking phosphogypsum and removing impurities such as organic matter and soluble impurities in phosphogypsum.As a result, the modified phosphogypsum powder was obtained, achieving the purpose of reducing heavy metals and other harmful components of phosphogypsum.

3.3 Preparation and application of amendment

3.3.1Preparation.Two amendments(A1 and A2)and control(CK)were set according to different proportions of raw materials.The specific proportion of A1 was: 200 copies of phosphogypsum, 50 copies of bentonite, 30 copies of vermiculite, 40 copies of fungus bran and 6 copies of attapulgite clay powder.The specific proportion of A2 was: 400 copies of phosphogypsum, 70 copies of bentonite, 40 copies of vermiculite, 45 copies of fungus bran and 9 copies of attapulgite clay powder.The specific proportion of control was: 500 copies of phosphogypsum, 100 copies of bentonite, 50 copies of vermiculite and 10 copies of starch.

The phosphogypsum was broken, and impurities such as organic matter and soluble impurities in phosphogypsum were removed to get powder of modified phosphogypsum.The fungus bran was crushed into lumps below 1.5 cm, and the attapulgite clay powder was mixed with water to form adhesive.The modified phosphogypsum, vermiculite powder and bentonite particles were evenly mixed with water, and loaded into a sphere mould with the diameter of 3-5 cm.After demoulding and predrying treatment, the fungus bran was dipped in the adhesive and glued to the sphere after demoulding, so that its surface had a number of bumps.The rough billets obtained was dried at 90-120 ℃ until the moisture content was less than 10% to obtain the soil amendment.

3.3.2Application.The soil amendment was evenly broadcasted on the soil surface at the dose of 500 kg/667 m2, and then ploughed under to great depth, with the ploughing depth greater than 40 cm, ensuring that the soil amendment was plowed into the soil below 20 cm to get the plough layer.The soil was watered thoroughly after the soil surface was flat, and crops were planted after 30-45 d.

3.3.3Application effect.Yellow soil, a representative soil type in Zunyi, Guizhou Province, was selected to measure and analyze the pH and porosity of soil before application of soil amendment, 3 months after application of soil amendment, and 1 year after application of soil amendment, respectively, and the results are shown in Table 1.

Table 1 Changes in pH and porosity of soil after application of soil amendment

Through the changes before and after application of soil amendment, it could be concluded that soil amendment improved the soil pH and was suitable for the demand of plant growth.Meantime, the porosity of soil was greatly improved, and high porosity could be maintained even after a year.However, there was no fungus bran bumps on the surface of CK, and there was little difference in pH after application of soil amendment, but the increase of porosity was significantly lower than that in A1 and A2.

4 Conclusions

In this study, modified phosphogypsum, bentonite and vermiculite were mixed and processed into three-dimensional form with a size of 3-7 cm, and evenly broadcasted on soil surface at the dose of 500 kg/667 m2.Afterwards, the soil was ploughed under to great depth, with the ploughing depth greater than 40 cm.The soil was watered thoroughly as the soil surface was flat, and crops were planted after 30-45 d.After application, it was found that the soil pH was improved and the soil porosity was increased, and soil porosity could be maintained for a long time, so as to achieve the functions of increasing fertilizer, fertilizer retention, water retention and air permeability.