The best cultivable land during the Mughal period was known as "Polaj". Polaj was highly fertile land that was suitable for cultivation and yielded abundant crops. It was considered the most desirable land for agricultural purposes.

Here is an explanation of why "Polaj" was considered the best cultivable land during the Mughal period:

1. Definition of Polaj:
- Polaj refers to the land that was suitable for cultivation and could be sown and reaped in the same agricultural year.
- It was characterized by fertile soil, adequate irrigation facilities, and favorable climatic conditions, which allowed for the cultivation of multiple crops in a year.

2. Abundant Crop Yield:
- Polaj lands were known for their high productivity and yielded abundant crops.
- The fertility of the soil in these lands ensured that the crops received an adequate supply of nutrients, resulting in healthy plant growth and higher yields.
- The availability of water through irrigation facilities further supported the growth of crops and ensured a consistent water supply throughout the year.

3. Multiple Crops in a Year:
- The fertility and favorable conditions of Polaj lands allowed for the cultivation of multiple crops in a year.
- Farmers could harvest one crop and immediately sow another, maximizing their agricultural output and ensuring a continuous supply of food and other agricultural products.

4. Economic Importance:
- Polaj lands played a crucial role in the economic prosperity of the Mughal Empire.
- The surplus agricultural produce from these lands helped in meeting the food requirements of the growing population.
- The abundance of crops also contributed to the revenue of the state through taxation and trade.

5. Preference of the Mughal Emperors:
- The Mughal emperors recognized the significance of Polaj lands and took measures to promote their cultivation.
- They encouraged the construction and maintenance of irrigation systems to ensure a regular water supply to these lands.
- The revenue policies of the Mughal administration also favored the cultivation of Polaj lands, providing incentives to farmers and landowners.

In conclusion, "Polaj" was considered the best cultivable land during the Mughal period due to its fertility, high crop yields, ability to sustain multiple crops in a year, and its economic importance. The Mughal emperors recognized its significance and took measures to promote its cultivation.