The Air Force will dig two ditches in the Arizona desert next year to test how well they would protect the deadliest missile under development.

But in the process, the Air Force promised without conscious irony yesterday, it will be sure to protect the rare cactus, bighorn sheep and lizards around the test site.

The service stopped short of making the same promise for rodents, however.

In an environmental impact statement, the Air Force assured the White House Council on Environmental Quality that it would dig with care, even to taking out the rare Suhuaro cacti at the site and turning them over to state horticulturists for "whatever disposition they recommend."

Conceding that "it is probably" that the bighorn sheep in the nearby Mohawk Mountains will get nervous once they hear the digging, the Air Force promised to keep things quiet January through April when "sheep are lambing and are most likely to be sensitive to disturbance."

The Air Force noted that although there are snakes and lizards where it wants to dig, rare species already are protected by law.

But as for the rodents, the Air Force statement said: "The rodents of the project site are the only group of mammals present that may be killed by construction activities" at the test ditches for the MX nuclear block-buster.

Whether to deploy the MX missile is the most controversial strategic weapon decision left for President Carter to make, now that he has canceled the B-1 bomber.

Putting MX missiles in covered ditches, or tunnels, would present the Soviet Union with more target area that their contemplated force of missiles could destroy, the Air Force states.

But it is still a question, the Air Force says, of how much it would cost to build the operational tunnels and how well missiles could be maneuvered inside them.

Therefore, the plan is to spend $20 million to build two test ditches next year on the Luke Air Force range near Yuma, Ariz. A dummy missile in one test will be raised from its wheeled carrier with enough force to punch a hole in the cement roof of the tunnel, the Air Force says in the environmental impact statement.

In a war, the MX would be fired through a hole punched in the roof of the tunnel. The MX, if produced, would carry about 12 H-bombs of 200 kilotons each and have unprecedented accuracy for destroying missile silos.

MX critics contend these attributes would make the missile a first-strike weapon in Soviet eyes. Therefore, they argue, Soviet leaders would be tempted to launch at the first, but perhaps false, sign of an American attack so their missiles would escape destruction.

Besides this possibility of putting a hair trigger on nuclear warfare and spending about $34 billion to produce and deploy 300 MX blockbusters, Carter will also confronted with the fact that very few communities would want ditches 13 to 20 miles long dug in their back yards.