and GOD-FREY and NICHOLS, Justices, dissenting.
We would affirm the judgment of the Superior Court, which upheld the Board of Environmental Protection’s denial of Va-lente’s topsoil mining permit.
Section 484 of the Site Location Law states that the Board
shall approve a development proposal whenever it finds that:
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3. ... The development will not adversely affect existing uses, scenic character, or natural resources in the municipality or in neighboring municipalities.
Each of the specific criteria listed in section 484 is “mandatory”; to obtain a site permit, a developer must meet them all. In re International Paper Co., Androscoggin Mill Expansion, 363 A.2d 235, 240 (Me.1976). Failure by an applicant to “affirmatively demonstrate compliance ... as to any one of them constitutes a basis for denial of the application.” In re Maine Clean Fuels, Inc., 310 A.2d 736, 741 (Me.1973) (emphasis in original).
The Board in this case identified the entire 40-acre parcel that Valente wished to strip-mine as “prime cropland” because of the high quality of its topsoil, and found that the parcel had “a long history of successful agricultural use.” The Board also found that Valente’s land had good drainage, resulting in a relatively long frost-free growing season, but that removal of the topsoil would significantly reduce the land’s *721water holding capacity and rooting depth.1 Finally, the Board found that there were only 142 acres of cropland (in parcels of 10 acres or more) in recent production in the town of Poland, including the 40 acres Va-lente proposed to mine for topsoil. Based on those findings, the Board determined that:
The stripping of 12 to 18 inches of loam from the site will drastically reduce the ability of the property to grow food stuffs, thus substantially reduce [sic] the base' of farmland available for commercial agriculture in a rural area. The loam removal will adversely affect existing and potential agricultural uses of the site in the local farming community, and will eliminate substantial natural farmland resources important to the municipality and neighboring municipalities.
By finding that Valente’s proposed mining of topsoil would “adversely affect existing ... agricultural uses ... and [would] eliminate substantial natural farmland resources,” the Board held by clear implication that, within the meaning of section 484(3), farming is an “existing use” of the site and topsoil or prime cropland is .a “natural resource.” We agree with the Board’s interpretation of the terms used in section 484(3).
Nothing in the language of the Site Location Law suggests that agriculture does not warrant Board protection as an “existing use” or that topsoil is not statutorily protected as a “natural resource.” Without any contrary indication appearing on the face of the law or in its legislative history, a court must give the words used in a statute their ordinary and plain meaning. See Franklin Property Trust v. Foresite, Inc., 438 A.2d 218, 222 (Me.1981). A natural resource such as the topsoil on the limited prime cropland in the town of Poland and its existing use for farming are in terms mandated for protection by the unambiguous language of section 484(3) of the Site Location Law. In view of the findings of fact made by the Board, setting forth the drastic adverse consequences that would result from the proposed topsoil stripping, the Board acted well within its statutory authority in holding that the project did not qualify for a development permit.
By a process of statutory construction, the court’s opinion (p. 719) reads the protections of section 484(3) to apply only to “existing uses” and “natural resources” in an area outside the development site itself. That holding considerably limits the effectiveness of the environmental protection laws. Strangely enough, it means that the larger the area that a proposed development encompasses, the more likely is its approval. Also, it means that a rare resource located only within the confines of a proposed development site could be completely destroyed or removed without any regulatory scrutiny by the Board of Environmental Protection. We do not find that statutory construction either reasonable or necessary.
Valente has argued that the Board’s interpretation and application of section 484(3)’s language in this case would lead to unreasonable results never intended by the legislature. We see no such danger. Contrary to the premise underlying Valente’s argument, court approval of the Board’s decision in this case will not give the Board the green light to turn down any future development proposal that has only the incidental and minimal effect of taking some cropland out of production. The Site Location Law does not authorize the Board to deny a permit whenever a proposed development will have any “adverse effect” upon the natural environment; rather, the Board *722has been vested with the “discretion” to “insure that such developments will be located in a manner which will have a minimal adverse impact on the natural environment of their surroundings.” 38 M.R.S.A. § 481 (emphasis added). See also Brennan v. Saco Construction, Inc., 381 A.2d 656, 661-62 (Me.1978); In re International Paper Co., Androscoggin Mill Expansion, 363 A.2d at 240. Furthermore, section 484(3) specifically directs the Board to limit its concern to a proposal’s adverse effect on existing uses or natural resources “in the municipality or in neighboring municipalities.” In the light of that statutory language. The Board acted properly in turning down a topsoil excavation project that would render over 28 percent of the town of Poland’s prime cropland nearly useless for agriculture for an indefinite period of time; on the other hand, the Board is not empowered to protect prime cropland from any and all developments that might have an insignificant, minimal, or remote adverse effect upon the topsoil natural resource or upon the existing agricultural use of land in the affected municipalities.
Nothing in the Board’s own regulations prohibit it from denying the topsoil mining application on the ground that it adversely affects the existing agricultural use and the natural resource of prime cropland. In 1979, pursuant to 38 M.R.S.A. § 343(1),2 the Board promulgated regulations designed to “explain and clarify” the meaning of section 484(3). Me.Dept. of Env.Protection Regs., ch. 375 (Nov. 1, 1979). The regulations list fifteen “specific areas of concern” that the Board will consider when it determines whether an applicant has met the requirements of section 484(3), detailing the Board’s policy and procedure as to each area. Farmland conservation is not mentioned as an area of concern for the Board,3 although an early draft of the regulations included several paragraphs indicating that the Board recognized “the increasing importance of protecting prime agricultural lands.” Yalente contends that the Board, because it considered the inclusion of farmland conservation among its listed concerns under section 484(3) but then promulgated regulations that are silent on the issue, is now foreclosed from denying a permit application on farmland conservation grounds. We are not impressed with that argument. To foreclose the Board in such a manner by the alleged negative implication flowing from an omission from its regulations is inconsistent with the Board’s continuous duty in adjudicatory proceedings to enforce the statutory criteria protecting existing uses and natural resources in the municipalities involved.
It is true, of course, that an agency is ordinarily bound by its own “legislative” rules and regulations.4 See 2 K. Davis, Administrative Law Treatise § 7:21, at 98-99 (2d ed. 1979). In this case, however, the Board did not violate the regulations it promulgated in 1979 when it denied Va-lente’s topsoil mining application on farmland conservation grounds. Those regulations by their terms were not exhaustive or exclusive. The Board’s own accompanying note stated plainly that the regulations were intended only to cover “several specific areas of concern” that “the Board has identified.” That language was enough to put permit applicants on notice that the Board considered itself free, in ruling on *723future permit applications, to take cognizance of factors not yet administratively identified as legitimate criteria under section 484(3). Like its predecessor, the Environmental Improvement Commission, the Board of Environmental Protection is ordinarily free to apply the Site Location Law “on a case-by-case basis ... under the guidance of the explicit criteria of the statute.” In re Spring Valley Development, 300 A.2d 736, 754 (Me.1973). While a valid regulation promulgated in a quasi-legislative proceeding may not be violated or ignored in an adjudicatory context, the Board’s authority to make adjudicatory decisions “under the guidance of the explicit criteria of the statute” is not limited by the existence of nonexhaustive regulations such as those at issue here.
The fact that the Board apparently made a conscious decision not to include “protecting prime agricultural lands” among its listed “areas of concern” would not alter our analysis. Administrative silence, like legislative silence, means different things in different contexts; an administrative agency may have many reasons for deciding not to promulgate regulations on a particular statutory subject.5 “Whether the expression of one thing is to operate as the exclusion of another, is ordinarily a question of intention, to be gathered from an examination of all parts of a statute by the aid of the usual rules of interpretation.” City of Portland v. New England Telephone and Telegraph Co., 103 Me. 240, 249, 68 A. 1040, 1043 (1907). Examination of all parts of the Board’s 1979 regulations convinces us that those regulations were not intended to exclude from Board consideration under section 484(3) factors other than those listed in the regulations themselves. Consequently, the Board was free to take into account, when considering a development permit application, any factors that come within the scope of the statute.
We conclude that the adverse effects of a development upon the existing agricultural use of a site and upon the total amount of prime cropland and topsoil resources “in the municipality” come within the scope of 38 M.R.S.A. § 484(3). It is, therefore, our view that the Board did not abuse its discretion in denying Valente’s topsoil mining application.
. The Board’s finding of fact as to the effect of the topsoil mining on the 40-acre lot is as follows:
A very firm hardpan is present throughout the site at a depth ranging from 18 to 36 inches from the surface. Removal of 12 to 18 inches of the fine sandy loam leaves a thin veneer of original soil over the hardpan. The seasonal water table which is perched over the hardpan will be significantly closer to the soil surface. The soil’s available water holding capacity and rooting depth will be significantly reduced.
. 38 M.R.S.A. § 343(1) (1978) states:
The Board of Environmental Protection may, after public hearing, adopt ... reasonable rules and regulations necessary for the proper ... interpretation of any provision of law that the department [of environmental protection] is charged with the duty of administering. Rules duly promulgated shall have the full force and effect of law.
. The regulations did detail the Board’s concern for erosion of topsoil, stating that the Board “considers topsoil to be a natural resource which should be properly managed.” The proposed topsoil mining presents a different question, however, as it would eliminate, rather than erode, the topsoil of the applicant’s 40-acre lot.
.“A legislative rule is the product of an exercise of delegated power to make law through rules.” 2 K. Davis, Administrative Law Treatise § 7:8, at 36 (2d ed. 1979).
. See, e.g., Shapiro, The Choice of Rulemaking or Adjudication in the Development of Administrative Policy, 78 Harv.L.Rev. 921, 927-28 (1965) (“the accumulation of experience in individual cases is [often] a necessary prelude to any effort to elaborate statutory standards in a manner that deals realistically with actual problems rather than with hypothetical cases that may never arise”).