**2. Comprehensive soil fertility and the functions of SOM**

## **2.1. Soil fertility aspects**

**1. Introduction**

conditions.

organic or low input farming systems.

290 Organic Fertilizers - From Basic Concepts to Applied Outcomes

or intercropping, and cover crops [5].

Organic or low input farm is a production system that sustains agricultural productivity by avoiding or restricting synthetic fertilizers and pesticides. It takes soil fertility, which governs the plant productivity of the soil, as a key measure in gaining an optimum yield from a longterm point of view. The establishment and maintenance of soil fertility is a major issue within

Incorporation of organic residues and manures are key approaches to many integrated soil management strategies [1], including that of nitrogen, one of the key plant nutrients in organically or low input managed farming systems. Green manure, manure, and litter from animal husbandry are considered as soil amendments and major mineral nutrient sources after mineralization. Increased soil organic matter (SOM) is a key issue in maintaining soil fertility and provides plant nutrients. Thus, SOM and N availability are important indices of soil fertility [2]. However, taking economic issues into consideration, industrial N fertilizer is of more benefit than biological N fixation in current agricultural management [3]. In 1987, James indicated that from 1960 to 1977 (during the "green revolution"), legume seed production declined dramatically from 170,000 to 70,000 tons worldwide [4]. Because planting legumes requires land, water, and other resources, the ability to fix N is limited by agricultural

However, due to the contribution to soil fertility through its effects on the physical, chemical, and biological properties of soils, the role of green manure has been rediscovered and is receiving more attention in soil fertility maintenance and enhancement by farmers, agrono‐ mists, and governments around the world. Under current conditions, several opportunities exist for the use of legumes in short-term situations, such as simple rotation, double cropping

The method of growing more than one agricultural species mixture together, as intercrops, is generally regarded as one measure to increase the productivity of crop systems. Cover cropping can reduce soil and water erosion, the process by particles detached from the soil mass are transported by running water and wind. Intercropping enhances ecosystem services including crop yield, N use efficiency, pest and weed management, and reduces nitrogen losses to the environment [6]. Thus, the method of intercropping with green manure is of interest in organic or low input farming systems, especially in non-animal husbandry farm systems.

Regarding the question of acceptable long-term productivity with major crop rotation or in‐ tercropping with legumes [7], this article discusses soil fertility and the functions of SOM, leguminous green manure as a source of SOM, and its capability to modify the C:N ratio of added organic matter. Increasing N availability and other plant nutrients, the efficiency of intercropping and living mulch, and soil and water conservation are also considered. The objective of the review is to present a way to maintain and enhance soil fertility with green

manure intercropping in an organically managed farming system.

Soil fertility is the crop productivity capacity of the soil due to the supply of plant nutrients and growth media. Long-term productivity can be taken into consideration instead of the yield in one growing season or year. Soil fertility includes sustainable availability and balanced forms of plant nutrients, soil water conservation, and aeration. It covers three aspects: physical, chemical, and biological properties. The physical property aspects mentioned in Table 1 [8] are related to soil texture and structure, which are related to the organization of particles and pores, reflecting effects on root growth, speed of plant growth, and water infiltration. Physical indicators include depth, bulk density, porosity, aggregate stability, texture, and compaction [9]. Loss of soil structure can occur through slaking and dispersion, often linked to intensive cultivation [10], compaction, and vital loss of the pore size distribution needed to maintain soil fertility [1]. Aggregates are the most profitable structural units of soil, offering water, air balance for root development, and the synthesis of complex organic compounds binding soil particles into structural units directly helps to build a loose, open, granular state with mediumto large-sized pores [11].

Chemical aspects include pH, salinity, organic matter content, phosphorus (P) availability, cation exchange capacity, nutrient cycling, and the presence of contaminants, such as heavy metals, organic compounds, and radioactive substances. These indicators determine the presence of soil-plant-related organisms, nutrient availability, water for plants and other organisms, and the mobility of contaminants [9].


**Table 1.** Physical, chemical, and biological soil indicators that may be included in a minimum data set for assessing soil quality.

Biological indicators include biomass of micro- and macroorganisms, their activities, and functions. Concentrations or populations of earthworms, nematodes, termites, and ants, as well as microbial biomass, fungi, actinomycetes, or lichens, can be used as indicators [9]. Soil biological properties are based on the soil being a living system; many kinds of organisms are involved in complex biological, chemical, and physical processes. A living soil is regarded as a healthy soil and favorable to plant growth because of the organisms' roles in soil development and conservation, specifically nutrient cycling and determining soil fertility.
