* Incessant rains follow worst drought in 25 years
* Staple maize and tobacco worst hit by rains
* Fertiliser shortages add to maize farmers' problems
By MacDonald Dzirutwe
HARARE, Feb 1 (Reuters) - Heavy rains have battered crops in Zimbabwe, threatening its food staple, maize, and its cash tobacco crops just months after the worst drought in a quarter century, the head of the commercial farmers' union said on Wednesday.
More than 4 million Zimbabweans need food aid until the end of March because of the 2016 drought which also scorched crops in other southern African countries.
Wonder Chabikwa, president of the Zimbabwe Commercial Farmers Union, said incessant rains had caused water logging and leaching in light soils, washing away nutrients that crops need.
Chabikwa said maize and tobacco, the biggest export earner ahead of gold and platinum, were most affected by leaching.
"Leaching is taking its toll on the crops and there is also false ripening of the tobacco crop," Chabikwa told Reuters, referring to premature maturing of tobacco.
"Maize (plants) in sandy soils are turning yellow and this deficiency in nutrients means the crop cannot have a cob," he said.
Zimbabwe's Civil Protection Unit says floods have hit homes and crops across the country, including houses built on wetland in the capital Harare.
The Ministry of Agriculture said it was carrying out its first crop assessment, which would be completed by the end of February.
President Robert Mugabe's government has set a target of 2 million tonnes of maize under a special programme, but heavy rains and shortages of the nitrogen-giving ammonium nitrate fertiliser could scuttle the plan, Chabikwa said.
He said farmers queued for days outside the premises of fertiliser companies, who blamed the shortages on lack of foreign currency to import ammonia.
An acute shortage of foreign currency has left companies, including mines, struggling to pay for imports.
"We could have had a bumper crop and done away with maize imports in just one season if only we could sort out the fertiliser situation," said Chabikwa.
He said early planted maize that had fallen prey to the fall armyworm - an invasive Central American species that is harder to detect and eradicate than its African counterpart - had recovered after farmers used pesticides.
Chabikwa said, however, that this year's maize crop was still expected to be higher than last year, when only 511,000 tonnes was harvested against national requirements of 1.8 million tonnes.
(Editing by James Macharia/Ruth Pitchford)