Is There Zakah on Grains and Fruits?
Question

What is the Zakah on grains and fruits, and what is the Nisab (minimum threshold at which Zakah becomes due) for them?
Praise be to Allah.Zakah on grains and fruits
Zakah must be paid on grains and fruits according to scholarly consensus. Ibn Qudamah (may Allah have mercy on him) said in Al-Mughni (2/294): 
“The scholars unanimously agreed that Zakah is obligatory on wheat, barley, dates, and grapes. This was stated by Ibn Al-Mundhir and Ibn ‘Abd Al-Barr.”
The fact that Zakah must be paid on grains and fruits is indicated by the words of Allah, may He be Exalted (interpretation of the meaning): “but pay the due thereof (its Zakah, according to Allah’s Orders 1/10th or 1/20th) on the day of its harvest.” [Al-An`am 6:141] 
Zakah must be paid on grains and fruits that can be measured and stored , whether they are foods or otherwise, because of the report narrated by Al-Bukhari (1483) from ‘Abdullah ibn ‘Umar (may Allah be pleased with him) from the Prophet (blessings and peace of Allah be upon him) who said: “With regard to that which is irrigated by rainfall and springs, or has a taproot, (the Zakah) is one tenth, and with regard to that which is irrigated by drawing water, (the Zakah) is half of one tenth.” This Hadith is general in meaning and applies to everything that is produced by the earth, whether it is food or not. 
Muslim (979) narrated from Abu Sa`id Al-Khudri (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “There is no Zakah due on grains or dates unless they reach five Wasqs.” One Wasq equals sixty Sa`, and each Sa` is four Mudds, and a Mudd is what may be held in the cupped hands of man of average build. This indicates that what matters is measuring its volume using Wasq, which is a measuring unit. As for storing, that is because the blessing is only completed by that which could be stored, because the benefit thereof lasts for a longer time. 
Al-Buhuti (may Allah have mercy on him) said in Kashshaf Al-Qina‘ (2/205): 
“Zakah must be paid on every kind of fruit that can be measured or stored, such as dates, raisins, almonds, pistachios and hazelnuts.” 
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said in Ash-Sharh Al-Mumti` (6/70): 
“Zakah must be paid on grains and fruits , on condition that they can be measured and stored. If that is not the case, then no Zakah is due on them.”
What is the Nisab for grains and fruits?
It is not obligatory to give Zakah on grains and fruits unless they reach the minimum threshold (Nisab) , which is five ًasqs. Muslim (979) narrated from Abu Sa`id Al-Khudri (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “There is no Zakah due on grains or dates unless they reach five Wasqs.” 
The rate of Zakah to be given on crops and fruit varies according to the method of irrigation. If it is irrigated without any expenses, such as if it is irrigated by rainwater or springs, then the rate is one tenth. If it is irrigated with expenses, such as if it requires mechanical means of lifting up the water (such as water pumps), then the rate is one half of one tenth. 
The evidence for that is the Hadith of Ibn ‘Umar (may Allah be pleased with him) quoted above: “With regard to that which is irrigated by rainfall and springs, or has a taproot, (the Zakah) is one tenth, and with regard to that which is irrigated by drawing water, (the Zakah) is half of one tenth.” 
Al-Hafiz (may Allah have mercy on him) said: 
“has a taproot” – Al-Khattabi said: This refers to plants that take up water through their roots and veins, without being irrigated 
“drawing water” – this refers to using camels to carry water. The camel is mentioned by way of example, otherwise oxen and other animals come under the same ruling. This is equivalent to irrigation by digging irrigation channels nowadays. 
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said in Ash-Sharh Al-Mumti` (6/77): 
“The wisdom behind that is because there are many costs involved in that which is irrigated artificially, and few costs involved in that which is irrigated without artificial means, so the Lawgiver (Allah) considered these costs and reduced the rate of Zakah on that which is irrigated with extra costs.” 
Shaykh Ibn Baz (may Allah have mercy on him) said (14/74): 
“With regard to that which is irrigated by means of rainfall, rivers and springs, such as grains and fruits such as dates, grapes, wheat, and barley, the rate of Zakah is one tenth. As for that which is irrigated by machines and other artificial means, the rate is one half of one tenth.”
For more details, please see this category: Zakah on crops and fruits
And Allah knows best.