Major reported crime in the District of Columbia decreased 4 percent last month compared to January 1981, Police Chief Maurice T. Turner Jr. said yesterday, crediting new police anticrime tactics and this January's bitterly cold weather for the decline.

However, there were five more homicides last month than in January of 1981--a total of 24, compared with 19 a year ago--adding up to a 26 percent increase that Turner said represented a jump in drug-related homicides that have accompanied increased heroin traffic on the District's streets.

At a police headquarters press conference, Turner said recently deployed police "plants" in commercial establishments, plus the longer-term effects of phony police fencing operations in the Washington area, contributed to significant decreases in reported robberies, burglaries and larcenies.

"The word is out" among street-wise criminals, said Turner, that convenience stores and other commercial targets are planted with shotgun-carrying police officers. Also, he said, arrests made in phony fencing rings in Washington and Fairfax County have made thieves more hestitant to burglarize dwellings and offices with the idea of selling stolen goods.

The January drop, which was the first monthly decline since last September, may have another cause: the weather. Record-breaking cold "played a big part" in keeping people--both criminals and their victims--off the street, Turner said in an interview after the press conference. It is a police rule of thumb throughout the country that severe weather, especially extreme cold, tends to blunt criminal activity.

The reported January decline comes after an increase of 7 percent in reported crimes in 1981 as a whole compared to 1980. That increase, however, was not as steep as annual increases in the two previous years, and Turner voiced cautious optimism that the figures in 1982 will "show a positive decrease."

The January figures released by Turner yesterday show that the total number of reported crimes--homicides, rapes, robberies, aggravated assaults, burglaries, larcenies and auto thefts--decreased from 5,611 in January 1981 to 5,390 last month, a drop of almost 4 percent.

Within that total, rapes, robberies, burglaries and larcenies all showed decreases, while the numerically smaller categories of homicide, aggravated assault and auto theft went up by varying amounts.

Robbery, the street crime most feared by citizens, according to police, decreased 9 percent from 1,024 to 932. Burglaries dropped 4 percent from 1,389 to 1,328; larcenies 5 percent from 2,624 to 2,484; and rapes 24 percent from 29 to 23, according to police figures.

Homicides increased 26 percent from 19 to 24, according to the figures, while aggravated assault went up 8 percent from 223 to 241 and auto theft 18 percent from 303 to 358.

Despite what Turner called hopeful signs in the January crime figures, he said illicit drugs, especially heroin, remain the "biggest motivator of crime" in the city. He estimated there are 16,000 to 18,000 heroin users in the District, and said many of them must steal to support their habits.

Drug traffic generates not only robberies and thefts but also homicides, Turner said. With the influx of heroin here in recent months, he said, there has been a noted increase in drug-related murders, and they now account for one-third of all homicides, far above normal.

Such slayings are particularly difficult to solve, Turner said. "Witnesses are few," he said. "The homicides themselves are more complicated . . . and on the whole it takes longer to get information."

Turner said drug traffickers may kill each other over nonpayment for heroin shipments, delivery of inferior goods and other disputes among dealers.

He noted that the department's special drug task force of undercover officers is still on the street and has made more than 1,889 arrests since last August. He acknowledged, however, that much of the crackdown's effect has been to "displace criminal activity," moving it from one section of the city to another rather than reducing it.

On the positive side, he said, police have arrested and successfully prosecuted a number of major drug dealers in recent months and hope to snare still more, a move police hope will stem the flow of heroin into the city.

Also, said Turner yesterday, in an effort to bolster street patrols, he plans to transfer about 100 officers from administrative jobs to street duty in the next few weeks.