Activating Rice fallows in Eastern India
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Rice is grown in Kharif in Eastern India in 26.8 mHa, or 67% of total rice acreage. About 11.7 mHa remains fallow in the rabi or winter season. The unutilised lands are amenable to growing pulses and oilseeds, which in turn can reduce or even eradicate poverty and malnutrition in the region. According to this article, around 4 mHa can be brought into production (3 mHa pulses, 1 mHa oilseeds), with suitable policy interventions. Short duration crops like pulses (Chickpeas, Lentils, Lathyrus, moong beans, urad beans) and oilseeds (safflower, linseed, mustard) can be grown successfully, with life-saving irrigation. The choice of crop will depend on soil properties, among others. 

Scientists are working on how to "alleviate location specific constraints":
1. Extra early-duration and drought tolerant varieties of pulses and oilseeds are vital
2. Careful selection of crops and cropping patterns
3. Adoption of improved farming techniques - that aims to preserve soil moisture and reduce abiotic stress.
https://www.researchgate.net/publication/332343129_Rice_fallows_in_the_eastern_India_Problems_and_prospects

Outline of agriculture and rice fallows in E. India
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Eastern India is 22% of total geographical area of the country, has 38% of the total national population and produces 34.6% of total national food. There is enough rainfall at 1100-1200mm per annum, but cultivated land is only 45% of total or 33.6 mHa, with lower crop intensity (from 115-170%). As a result, per capita accessibility of cultivated land is the lowest (0.15 ha) in the country. Agriculture is much below its potential and so mass of people stay below poverty and have malnutrition.

Rice monocrop is widely practiced in Eastern state (82%) such as Chhattisgarh, Jharkhand, Upper Assam, Bihar, Eastern Uttar Pradesh, Odisha and West Bengal. Rice is grown for 180-210 days extending to more than 240 in WB! As there is little time & rains left for the winter crop, rice fallows are best suited for short duration (< 3 months) and low water-consuming rabi crops. Grain legumes and oilseeds are ideal as they can also improve soil health.

Why make a change (ie crop intensification)?
i) higher demand for food, and shortage of pulses and oilseeds
ii) lack of cropping area elsewhere
iii) need for higher farmers' incomes and diversification of incomes
iv) improvement in soil fertility due to nitrogen fixing
v) higher local fodder availability in rabi season will support livestock farming

Reasons for not using rice fallows (ie. vacant rabi season)
i) lack of irrigation & early withdrawal of monsoon causes moisture stress (rapid loss of soil moisture)
ii) late harvesting of rice (long-duration, high yielding rice varieties used)
iii) waterlogging and excessive moisture in Nov/ Dec
iv) stray cattle and blue bulls
v) series of abiotic and biotic stresses incl. poor soil structure

Summary: Lack of irrigation facilities and poor soil moisture retention cause severe abiotic constraints and are the main limiting factor in cultivating rice fallows. Soil compaction due to puddle rice cultivation badly alters the physical properties of soil which adversely affects soil moistures, seed-soil contact and root growth of a rabi crop. Ploughing dries out the soil even more. Many lowland regions have adequate soil moistures for growing lentil and lathyrus, and other pulses & oilseed.

Narrative: Much spatial and temporal variation is found in the rainfall pattern and water distribution - which leads to volatility in the farming. Rice is transplanted using puddle method, but this compacts the soil (ie. reduces macro-pores, which stops water percolation and increases evaporation), that frequently dries out and cracks by end of Kharif season. So there is not enough soil moistures for support the winter crops. Ploughing of these soils creates big clods (due to soil hardness) that dries the soil, slows seed germination, restricts root growth and decreases yields of the winter crop. Farmers are not able to afford irrigation and fertilizers expenses for rabi crop. Thus, a rabi crop in rice fallows must use existing (or residual) soil moisture & nutrients more efficiently (eg zero-tilling method). Conventional rice-pulse relay systems being practiced in Odisha, WB, Chhattisgarh and Jharkhand is not conducive to good pulses yields.