A field experiment was set up at the onset of the first rain season this year in early April. Two farmers were selected, constituting sites one and two. At each site, four treatments were set up: maize continuous (every season, maize will be planted in the same plots), maize rotation with grazing (in this plot maize will be planted, but in the next season, the plot will be left under grazing), bare ground (neither crop nor other plants, including weeds, will be allowed to grow), and grazing continuous (the plot will be under continuous grazing every season). At each site, each of the treatments was replicated twice with each plot measuring 10 m x 10 m. From the design, only four plots were planted with maize this season (maize continuous and maize/grazing). The variety of maize used was Longe 5. Prior to planting, all the plots, including the bare and grazing ones, were bracket fertilized with 120 kg ha-1 of NPK at a 17:17:17 ratio with a broadcasting method. At knee height, the maize was supplied with urea at a rate of 40 kg ha-1. The plots under continuous grazing and maize grazing rotation were fenced off with poles and barbed wires. What is being tested?

It seems to me that the only thing being tested here is the two different farmers.
When you get a question like this, you want to look for a single changing variable. If there are several changing variables, the experiment is going to be inconclusive. If there are no changing variables, you have no purpose to experiment. We can see that both farmers have the same setups. Each has two identical sets of four plots with the different types of farming methods. Every plot of land is being treated the same with identical doses of fertilizer prior to the test starting, and the crops grown are genetically identical. The only thing changing between cases here is the farmer's land. One might be in a mountainous area with rocky soil and dry climate. The other might be on a coastal plain with high salt content and intense heat and humidity. These two variables being tested are the root of the problem.
I would say that the thing being tested is the climate of the two farms in order to determine the most viable method of agriculture for them.