**1. Introduction**

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 organic or low input farming systems.

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 conditions.

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 or intercropping, and cover crops [5].

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.
