(Adds farmer and weather forecaster quotes)
BUENOS AIRES, March 26 (Reuters) - Rains expected to relieve Argentina's drought-hit soy and corn crops failed to materialize over the weekend, all but ending hope that yields might recover from four months of unrelenting sun with more heat and dryness expected over the days ahead.
Adverse weather in the world's No. 3 soybean and corn supplier has put upward pressure on food prices, making it harder for poor countries to feed themselves.
Chicago soybeans rose on Monday as concerns over trade tensions between China and the United States eased, shifting attention back to crop damage in Argentina.
"We had a few showers this weekend, but none of them were important," said Francisco Abello, a partner in the TraulenCO SA farm management company. "Even if it rains going forward, yields are not going to change much. Most of our soybeans are done."
As of Monday Argentina looked increasingly dry over the 10 days ahead, said Isaac Hankes, a weather research analyst at Thomson Reuters' Lanworth commodities and weather forecaster.
"The warmth won't fully kick in until the April 6 to 10th period, but it could be intense and dryness is likely throughout the 10-day forecast," Hankes added.
Soy crop estimates started the 2017-18 season in the 55 million tonne range, but have been slashed to around 40 million.
Meteorologists had projected weekend rains of one to two centimeters (0.4-0.8 inch) in key farm areas of Cordoba, Santa Fe and Entre Rios provinces. But the showers never came.
"The forecasts let us down again," said Sofia Corina, farm analyst at the Rosario grains exchange.
"No matter how much it rains now, it won't help. Yields are fixed. The first soy to be harvested shows a lot of variability from two to five tonnes per hectare. Soy that was planted later in the season is more affected by the drought, and a lot of those fields will not be harvested at all," Corina said.
The first corn to be harvested has shown better-than-expected yields but later-planted corn was harder hit by the dryness. Farmers were expected not to try to harvest many of their later-planted corn fields, Corina added.
The scant rains that fell on parts of the normally fertile Pampas grains belt did nothing to relieve fields parched by unusually dry weather that started in November. The Southern Hemisphere autumn harvesting season started last week.
"We had practically no rain on our land over the weekend," said Pedro Vigneau, who operates a 1,400-hectare (3,500-acre) farm in the central Buenos Aires district of Carlos Casares.
Early planted soy benefited from good ground moisture at the start of the season, Vigneau said. But as the harvest continues, he said yields will fall as later crops, planted under drier conditions, are brought in.
This month the Rosario exchange slashed its soy crop forecast to 40 million tonnes from a previous 46.5 million while cutting its corn estimate to 32 million tonnes from 35 million.
The Buenos Aires Grains Exchange meanwhile cut its soybean harvest estimate to 39.5 million tonnes from 42 million and reduced its corn crop forecast to 32 million tonnes from 34 million tonnes. (Reporting by Hugh Bronstein Editing by Jeffrey Benkoe and Marguerita Choy)