We'll talk about what type of grain is used for the menachot (meal offerings). The Mishnah says: “You should not bring grain that was taken from a field that requires fertilization,” because a field that requires extra fertilizer means the grain doesn't grow well on its own. Maybe not enough fertilizer was used, and the grain will be of inferior quality. So you shouldn't use grain from that type of field. You shouldn't use grain from a field that requires irrigation either. Some fields require only rainwater, but this field needs to be watered by sources beyond the rain. Therefore, it's possible that not enough water was given to this field, and the grain coming from it is of inferior quality. Also, you should not use grain that grows in a field with trees, because grain in these types of fields is of poor quality: the trees and their roots soak up all the water, and the grain doesn't get what it needs. But if somebody did bring grain from one of these fields for a Mincha (meal offering), the Mincha is still kosher (valid).
What does a person who wants to have a field that will produce very high-level grain do to prepare it properly? For the menachot, he plows the entire field the first year and only plants on half of it. He doesn't use half of the field; on the other half, he only plants in one half of it. In the second year, he plows the whole field again and only plants on the other half of the field 70 days before Pesach. The second planting is going to produce a lot of fine flour that is fit for a Mincha. What he does here is first plow the entire field, and everything that's growing there—all the roots and weeds—is dug up, so they won't use the nutrients of the soil whatsoever. The unplanted part of the soil then regains strength for a year, and it's only planted after a second plowing, which makes sure that the seeds take root in a very short period of time. So it's going to produce a lot of fine flour that is fit for menachot. What he does here is first plow the entire field, and a lot of fine flour that is fit for menachot is going to be produced. Since they didn't have anything growing there for a year, the strong soil that's there will give forth a beautiful product. Also, we said that they wait until 70 days after Pesach. Even though generally they plant earlier on in the winter, here there's a lot of sunlight 70 days before Pesach, enough to make sure that the crop is going to grow in a beautiful way. And they would switch of years. They would always use half of the field: one year, the other half; another year, in order to make sure that the soil has the ability to regain its strength after the nutrients were pulled from it one year at a time.
Ketzad hu bodek? The treasurer of the Beit HaMikdash was the person who bought all of the grain, and then eventually he sold it to anybody who wanted to bring a Mincha. So how did he examine the flour to make sure that it was considered to be at a high enough level to be offered in the Beit HaMikdash? HaGizbar machnis et yado letochah. The treasurer of the Beit HaMikdash put his hand into the flour. Alah bah avak pesulah ad sheyenapeh. Dust, which consists of tiny particles of very low-level flour, if it came up in his hand, made the flour invalid until he sifted it again to remove the dust. He would have a very fine sifter where he would put the flour, and the flour would stay in while all the small particles considered inferior would fall out. So he could buy it, but he had to make sure that he sifted it once more.
If the flour or the grain was wormy, it becomes invalid permanently. There's nothing he could do. Even after he removes the worms, it's still considered invalid. Just like an animal that has a blemish isn't allowed to be used for a korban (sacrifice), grain here that is blemished with worms is not respectful to use as a Mincha. So we consider this wormy type of food invalid for a Mincha.