Maaser Sheni, Chapter 5, Mishnah 1. Chapter 5 of Maaser Sheni opens with the laws of kerem revai. Kerem revai, or neta revai, refers to the laws that apply in the fourth year after a fruit tree is planted. The halachah is that the fourth-year fruit must be brought up to Jerusalem exactly like Maaser Sheni, or alternatively it may be redeemed onto money and the money brought up to Jerusalem - again, exactly like Maaser Sheni. That is why these laws are placed side by side, since they share so many halachos in common.
The Mishnah opens with the laws of kerem revai - the fourth year of a vine in a vineyard. The Gemara records a dispute as to whether these laws were stated exclusively regarding grapes, or whether they apply to all fruit trees. Our Mishnah uses the term 'kerem revai', and there is room to discuss whether the law applies to neta revai in general. The specific concern of our Mishnah is how one identifies that a tree is in its fourth year, so that this should be public knowledge, and in that context we will also mention how burial places are marked.
The words of the Mishnah:
"Kerem revai - metzayenin oso b'kozezos adamah" - the grapes of the fourth year would be marked, meaning a sign was made for them, by placing clods of earth around them. These were distinct clods, recognizable as not being part of the ground itself but as having been deliberately placed there. The reason, according to the Gemara in Bava Kamma, is to symbolize that just as one derives no benefit from the earth until a process has taken place - sowing and the like - so too here, only after redemption may one benefit from the fruit.
"V'shel orlah - b'charsis" - in the first three years, when the fruit is entirely forbidden to benefit from, they would identify it by placing charsis - the clay used for making earthenware vessels. Just as one who plants in it grows nothing, so too this fruit is something from which no benefit may be derived.
"V'shel kevaros - b'sid, u'memacheh v'shofech" - burial places would be marked with lime, which is white in color. They would dissolve it and then pour it, since dissolving it made it whiter still. The white symbolized bones, which are white, and the entire marking came to indicate that there are bones there.
The opinion of Rabban Shimon ben Gamliel:
Rabban Shimon ben Gamliel says: "Bameh devarim amurim? Bishviis" - the need to mark the fruit for the sake of other people exists specifically in the seventh year, when others are permitted to enter the field and take from its produce. But in the other years of the cycle, what business do they have in a field that is not theirs? They do not belong there. To such a person we apply the principle "let the wicked one take and die" - that is his own problem, and we do not solve the problem for him so that he should not eat something forbidden. True, he does not know that it is forbidden to him, but he should not be eating from it at all, since he is stealing. Therefore we do not trouble ourselves to take care of it for him. But in shemittah, when he is permitted to take, it falls upon us to deal with the matter and make a sign.
The practice of the scrupulous:
The Mishnah continues and says that the tzenuim - and although we usually associate this term with modesty, the intent here is those people who were meticulous in mitzvos - would set aside coins. They were not satisfied with marking alone; rather, they would set money aside and declare: whatever will be gathered from this, meaning whatever any other person will gather from this vineyard in the fourth year, should automatically be redeemed onto these coins. They would then take the coins and eat with them in Jerusalem. In this way they themselves took care of the problem on behalf of those people: they did not merely hang up a sign, but actually resolved the difficulty that existed there, so that no problem would fall into the hands of others. This was conduct beyond the letter of the law.
Tosafos notes that the reason they said this specifically regarding those items that other people take, and did not simply declare that everything should be redeemed, is that they themselves might want to take some of the fruit and bring it up with them to Jerusalem, and therefore they did not want to redeem everything. Instead they declared that whatever is gathered by others should be redeemed automatically.
Opinions differ as to whether the practice of the scrupulous applied specifically in shemittah, or whether they conducted themselves this way even in the other years of the shemittah cycle, when people were not permitted to enter and yet the scrupulous continued their practice: according to some opinions they indeed did so, and according to other opinions they did so only in shemittah.
In summary: in this Mishnah we learned the ways of marking forbidden fruit and places of tumah - clods of earth for kerem revai, charsis for orlah and dissolved lime for graves, with each sign carrying its own symbolism. We examined the opinion of Rabban Shimon ben Gamliel, that marking is required specifically in the seventh year, when others are permitted to take from the fruit, and the practice of the scrupulous who acted in advance and set aside coins to redeem whatever would be gathered - conduct beyond the letter of the law - as well as the words of Tosafos explaining the reason for this and the question of in which years they conducted themselves this way.