Size and makeup of the island
How large is your island? How many people are on it -- in the city and in the farmland surrounding? How is the land divided between urban spaces, farmland, canals/waterways, roads, etc., rural housing and so on? How deep is the soil? Is there bedrock below the soil layer or sand or air? What kind of soil makes up the farmland?
If 75% of the island is urban spaces + infrastructure spaces (roads, canals, etc.) then you won't have enough farmland to support the population. If the urban population is too high, you won't be able to grow sufficient crops to feed them.
If the soil is not good farming soil with proper water tables and sufficient depth, etc., then crops won't grow well. Soil is a complex combination of things that must maintain proper balances to function.
I can't see what technology level your island has. Farming is different based on the technology level. Medieval farming required crop rotation, fallow fields, and so on to support the population. Modern farming has a much higher yield per acre/hectare, because we use modern pesticides, herbicides, and fertilizers. And our crops, whether GMOs or through centuries of selective breeding, are much more nutrient-rich than they used to be. Yields have risen over time.
Again, technology levels play into how much land a given number of people can farm effectively. Medieval farmers worked by hand with the help of oxen. It is the foundation for the word acre, which was defined as the amount of land a farmer could till with an ox in one day. Modern farmers use tractors and can, with sufficient crews, manage farms covering thousands of acres.
If your have no oxen or equivalent and no machinery, then farming becomes a highly labor-intensive job. It will mean many hands are required to grow food. Without livestock or machinery, you may not be able to grow enough food to support an urban population -- everyone must farm for subsistence. But livestock require food too...
You also have to have sufficient land to grow multiple kinds of crops. If your island only eats corn, then malnutrition will eventually set in. So you must supply a variety of crops to give a balanced diet.
With no seasons, your farmers can always have a crop growing. With proper, centralized, planning, you can even stagger crops so that there are always fresh crops ready for harvest at any given time -- provided you have enough land and farmers available.
Fauna matter, too
At the very least, your crops will require bees or some equivalent species to cross-pollinate. Your island either requires insects as well, or requires some other means to pollinate your crops. (Bees also provide honey, which allows for mead...)
Your soil also requires various living organisms to support plant life. This ranges from bacteria to worms and other creatures that help transform nutrients, aerate the soil, etc.
water, water, everywhere
Your crops require water. Your world provides no rain. Therefore, water must get to the plants via irrigation. Irrigation has been in use for thousands of years by different cultures using different techniques, but your people will have no alternative to using this.
Your crops require nutrients. In ancient times, soil depletion was countered with crop rotation. This allows different crops to extract different nutrients. You will need to either supply massive amounts of chemical fertilizer, or use some combination of natural fertilizer with crop rotation. Otherwise, your soil will become useless within a few generations.