Shading is visible through object using surface shader

I currently have the following surface shader:

 Shader "Custom/Blend 4 Textures, Vertex Color" {
     Properties
     {
         _Layer1("Texture 1", 2D) = "black" {}
         _Layer2("Texture 2", 2D) = "black" {}
         _Layer3("Texture 3", 2D) = "black" {}
         _Layer4("Texture 4", 2D) = "black" {}
     }
         SubShader
     {
 
         Tags{ "Queue" = "Geometry" }
 
         Lighting Off
 
         CGPROGRAM
 #pragma surface surf Lambert
 
         sampler2D    _Layer1;
     sampler2D    _Layer2;
     sampler2D    _Layer3;
     sampler2D    _Layer4;
 
     struct Input
     {
         fixed4 color : COLOR;
         fixed2 uv_Layer1 : TEXCOORD0;
     };
 
     void surf(Input i, inout SurfaceOutput o)
     {
         half4 l1 = tex2D(_Layer1, i.uv_Layer1);
         half4 l2 = tex2D(_Layer2, i.uv_Layer1);
         half4 l3 = tex2D(_Layer3, i.uv_Layer1);
         half4 l4 = tex2D(_Layer4, i.uv_Layer1);
 
         float4 color = float4(1, 1, 1, 1);
         color.rgb = l1.rgb;
         color.rgb = l2.rgb * (1 - i.color.r) + color.rgb * i.color.r;
         color.rgb = l3.rgb * (1 - i.color.g) + color.rgb * i.color.g;
         color.rgb = l4.rgb * (1 - i.color.b) + color.rgb * i.color.b;
         o.Albedo = color.rgb;
         o.Alpha = color.a;
     }
         ENDCG
     }
 
 }
 

I'm using this for a model of terrain, so that I can blend four different terrain textures. However, the shading on other objects seems to be visible through the object with the surface shader:

The tree here is using the Tree Creator shaders, but with Standard it does the same thing. Therefore, I suspect the issue is with the custom surface shader. Does anyone have an idea what is wrong?