American Motors Corp. reported yesterday it lost $19.9 million in the second quarter of this year, a substantial improvement from its deficit of $88.1 million in the same period last year.

It also marked an improvement from the first quarter of this year, when losses mounted to $52.7 million.

AMC is the first of the domestic automakers to report second-quarter results, but it won't be alone in announcing better -- though still weak -- results.

General Motors Corp., Ford Motor Co. and Chrysler Corp. are expected to report during the next two weeks that they managed to increase profits or trim losses in the second quarter.

Earnings per share of common stock at Monsanto Co. surged to $2.35 in the second quarter of 1981, up from the 64 cents reported in the same period a year ago.

The 267 percent gain was termed "acceptable progress" by John W. Hanley, chairman and chief executive officer of the St. Louis-based chemical giant.

Monsanto reported consolidated net sales of $1.85 billion in the second quarter of 1981, up more than $3 million from the same period in 1980. The sales resulted in net income of $93.8 million in the second quarter of 1981, up from the net income of $23.2 million in the second quarter of 1980.