I agree with Vincent Wong's identification of "them", but want to discuss the clues, because I think this is important.
The main clue is that you are the land. The word of God is planted in the hearer, grows in the hearer, and so the quality of the soil depends on the hearer's own life.
This is clear to us now as personal faith is popular, but to people back then, "the land" was a synecdoche for Israel as a national and political body (this is still the interpretation of some).
Because they thought of the land in national and ethnic terms, they didn't understand the interpretation of the rocky or thorny soil, neither did they understand that seeds dropped into the land correspond to the Word entering their hearts.
This desire for national and political salvation was a series of endless misunderstandings, and one could argue it was the root cause of why the public didn't understand any of the other parables as well. As it says in Mark 4.13:
And he said unto them, Know ye not this parable? and how then will ye
know all parables? (KJV 1900)
It is also why Jesus kept preaching that the Kingdom of God was at hand, but they could only see that in terms of political revolution, not that the Kingdom was entering their hearts:
And when he was demanded of the Pharisees, when the kingdom of God
should come, he answered them and said, The kingdom of God cometh not
with observation: Neither shall they say, Lo here! or, lo there!
for, behold, the kingdom of God is within you. Luke 17:20–21 (KJV 1900)
In fact all of the disputes that Christ had with the Pharisees and other groups were that he kept insisting that the action was always in the heart, whereas they kept focusing on outward forms.
Matthew 5:28 (KJV 1900)
But I say unto you, That whosoever looketh on a woman to lust after
her hath committed adultery with her already in his heart.
Matthew 15.16-20 (KJV 1900):
And Jesus said, Are ye also yet without understanding? 17 Do not ye
yet understand, that whatsoever entereth in at the mouth goeth into
the belly, and is cast out into the draught? 18 But those things which
proceed out of the mouth come forth from the heart; and they defile
the man. 19 For out of the heart proceed evil thoughts, murders,
adulteries, fornications, thefts, false witness, blasphemies: 20 These
are the things which defile a man: but to eat with unwashen hands
defileth not a man.
But if they would have paid attention to the new covenant promised by Jeremiah, then it would have been clear where God would be intervening, and they would have understood all the parables.
31 Behold, the days come, saith the LORD,
That I will make a new covenant
With the house of Israel, and with the house of Judah:
32 Not according to the covenant that I made with their fathers
In the day that I took them by the hand
To bring them out of the land of Egypt;
Which my covenant they brake,
Although I was an husband unto them, saith the LORD:
33 But this shall be the covenant that I will make with the house of Israel;
After those days, saith the LORD,
I will put my law in their inward parts,
And write it in their hearts;
And will be their God,
And they shall be my people.
34 And they shall teach no more every man his neighbour,
and every man his brother, saying,
Know the LORD:
For they shall all know me,
From the least of them unto the greatest of them, saith the LORD:
For I will forgive their iniquity,
And I will remember their sin no more. (KJV 1900)
But once you understand that you are the land, everything else follows because the other terms in the parable had well known types in scripture:
the seed that comes from Heaven is the manna, and no one recognizes it. (Numbers 11.7: And the manna was as coriander seed, and the colour thereof as the colour of bdellium.) and yet the seed is also the promised one In Gen 3.15 that crushes the head of the serpent, and yet it is also the seed of David, which is Messiah, and the seed that is planted in the earth, that is, in you.
the thorns in the land are elements of the world not cleared from the land: Numbers 33:55 (KJV 1900)
But if ye will not drive out the inhabitants of the land from before you; then it shall come to pass, that those which ye let remain of them shall be pricks in your eyes, and thorns in your sides, and shall vex you in the land wherein ye dwell. And also 2 Sam 3.6: But the sons of Belial shall be all of them as thorns thrust away, Because they cannot be taken with hands
rocks in the ground correspond to hard hearts in which God's word cannot penetrate deeply: Zechariah 7:12 (KJV 1900) Yea, they made their hearts as an adamant stone, lest they should hear the law, And the words which the LORD of hosts hath sent in his spirit by the former prophets: Therefore came a great wrath from the LORD of hosts. and also Ezekiel 11.19: And I will give them one heart, and I will put a new spirit within you; and I will take the stony heart out of their flesh, and will give them an heart of flesh
So there was enough biblical imagery to explain the parable for all aspects except one -- understanding that you are the land. That's the secret one.