Firstly: Nisab as a Condition for Charity on Crops
It is a condition for charity to be wajib on crops that it reaches its nisab, which is five, which is equal to 612kg. There is nothing payable in any amount less than that. That is the position of the majority: Malikis, Shafi`is, Hanbalis, Muhammad and Abu Yusuf the companions of Abu Hanifah, chosen by Ibn Hazm, and held by the majority of scholars.
Secondly: Charity Payable over what is Greater than the Nisab
Charity is wajib upon any amount that is greater than the nisab, regardless of whether it is significantly or barely over it. Al-Mawardi, al-Nawawi, and Ibn Hajar have quoted consensus over this.
Thirdly: Completing the Nisab by Joining Different Fruits of the Same Type Together
Fruits of the same type are joined together in order to reach the nisab. Consensus has been quoted on this by: Ibn Hazm, Abu al-Walid al-Baji, and Ibn Qudamah.
Fourthly: The Impact of Debt on Charity on Crops
Debt has no effect on charity being made payable on crops. This is the position of the majority: Hanafis, Malikis, Shafi`is, Zahiris, and a narration from Ahmad.
Fifthly: The Amount Payable
1. What is irrigated naturally and artificially
Whatever is irrigated from crops naturally, then the charity that is wajib from it is a tenth. Whatever is irrigated artificially (through the use of resources), then the charity that is wajib from it is half a tenth, or a twentieth. Consensus has been quoted over this by: al-Shafi`i, Ibn Battal, Ibn Hazm, al-Bayhaqi, Ibn `Abd al-Barr, Ibn Rushd, Ibn Qudamah, and al-Nawawi.
2. What is irrigated artificially some of the year and naturally the rest
That which is irrigated artificially during some of the year and naturally the rest will fall under one of two scenarios:
The first: is that the irrigation is natural for half the year and artificial for the other half. In such a case, the charity that is wajib is three quarters of a tenth. Ibn Qudamah and al-Shinqiti have quoted consensus over this.
The second: is that it is predominantly one or the other, in which case the ruling is based on the nature of the irrigation for the majority of the year. If the irrigation mainly takes place by rainfall or streams,  Sayh is flowing but flat water on the face of the earth. then a tenth is wajib. If it is primarily through some sort of spraying mechanism,  Nadih is when camels are used for irrigation. then a half of a tenth is wajib. This is the position of the Hanafis, Hanbalis, and one of two known positions of the Malikis. It is also a position among Shafi`is and some of the Salaf.
Sixthly: The Time Charity is Wajib
Charity becomes wajib at the appearance of ripeness in fruits,  Ibn Taymiyyah said: “The appearance of ripeness in fruits differs, sometimes it is through moisture after previous dryness, sometimes through softness after hardness, and sometimes by the changing of its colour to red, yellow, or white, and sometimes it does not change at all.” Al-Fatawa al-Kubra (53/4) and strengthening of grains. This is the position of the majority: Malikis, Shafi`is, and Hanbalis.