THE REAGAN administration's tax bill promises to change, more or less at random, the basic terms of competition throughout American industry. The past week's brief squall among the business lobbyists, over minor changes in the depreciation schedules, is a useful reminder that there is more -- much more -- to this bill than the personal tax cuts. The business end of this bill a radical simplification of the depreciation schedules that will sharply tilt the relative profitability of different kinds of equipment, and different kinds of companies.

The law lets companies deduct from their taxable income the cost of plant and equipment, and the issue here is the number of years over which they must spread those deductions. The shorter the depreciation schedule, the larger the deduction each year and the lower the company's tax. Traditionally, tax depreciation has been at least roughly linked to the actual productive life of the equipment. The Reagan bill would abolish that link.

A truck's tax life is now commonly four years. Under the bill, it would drop to three. What about the truck's competitors? The present tax life of a river barge is 18 years, a freight train 14 years, and a pipeline 22 years. Under the bill, all would drop to five years, providing their owners with much larger tax benefits than the truckers would get. In this case, transportation policy will be made, more or less unconsciously, by tax policy.

The utilities will be big winners. A water company's plant is now generally written off in 50 years, a phone line in 35 years, a nuclear reactor in 20 years. All will drop to 10 years under the bill. The electric companies need the help, but the water and phone companies --which will get the bigger advantages -- do not.

This sudden redistribution of tax benefits, and profitability, is likely to incite a new wave of corporate marriages. Companies with cash will begin looking for merger partners with new wealth in the form of depreciation allowances to act as shelters.

The greatest defect in this bill is the irrational shower of benefits on real estate development and speculation. A shopping center's depreciation is now calculated case by case, with a normal life ranging from 30 to 60 years. The Reagan bill would give commercial buildings a uniform, "audit-proof" life of 15 years. Why this bonanza for the developers, in contrast with the much more modest benefits for high technology and the export industries -- electronics, aircraft, computers and agriculture?

The business end of this tax bill originated in a hasty and desperate attempt to offset some of the effects of inflation on investment. In practice it will overcompensate some business, undercompensate others and skew the tax advantages among them -- the precise dimensions of those inequities depending on future rates of inflation. It would have been entirely possible to design a simple depreciation system that was neutral, in regard to both inflation rates and differences among assets. Congress, voters and investors are well advised to note that the Reagan bill will be anything but neutral.