One category of wealth cannot be added to another category to complete the minimum threshold (nisaab)
Question

I have land on which there is wheat and barley. Do I have to give zakaah on it?
Praise be to Allah.Zakaah is due on grains and crops, if they reach the minimum threshold (nisaab), which is five wasqs. The wasq is sixty saa‘s, the saa‘ is four mudds, and the mudd is what can be scooped up in the two cupped hands of a man of average build. That is because of the report narrated by Muslim (979) from Abu Sa‘eed al-Khudri (may Allah be pleased with him), according to which the Prophet (blessings and peace of Allah be upon him) said: “No zakaah is due on grains or dates unless they reach five wasqs.”
Wheat and barley are two categories of wealth on which zakaah must be given, according to scholarly consensus. So if the land produces enough wheat or barley to reach the minimum threshold, then you must give zakaah on it. If the yield of wheat and barley does not reach the minimum threshold, but if one of them is added to the other it does reach the minimum threshold, you do not have to give zakaah, because you do not own the minimum amount of barley or the minimum amount of wheat.
To go into further detail: with regard to adding different categories of grains or crops together to complete the minimum threshold, there can only be two scenarios:
The first scenario is when the category is the same, but there are different varieties. In this case, they can be added together to complete the minimum threshold. So sukkari dates can be added to barhi dates and, by the same token, different varieties of wheat should be added together, and different varieties of raisins should be added together, and so on.
The fact that different varieties may be added together is supported by the general meaning of the hadith of Abu Sa‘eed al-Khudri that is quoted above. That is because the Prophet (blessings and peace of Allah be upon him) enjoined zakaah on dates as a category without further qualification, and it is well known that dates are of different varieties, but he did not enjoin separating one variety from another.
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (2/316): There is no difference of opinion among the scholars regarding the fact that all varieties of a category may be added to one another to complete the minimum threshold. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said in ash-Sharh al-Mumti‘ (6/73): Different varieties may be added to one another, so sukkari [dates] must  be added to barhi [dates]. The same also applies to wheat, so different varieties of wheat must be added together. End quote.
The second scenario is when the categories differ. In this case, they cannot be added to one another to complete the minimum threshold. So wheat cannot be added to barley, dates cannot be added to raisons, and rice cannot be added to wheat to complete the minimum threshold, because they are different categories, just as cattle cannot be added to camels or sheep, because they are different categories.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said in ash-Sharh al-Mumti‘ (6/73): One category cannot be added to another. So if a person has a farm, half of which grows barley and the other half wheat, and each of them reaches half of the minimum threshold, he cannot put them together, because they are of different categories, just as cattle cannot be put together with camels or sheep, because they are of different categories. End quote.
Based on that, whatever reaches the minimum threshold of the yield of wheat or barley that you have, you must give zakaah on it. Whatever does not reach the minimum threshold is not subject to zakaah.
And Allah knows best.