  We show that the matrix query language $\mathsf{MATLANG}$ corresponds to a
natural fragment of the positive relational algebra on $K$-relations. The
fragment is defined by introducing a composition operator and restricting
$K$-relation arities to two. We then proceed to show that $\mathsf{MATLANG}$
can express all matrix queries expressible in the positive relational algebra
on $K$-relations, when intermediate arities are restricted to three. Thus we
offer an analogue, in a model with numerical data, to the situation in
classical logic, where the algebra of binary relations is equivalent to
first-order logic with three variables.
