iOS+SceneKit: How to apply toon shader on texture?

I want to use a toon-shader with SceneKit on iOS8+ to render a sphere with an Earth texture on it. I also want to add a toon shader for rendering the Earth. So far the shader works on the lighting, but the texture isn't shaded by the toon-shader (cp. to the image below, the texture should also be "tooned").

Someone any ideas?

Here's my view controller code (self.sceneKitView is an instance of SCNView):

@implementation ToonViewController

- (void)viewDidLoad {

    [super viewDidLoad];

    SCNScene *scene = [SCNScene scene];

    // create and add a camera to the scene
    SCNNode *cameraNode = [SCNNode node];
    cameraNode.camera = [SCNCamera camera];
    [scene.rootNode addChildNode:cameraNode];
    // place the camera
    cameraNode.position = SCNVector3Make(0, 0, 15);

    // create and add a light to the scene
    SCNNode *lightNode = [SCNNode node];
    lightNode.light = [SCNLight light];
    lightNode.light.type = SCNLightTypeOmni;
    lightNode.position = SCNVector3Make(0, 10, 10);
    [scene.rootNode addChildNode:lightNode];

    // create and add an ambient light to the scene
    SCNNode *ambientLightNode = [SCNNode node];
    ambientLightNode.light = [SCNLight light];
    ambientLightNode.light.type = SCNLightTypeAmbient;
    ambientLightNode.light.color = [UIColor darkGrayColor];
    [scene.rootNode addChildNode:ambientLightNode];

    // set up the scene
    self.sceneKitView.scene = scene;
    self.sceneKitView.allowsCameraControl = YES;
    self.sceneKitView.showsStatistics = NO;
    self.sceneKitView.backgroundColor = [UIColor clearColor];

    NSMutableDictionary* shaders = [[NSMutableDictionary alloc] init];
    shaders[SCNShaderModifierEntryPointLightingModel] = [[NSString alloc] initWithContentsOfURL:[[NSBundle mainBundle] URLForResource:@"fixed_toon" withExtension:@"shader"]
                                                                                       encoding:NSUTF8StringEncoding
                                                                                          error:nil];

    SCNNode* earth = [SCNNode nodeWithGeometry:[SCNSphere sphereWithRadius:5.0]];
    earth.position = SCNVector3Make(0.0, 0.0, 0.0);
    [scene.rootNode addChildNode:earth];

    [earth runAction:[SCNAction repeatActionForever:[SCNAction rotateByX:0.0 y:0.25 z:0.0 duration:1.0]]];

    SCNMaterial* earthMaterial = [SCNMaterial material];
    earthMaterial.diffuse.contents = [UIImage imageNamed:@"Earth.png"];
    earthMaterial.specular.contents = [UIColor whiteColor];
    earthMaterial.specular.intensity = 0.2;
    earthMaterial.locksAmbientWithDiffuse = NO;
    earthMaterial.shaderModifiers = shaders;

    earth.geometry.firstMaterial = earthMaterial;

}

@end

And this is the fixed_toon.shader file:

vec3 lDir = normalize(vec3(0.1, 1.0, 1.0));
float dotProduct = dot(_surface.normal, lDir);

_lightingContribution.diffuse += (dotProduct*dotProduct*_light.intensity.rgb);
_lightingContribution.diffuse = floor(_lightingContribution.diffuse*4.0)/3.0;

vec3 halfVector = normalize(lDir + _surface.view);

dotProduct = max(0.0, pow(max(0.0, dot(_surface.normal, halfVector)), _surface.shininess));
dotProduct = floor(dotProduct*3.0)/3.0;

//_lightingContribution.specular += (dotProduct*_light.intensity.rgb);
_lightingContribution.specular = vec3(0,0,0);