## 1 Introduction  
### Background and Motivation  
Agriculture is the foundation of and is essential for human survival. The total global population has exceeded 7.5 billion, and it is expected to reach 11 billion by 2100 [1]. By the end of 2021, 702 million to 828 million people in the world will face hunger [2]. With the development of human society and economy, limited resources cannot meet the needs of the ever-increasing population [3]. Under the influence of multiple factors, such as nature and social economy, productivity declines, land degradation, soil erosion, and other problems are emerging sequentially, and mankind is facing severe challenges [4, 5]. As a natural resource for agricultural production, soil is a natural complex that contains a variety of ingredients, which provide necessary moisture and nutrients for plant growth [6, 7]. Soil organic matter (SN) comprises essential nutrients provided by the soil for plant growth. Mineral nutrients in the soil can be absorbed by plant roots directly or after transformation. These include 13 elements, such as nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, boron, molybdenum, zinc, manganese, copper, and chlorine [8, 9]. Generally,the contents of nitrogen, phosphorus, and potassium are used as an index to measure soil fertility. Fast and accurate monitoring of SN content plays an important role in research fields, such as precision fertilization of farmland [10,11]. Soil moisture (SM) changes have an important impact on the physical and chemical properties of the soil and the growth and development of vegetation [12,13]. The accurate control of the temporal and spatial distribution and changes of soil moisture is in the monitoring of crop growth, agricultural moisture, and soil drought. The field is of great significance. SN mainly comes from plants, animals, and microbial residues, among which higher plants are the main source. The organisms that first appeared in the parent material in the original soil were microorganisms. With the development of biological evolution and soil-forming process, animal and plant residues and their secretions became the basic source of SN [14]. In natural soil, ground vegetation residues and roots are the main sources of SN, such as trees, shrubs, grasses, and their residues, which provide a large amount of organic residues to the soil every year [15]. In agricultural soil, the source of SN is wide, mainly including crop stubble, straw returned to the field, and green manure turned over; the manure of humans and livestock; leftovers of industrial and agricultural by-products (such as distillers' grains, ammonium sulfite papermaking waste liquid, etc. ); urban domestic garbage and sewage; the remains and secretions of soil microorganisms and animals (such as earthworms, insects, etc. ); and various artificial organic fertilizers (manure, humic acid fertilizer, sludge, soil miscellaneous fertilizer, etc.) [16]. Among them, there is no natural vegetation in the cultivated soil, which mainly comes from the exudates of crop roots, root stubbles, dead leaves, and organic fertilizers (green manure, compost, compost and manure, etc.) that people apply every year [17]. Soil moisture is the main source of water absorption by plants (except for hydroponic plants). In addition, plants can also directly absorb a small amount of water that falls on the leaves [18]. The main sources of soil moisture are precipitation and irrigation water, which participate in the large water cycle of the lithosphere-biosphere-atmosphere-hydrosphere. At present, mudslides, earthquakes, and droughts are common [19]. Among them, drought is the most serious because it leads to the loss of land and water resources. Compared with other natural disasters, the scope of drought caused by soil moisture loss is wider, and the experience lasts longer [20]. It severely damages the regional ecological environment and affects the normal economic production and life of human beings.