Menachos Chapter 8, Mishnah 2. The Mishnah discusses the ideal places from which a person should bring his grain for making the menachos, and how the field is prepared and the fine flour is inspected.
The places from which one does not bring:
"Ein mevi'in lo mibeis hazevalim" - one may not bring the grain from a place that has been fertilized with manure. According to the Bartenura, the reason is that the manure itself is liable to give the grain a bad taste and smell. Others explain that fertilizing indicates that the soil itself is not of high quality and needs a supplement, and perhaps the supplement was not given properly; therefore it is preferable to bring from a place that needs no supplement at all, where the grain grew without any fertilizing.
"Velo mibeis hashelachin" - a similar idea: one may not bring the wheat from a place that requires irrigation, such as through water channels, since there are places that grow on natural rainwater alone, and that is the proper nourishment. When the grain comes from an irrigated place, there is reason to suspect that the soil is not sufficiently moist, and because of this the grain will not be as strong and flavorful as it should be.
"Velo mibeis ha'ilan" - one may not bring from a place where trees grow close to the grain, such as an orchard. The reason is that the trees draw the nutrients out of the soil for themselves, and at that cost the grain is not sufficiently nourished. This applies primarily to wheat, and to barley as well.
"Bedi'avad kasher" - if this is the place from which he received his wheat, and likewise in the case of the barley minchah of the sotah, the minchah is valid. After the fact there is no disqualification, for as long as it is fine flour - it is fine flour.
How the field is prepared:
What is the ideal way to prepare a field for someone who wishes to grow grain, and especially the barley for the minchah of the Omer? The Mishnah says: "Nir shanah rishonah, uvashniyah zore'a" - in the first year one only plows the field, and in the second year one sows it.
In practice the field is divided into two parts: in the first year only one side is sown, while the other side is left fallow - merely plowed, with the weeds and everything else growing there and drawing the soil's nourishment removed from it. In the second year they switch: the barley is sown in the place that lay fallow the previous year, after it has recovered and gathered additional nourishment, and the other side is left fallow. And so they alternate back and forth, and the strongest grain is produced.
Furthermore: "Zore'a kodem lePesach shiv'im yom" - one should sow seventy days before Pesach. This applies especially to the barley used for the Omer offering, for sowing it seventy days before Pesach is the finest. Wheat, by contrast, is properly sown at the end of the winter and not at the beginning of the spring.
"Vehi osah soles merubah" - sowing in this manner produces the finest fine flour, both in the quality of the flour and in its quantity.
The treasurer's inspection:
How does the treasurer, the official appointed on behalf of the Beis HaMikdash, choose flour fit for purchase? The treasurer puts his hand into the vessel in which the flour was collected. "Alah bo avak" - if dust rises from his hand as he withdraws it, this is not fine flour but flour dust, and it must be disposed of, and therefore "pesulah" - it is disqualified. Certainly actual dirt disqualifies, but even mere dust - whether dust from the air or noticeable dust of the flour itself - disqualifies.
As was explained in the previous Mishnah, one must sift with a sieve whose holes are the smallest and finest, in order to remove the actual dust, and what remains is what serves as the fine flour. Dust cannot be mixed into it, since it is too fine and is not fine flour. Therefore it is disqualified "ad sheyenapenah" - until he sifts it and removes the dust, leaving the rest of the fine flour that is used for the minchah.
"Hitli'a - pesulah": If the grain became worm-infested, it is unfit. The Bartenura explains that this refers to the fine flour, that is, the ground flour, or to the wheat itself - the kernel before it is ground into flour - which has become disqualified and is no longer fit for use. However, this applies specifically when most of it became infested; but if only a small part of it became infested, such as a few worms, one can sift them out and use the rest, and the minchah is valid.
In summary: In this mishnah we learned the places from which grain may not be brought for menachos - a field fertilized with manure, an irrigated field, and a field with trees - and that after the fact the minchah is valid; the method of preparing the field by plowing it in the first year and sowing it in the second, seventy days before Pesach, which produces an abundance of fine flour; the treasurer's inspection, that if dust rose onto his hand the flour is unfit until he sifts it; and likewise the law of grain that became worm-infested.